The Different Forms of Flowers on Plants of the Same Species
by
Charles Darwin
Part 2 out of 6
Many of the seeds very poor, some good.
Column 3: Illegitimate union.
Long-styled oxlip, by pollen of long-styled primrose: 11 flowers fertilised,
produced four capsules, containing 10, 7, 5, and 6 wretched seeds. Average 7.0.
Column 4: Legitimate union.
Long-styled oxlip, by pollen of short-styled primrose: 5 flowers fertilised,
produced five capsules, containing 26, 32, 23, 28, and 34 seeds. Average 28.6.
TABLE 2.17. Both forms of the Cowslip crossed with Pollen of both forms of the
Oxlip.
Column 1: Illegitimate union.
Short-styled cowslip, by pollen of short-styled oxlip: 8 flowers fertilised, did
not produce one capsule.
Column 2: Legitimate union.
Long-styled cowslip, by pollen of short-styled oxlip: 8 flowers fertilised,
produced one capsule, containing 26 seeds.
Column 3: Illegitimate union.
Long-styled cowslip, by pollen of long-styled oxlip: 8 flowers fertilised,
produced three capsules, containing 5, 6 and 14 seeds. Average 8.3.
Column 4: Legitimate union.
Short-styled cowslip, by pollen of long-styled oxlip: 8 flowers fertilised,
produced 8 capsules, containing 58, 38, 31, 44, 23, 26, 37, and 66 seeds.
Average 40.4.
TABLE 2.18. Both forms of the Primrose crossed with Pollen of both forms of the
Oxlip.
Column 1: Illegitimate union.
Short-styled primrose, by pollen of short-styled oxlip: 8 flowers fertilised,
did not produce one capsule.
Column 2: Legitimate union.
Long-styled primrose, by pollen of short-styled oxlip: 8 flowers fertilised,
produced two capsules, containing 5 and 2 seeds.
Column 3: Illegitimate union.
Long-styled primrose, by pollen of long-styled oxlip: 8 flowers fertilised,
produced 8 capsules, containing 15, 7, 12, 20, 22, 7, 16, and 13 seeds. Average
14.0.
Column 4: Legitimate union.
Short-styled primrose, by pollen of long-styled oxlip: 8 flowers fertilised,
produced 4 capsules, containing 52, 52, 42, and 49 seeds, some good and some
bad. Average 48.7.
We see in Tables 2/14 to 2/18 the number of capsules and of seeds produced, by
crossing both forms of the oxlip in a legitimate and illegitimate manner with
one another, and with the two forms of the primrose and cowslip. I may premise
that the pollen of two of the short-styled oxlips consisted of nothing but
minute aborted whitish cells; but in the third short-styled plant about one-
fifth of the grains appeared in a sound condition. Hence it is not surprising
that neither the short-styled nor the long-styled oxlip produced a single seed
when fertilised with this pollen. Nor did the pure cowslips or primroses when
illegitimately fertilised with it; but when thus legitimately fertilised they
yielded a few good seeds. The female organs of the short-styled oxlips, though
greatly deteriorated in power, were in a rather better condition than the male
organs; for though the short-styled oxlips yielded no seed when fertilised by
the long-styled oxlips, and hardly any when illegitimately fertilised by pure
cowslips or primroses, yet when legitimately fertilised by these latter species,
especially by the long-styled primrose, they yielded a moderate supply of good
seed.
The long-styled oxlip was more fertile than the three short-styled oxlips, and
about half its pollen-grains appeared sound. It bore no seed when legitimately
fertilised by the short-styled oxlips; but this no doubt was due to the badness
of the pollen of the latter; for when illegitimately fertilised (Table 2.14) by
its own pollen it produced some good seeds, though much fewer than self-
fertilised cowslips or primroses would have produced. The long-styled oxlip
likewise yielded a very low average of seed, as may be seen in the third
compartment of Tables 2.15 to 2.18, when illegitimately fertilised by, and when
illegitimately fertilising, pure cowslips and primroses. The four corresponding
legitimate unions, however, were moderately fertile, and one (namely that
between a short-styled cowslip and the long-styled oxlip in Table 2.17) was
nearly as fertile as if both parents had been pure. A short-styled primrose
legitimately fertilised by the long-styled oxlip (Table 2.18) also yielded a
moderately good average, namely 48.7 seeds; but if this short-styled primrose
had been fertilised by a long-styled primrose it would have yielded an average
of 65 seeds. If we take the ten legitimate unions together, and the ten
illegitimate unions together, we shall find that 29 per cent of the flowers
fertilised in a legitimate manner yielded capsules, these containing on an
average 27.4 good and bad seeds; whilst only 15 per cent of the flowers
fertilised in an illegitimate manner yielded capsules, these containing on an
average only 11.0 good and bad seeds.
In a previous part of this chapter it was shown that illegitimate crosses
between the long-styled form of the primrose and the long-styled cowslip, and
between the short-styled primrose and short-styled cowslip, are more sterile
than legitimate crosses between these two species; and we now see that the same
rule holds good almost invariably with their hybrid offspring, whether these are
crossed inter se, or with either parent-species; so that in this particular
case, but not as we shall presently see in other cases, the same rule prevails
with the pure unions between the two forms of the same heterostyled species,
with crosses between two distinct heterostyled species, and with their hybrid
offspring.
Seeds from the long-styled oxlip fertilised by its own pollen were sown, and
three long-styled plants raised. The first of these was identical in every
character with its parent. The second bore rather smaller flowers, of a paler
colour, almost like those of the primrose; the scapes were at first single-
flowered, but later in the season a tall thick scape, bearing many flowers, like
that of the parent oxlip, was thrown up. The third plant likewise produced at
first only single-flowered scapes, with the flowers rather small and of a darker
yellow; but it perished early. The second plant also died in September; and the
first plant, though all three grew under very favourable conditions, looked very
sickly. Hence we may infer that seedlings from self-fertilised oxlips would
hardly be able to exist in a state of nature. I was surprised to find that all
the pollen-grains in the first of these seedling oxlips appeared sound; and in
the second only a moderate number were bad. These two plants, however, had not
the power of producing a proper number of seeds; for though left uncovered and
surrounded by pure primroses and cowslips, the capsules were estimated to
include an average of only from fifteen to twenty seeds.
From having many experiments in hand, I did not sow the seed obtained by
crossing both forms of the primrose and cowslip with both forms of the oxlip,
which I now regret; but I ascertained an interesting point, namely, the
character of the offspring from oxlips growing in a state of nature near both
primroses and cowslips. The oxlips were the same plants which, after their seeds
had been collected, were transplanted and experimented on. From the seeds thus
obtained eight plants were raised, which, when they flowered, might have been
mistaken for pure primroses; but on close comparison the eye in the centre of
the corolla was seen to be of a darker yellow, and the peduncles more elongated.
As the season advanced, one of these plants threw up two naked scapes, 7 inches
in height, which bore umbels of flowers of the same character as before. This
fact led me to examine the other plants after they had flowered and were dug up;
and I found that the flower-peduncles of all sprung from an extremely short
common scape, of which no trace can be found in the pure primrose. Hence these
plants are beautifully intermediate between the oxlip and the primrose,
inclining rather towards the latter; and we may safely conclude that the parent
oxlips had been fertilised by the surrounding primroses.
From the various facts now given, there can be no doubt that the common oxlip is
a hybrid between the cowslip (P. veris, Brit. Fl.) and the primrose (P.
vulgaris, Brit. Fl.), as has been surmised by several botanists. It is probable
that oxlips may be produced either from the cowslip or the primrose as the seed-
bearer, but oftenest from the latter, as I judge from the nature of the stations
in which oxlips are generally found (2/13. See also on this head Hardwicke's
'Science Gossip' 1867 pages 114, 137.), and from the primrose when crossed by
the cowslip being more fertile than, conversely, the cowslip by the primrose.
The hybrids themselves are also rather more fertile when crossed with the
primrose than with the cowslip. Whichever may be the seed-bearing plant, the
cross is probably between different forms of the two species; for we have seen
that legitimate hybrid unions are more fertile than illegitimate hybrid unions.
Moreover a friend in Surrey found that 29 oxlips which grew in the neighbourhood
of his house consisted of 13 long-styled and 16 short-styled plants; now, if the
parent-plants had been illegitimately united, either the long- or short-styled
form would have greatly preponderated, as we shall hereafter see good reason to
believe. The case of the oxlip is interesting; for hardly any other instance is
known of a hybrid spontaneously arising in such large numbers over so wide an
extent of country. The common oxlip (not the P. elatior of Jacq.) is found
almost everywhere throughout England, where both cowslips and primroses grow. In
some districts, as I have seen near Hartfield in Sussex and in parts of Surrey,
specimens may be found on the borders of almost every field and small wood. In
other districts the oxlip is comparatively rare: near my own residence I have
found, during the last twenty-five years, not more than five or six plants or
groups of plants. It is difficult to conjecture what is the cause of this
difference in their number. It is almost necessary that a plant, or several
plants belonging to the same form, of one parent-species, should grow near the
opposite form of the other parent-species; and it is further necessary that both
species should be frequented by the same kind of insect, no doubt a moth. The
cause of the rare appearance of the oxlip in certain districts may be the rarity
of some moth, which in other districts habitually visits both the primrose and
cowslip.
Finally, as the cowslip and primrose differ in the various characters above
specified,--as they are in a high degree sterile when intercrossed,--as there is
no trustworthy evidence that either species, when uncrossed, has ever given
birth to the other species or to any intermediate form,--and as the intermediate
forms which are often found in a state of nature have been shown to be more or
less sterile hybrids of the first or second generation,--we must for the future
look at the cowslip and primrose as good and true species.
Primula elatior, Jacq., or the Bardfield Oxlip, is found in England only in two
or three of the eastern counties. On the Continent it has a somewhat different
range from that of the cowslip and primrose; and it inhabits some districts
where neither of these species live. (2/14. For England, see Hewett C. Watson
'Cybele Britannica' volume 2 1849 page 292. For the Continent, see Lecoq
'Geograph. Botanique de l'Europe' tome 8 1858 page 142. For the Alps see 'Annals
and Magazine of Natural History' volume 9 1842 pages 156 and 515.) In general
appearance it differs so much from the common oxlip, that no one accustomed to
see both forms in the living state could afterwards confound them; but there is
scarcely more than a single character by which they can be distinctly defined,
namely, their linear-oblong capsules equalling the calyx in length. (2/15.
Babington 'Manual of British Botany' 1851 page 258.) The capsules when mature
differ conspicuously, owing to their length, from those of the cowslip and
primrose. With respect to the fertility of the two forms when these are united
in the four possible methods, they behave like the other heterostyled species of
the genus, but differ somewhat (see Tables 1.8 and 1.12.) in the smaller
proportion of the illegitimately fertilised flowers which set capsules. That P.
elatior is not a hybrid is certain, for when the two forms were legitimately
united they yielded the large average of 47.1 seeds, and when illegitimately
united 35.5 per capsule; whereas, of the four possible unions (Table 2.14)
between the two forms of the common oxlip which we know to be a hybrid, one
alone yielded any seed; and in this case the average number was only 11.6 per
capsule. Moreover I could not detect a single bad pollen-grain in the anthers of
the short-styled P. elatior; whilst in two short-styled plants of the common
oxlip all the grains were bad, as were a large majority in a third plant. As the
common oxlip is a hybrid between the primrose and cowslip, it is not surprising
that eight long-styled flowers of the primrose, fertilised by pollen from the
long-styled common oxlip, produced eight capsules (Table 1.18), containing,
however, only a low average of seeds; whilst the same number of flowers of the
primrose, similarly fertilised by the long-styled Bardfield oxlip, produced only
a single capsule; this latter plant being an altogether distinct species from
the primrose. Plants of P. elatior have been propagated by seed in a garden for
twenty-five years, and have kept all this time quite constant, excepting that in
some cases the flowers varied a little in size and tint. (2/16. See Mr. H.
Doubleday in the 'Gardener's Chronicle' 1867 page 435, also Mr. W. Marshall
ibid. page 462.) Nevertheless, according to Mr. H.C. Watson and Dr. Bromfield
(2/17. 'Phytologist' volume 1 page 1001 and volume 3 page 695.), plants may be
occasionally found in a state of nature, in which most of the characters by
which this species can be distinguished from P. veris and vulgaris fail; but
such intermediate forms are probably due to hybridisation; for Kerner states, in
the paper before referred to, that hybrids sometimes, though rarely, arise in
the Alps between P. elatior and veris.
Finally, although we may freely admit that Primula veris, vulgaris, and elatior,
as well as all the other species of the genus, are descended from a common
primordial form, yet from the facts above given, we must conclude that these
three forms are now as fixed in character as are many others which are
universally ranked as true species. Consequently they have as good a right to
receive distinct specific names as have, for instance, the ass, quagga, and
zebra.
Mr. Scott has arrived at some interesting results by crossing other heterostyled
species of Primula. (2/18. 'Journal of the Linnean Society Botany' volume 8 1864
page 93 to end.) I have already alluded to his statement, that in four instances
(not to mention others) a species when crossed with a distinct one yielded a
larger number of seeds than the same species fertilised illegitimately with its
own-form pollen, though taken from a distinct plant. It has long been known from
the researches of Kolreuter and Gartner, that two species when crossed
reciprocally sometimes differ as widely as is possible in their fertility: thus
A when crossed with the pollen of B will yield a large number of seeds, whilst B
may be crossed repeatedly with pollen of A, and will never yield a single seed.
Now Mr. Scott shows in several cases that the same law holds good when two
heterostyled species of Primula are intercrossed, or when one is crossed with a
homostyled species. But the results are much more complicated than with ordinary
plants, as two heterostyled dimorphic species can be intercrossed in eight
different ways. I will give one instance from Mr. Scott. The long-styled P.
hirsuta fertilised legitimately and illegitimately with pollen from the two
forms of P. auricula, and reciprocally the long-styled P. auricula fertilised
legitimately and illegitimately with pollen from the two forms of P. hirsuta,
did not produce a single seed. Nor did the short-styled P. hirsuta when
fertilised legitimately and illegitimately with the pollen of the two forms of
P. auricula. On the other hand, the short-styled P. auricula fertilised with
pollen from the long-styled P. hirsuta yielded capsules containing on an average
no less than 56 seeds; and the short-styled P. auricula by pollen of the short-
styled P. hirsuta yielded capsules containing on an average 42 seeds per
capsule. So that out of the eight possible unions between the two forms of these
two species, six were utterly barren, and two fairly fertile. We have seen also
the same sort of extraordinary irregularity in the results of my twenty
different crosses (Tables 2.14 to 2.18), between the two forms of the oxlip,
primrose, and cowslip. Mr. Scott remarks, with respect to the results of his
trials, that they are very surprising, as they show us that "the sexual forms of
a species manifest in their respective powers for conjunction with those of
another species, physiological peculiarities which might well entitle them, by
the criterion of fertility, to specific distinction."
Finally, although P. veris and vulgaris, when crossed legitimately, and
especially when their hybrid offspring are crossed in this manner with both
parent-species, were decidedly more fertile, than when crossed in an
illegitimate manner, and although the legitimate cross effected by Mr. Scott
between P. auricula and hirsuta was more fertile, in the ratio of 56 to 42, than
the illegitimate cross, nevertheless it is very doubtful, from the extreme
irregularity of the results in the various other hybrid crosses made by Mr.
Scott, whether it can be predicted that two heterostyled species are generally
more fertile if crossed legitimately (i.e. when opposite forms are united) than
when crossed illegitimately.
SUPPLEMENTARY NOTE ON SOME WILD HYBRID VERBASCUMS.
In an early part of this chapter I remarked that few other instances could be
given of a hybrid spontaneously arising in such large numbers, and over so wide
an extent of country, as that of the common oxlip; but perhaps the number of
well-ascertained cases of naturally produced hybrid willows is equally great.
(2/19. Max Wichura 'Die Bastardbefruchtung etc. der Weiden' 1865.) Numerous
spontaneous hybrids between several species of Cistus, found near Narbonne, have
been carefully described by M. Timbal-Lagrave (2/20. 'Mem. de l'Acad. des
Sciences de Toulouse' 5e serie tome 5 page 28.), and many hybrids between an
Aceras and Orchis have been observed by Dr. Weddell. (2/21. 'Annales des Sc.
Nat.' 3e serie Bot. tome 18 page 6.) In the genus Verbascum, hybrids are
supposed to have often originated in a state of nature (2/22. See for instance
the 'English Flora' by Sir J.E. Smith 1824 volume 1 page 307.); some of these
undoubtedly are hybrids, and several hybrids have originated in gardens; but
most of these cases require, as Gartner remarks, verification. (2/23. See
Gartner 'Bastarderzeugung' 1849 page 590.) Hence the following case is worth
recording, more especially as the two species in question, V. thapsus and
lychnitis, are perfectly fertile when insects are excluded, showing that the
stigma of each flower receives its own pollen. Moreover the flowers offer only
pollen to insects, and have not been rendered attractive to them by secreting
nectar.
I transplanted a young wild plant into my garden for experimental purposes, and
when it flowered it plainly differed from the two species just mentioned and
from a third which grows in this neighbourhood. I thought that it was a strange
variety of V. thapsus. It attained the height (by measurement) of 8 feet! It was
covered with a net, and ten flowers were fertilised with pollen from the same
plant; later in the season, when uncovered, the flowers were freely visited by
pollen-collecting bees; nevertheless, although many capsules were produced, not
one contained a single seed. During the following year this same plant was left
uncovered near plants of V. thapsus and lychnitis; but again it did not produce
a single seed. Four flowers, however, which were repeatedly fertilised with
pollen of V. lychnitis, whilst the plant was temporarily kept under a net,
produced four capsules, which contained five, one, two, and two seeds; at the
same time three flowers were fertilised with pollen of V. thapsus, and these
produced two, two, and three seeds. To show how unproductive these seven
capsules were, I may state that a fine capsule from a plant of V. thapsus
growing close by contained above 700 seeds. These facts led me to search the
moderately-sized field whence my plant had been removed, and I found in it many
plants of V. thapsus and lychnitis as well as thirty-three plants intermediate
in character between these two species. These thirty-three plants differed much
from one another. In the branching of the stem they more closely resembled V.
lychnitis than V. thapsus, but in height the latter species. In the shape of
their leaves they often closely approached V. lychnitis, but some had leaves
extremely woolly on the upper surface and decurrent like those of V. thapsus;
yet the degree of woolliness and of decurrency did not always go together. In
the petals being flat and remaining open, and in the manner in which the anthers
of the longer stamens were attached to the filaments, these plants all took more
after V. lychnitis than V. thapsus. In the yellow colour of the corolla they all
resembled the latter species. On the whole, these plants appeared to take rather
more after V. lychnitis than V. thapsus. On the supposition that they were
hybrids, it is not an anomalous circumstance that they should all have produced
yellow flowers; for Gartner crossed white and yellow-flowered varieties of
Verbascum, and the offspring thus produced never bore flowers of an intermediate
tint, but either pure white or pure yellow flowers, generally of the latter
colour. (2/24. 'Bastardzeugung' page 307.)
My observations were made in the autumn; so that I was able to collect some
half-matured capsules from twenty of the thirty-three intermediate plants, and
likewise capsules of the pure V. lychnitis and thapsus growing in the same
field. All the latter were filled with perfect but immature seeds, whilst the
capsules of the twenty intermediate plants did not contain one single perfect
seed. These plants, consequently, were absolutely barren. From this fact,--from
the one plant which was transplanted into my garden yielding when artificially
fertilised with pollen from V. lychnitis and thapsus some seeds, though
extremely few in number,--from the circumstance of the two pure species growing
in the same field,--and from the intermediate character of the sterile plants,
there can be no doubt that they were hybrids. Judging from the position in which
they were chiefly found, I am inclined to believe they were descended from V.
thapsus as the seed-bearer, and V. lychnitis as the pollen-bearer.
It is known that many species of Verbascum, when the stem is jarred or struck by
a stick, cast off their flowers. (2/25. This was first observed by Correa de
Serra: see Sir J.E. Smith's 'English Flora' 1824 volume 1 page 311; also 'Life
of Sir J.E. Smith' volume 2 page 210. I was guided to these references by the
Reverend W.A. Leighton, who observed this same phenomenon with V. virgatum.)
This occurs with V. thapsus, as I have repeatedly observed. The corolla first
separates from its attachment, and then the sepals spontaneously bend inwards so
as to clasp the ovarium, pushing off the corolla by their movement, in the
course of two or three minutes. Nothing of this kind takes place with young
barely expanded flowers. With Verbascum lychnitis and, as I believe, V.
phoeniceum the corolla is not cast off, however often and severely the stem may
be struck. In this curious property the above-described hybrids took after V.
thapsus; for I observed, to my surprise, that when I pulled off the flower-buds
round the flowers which I wished to mark with a thread, the slight jar
invariably caused the corollas to fall off.
These hybrids are interesting under several points of view. First, from the
number found in various parts of the same moderately-sized field. That they owed
their origin to insects flying from flower to flower, whilst collecting pollen,
there can be no doubt. Although insects thus rob the flowers of a most precious
substance, yet they do great good; for, as I have elsewhere shown, the seedlings
of V. thapsus raised from flowers fertilised with pollen from another plant, are
more vigorous than those raised from self-fertilised flowers. (2/26. 'The
Effects of Cross and Self-fertilisation' 1876 page 89.) But in this particular
instance the insects did great harm, as they led to the production of utterly
barren plants. Secondly, these hybrids are remarkable from differing much from
one another in many of their characters; for hybrids of the first generation, if
raised from uncultivated plants, are generally uniform in character. That these
hybrids belonged to the first generation we may safely conclude, from the
absolute sterility of all those observed by me in a state of nature and of the
one plant in my garden, excepting when artificially and repeatedly fertilised
with pure pollen, and then the number of seeds produced was extremely small. As
these hybrids varied so much, an almost perfectly graduated series of forms,
connecting together the two widely distinct parent-species, could easily have
been selected. This case, like that of the common oxlip, shows that botanists
ought to be cautious in inferring the specific identity of two forms from the
presence of intermediate gradations; nor would it be easy in the many cases in
which hybrids are moderately fertile to detect a slight degree of sterility in
such plants growing in a state of nature and liable to be fertilised by either
parent-species. Thirdly and lastly, these hybrids offer an excellent
illustration of a statement made by that admirable observer Gartner, namely,
that although plants which can be crossed with ease generally produce fairly
fertile offspring, yet well-pronounced exceptions to this rule occur; and here
we have two species of Verbascum which evidently cross with the greatest ease,
but produce hybrids which are excessively sterile.
CHAPTER III. HETEROSTYLED DIMORPHIC PLANTS--continued.
Linum grandiflorum, long-styled form utterly sterile with own-form pollen.
Linum perenne, torsion of the pistils in the long-styled form alone.
Homostyled species of Linum.
Pulmonaria officinalis, singular difference in self-fertility between the
English and German long-styled plants.
Pulmonaria angustifolia shown to be a distinct species, long-styled form
completely self-sterile.
Polygonum fagopyrum.
Various other heterostyled genera.
Rubiaceae.
Mitchella repens, fertility of the flowers in pairs.
Houstonia.
Faramea, remarkable difference in the pollen-grains of the two forms; torsion of
the stamens in the short-styled form alone; development not as yet perfect.
The heterostyled structure in the several Rubiaceous genera not due to descent
in common.
(FIGURE 3.4. Linum grandiflorum.
Left: Long-styled form.
Right: Short-styled form.
s, s: stigmas.)
It has long been known that several species of Linum present two forms (3/1.
Treviranus has shown that this is the case in his review of my original paper
'Botanische Zeitung' 1863 page 189.), and having observed this fact in L. flavum
more than thirty years ago, I was led, after ascertaining the nature of
heterostylism in Primula, to examine the first species of Linum which I met
with, namely, the beautiful L. grandiflorum. This plant exists under two forms,
occurring in about equal numbers, which differ little in structure, but greatly
in function. The foliage, corolla, stamens, and pollen-grains (the latter
examined both distended with water and dry) are alike in the two forms (Figure
3.4). The difference is confined to the pistil; in the short-styled form the
styles and the stigmas are only about half the length of those in the long-
styled. A more important distinction is, that the five stigmas in the short-
styled form diverge greatly from one another, and pass out between the filaments
of the stamens, and thus lie within the tube of the corolla. In the long-styled
form the elongated stigmas stand nearly upright, and alternate with the anthers.
In this latter form the length of the stigmas varies considerably, their upper
extremities projecting even a little above the anthers, or reaching up only to
about their middle. Nevertheless, there is never the slightest difficulty in
distinguishing between the two forms; for, besides the difference in the
divergence of the stigmas, those of the short-styled form never reach even to
the bases of the anthers. In this form the papillae on the stigmatic surfaces
are shorter, darker-coloured, and more crowded together than in the long-styled
form; but these differences seem due merely to the shortening of the stigma, for
in the varieties of the long-styled form with shorter stigmas, the papillae are
more crowded and darker-coloured than in those with the longer stigmas.
Considering the slight and variable differences between the two forms of this
Linum, it is not surprising that hitherto they have been overlooked.
In 1861 I had eleven plants in my garden, eight of which were long-styled, and
three short-styled. Two very fine long-styled plants grew in a bed a hundred
yards off all the others, and separated from them by a screen of evergreens. I
marked twelve flowers, and placed on their stigmas a little pollen from the
short-styled plants. The pollen of the two forms is, as stated, identical in
appearance; the stigmas of the long-styled flowers were already thickly covered
with their own pollen--so thickly that I could not find one bare stigma, and it
was late in the season, namely, September 15th. Altogether, it seemed almost
childish to expect any result. Nevertheless from my experiments on Primula, I
had faith, and did not hesitate to make the trial, but certainly did not
anticipate the full result which was obtained. The germens of these twelve
flowers all swelled, and ultimately six fine capsules (the seed of which
germinated on the following year) and two poor capsules were produced; only four
capsules shanking off. These same two long-styled plants produced, in the course
of the summer, a vast number of flowers, the stigmas of which were covered with
their own pollen; but they all proved absolutely barren, and their germens did
not even swell.
The nine other plants, six long-styled and three short-styled, grew not very far
apart in my flower-garden. Four of these long-styled plants produced no seed-
capsules; the fifth produced two; and the remaining one grew so close to a
short-styled plant that their branches touched, and this produced twelve
capsules, but they were poor ones. The case was different with the short-styled
plants. The one which grew close to the long-styled plant produced ninety-four
imperfectly fertilised capsules containing a multitude of bad seeds, with a
moderate number of good ones. The two other short-styled plants growing together
were small, being partly smothered by other plants; they did not stand very
close to any long-styled plants, yet they yielded together nineteen capsules.
These facts seem to show that the short-styled plants are more fertile with
their own pollen than are the long-styled, and we shall immediately see that
this probably is the case. But I suspect that the difference in fertility
between the two forms was in this instance in part due to a distinct cause. I
repeatedly watched the flowers, and only once saw a humble-bee momentarily
alight on one, and then fly away. If bees had visited the several plants, there
cannot be a doubt that the four long-styled plants, which did not produce a
single capsule, would have borne an abundance. But several times I saw small
diptera sucking the flowers; and these insects, though not visiting the flowers
with anything like the regularity of bees, would carry a little pollen from one
form to the other, especially when growing near together; and the stigmas of the
short-styled plants, diverging within the tube of the corolla, would be more
likely than the upright stigmas of the long-styled plants, to receive a small
quantity of pollen if brought to them by small insects. Moreover from the
greater number of the long-styled than of the short-styled plants in the garden,
the latter would be more likely to receive pollen from the long-styled, than the
long-styled from the short-styled.
In 1862 I raised thirty-four plants of this Linum in a hot-bed; and these
consisted of seventeen long-styled and seventeen short-styled forms. Seed sown
later in the flower-garden yielded seventeen long-styled and twelve short-styled
forms. These facts justify the statement that the two forms are produced in
about equal numbers. The thirty-four plants of the first lot were kept under a
net which excluded all insects, except such minute ones as Thrips. I fertilised
fourteen long-styled flowers legitimately with pollen from the short-styled, and
got eleven fine seed-capsules, which contained on an average 8.6 seeds per
capsule, but only 5.6 appeared to be good. It may be well to state that ten
seeds is the maximum production for a capsule, and that our climate cannot be
very favourable to this North-African plant. On three occasions the stigmas of
nearly a hundred flowers were fertilised illegitimately with their own-form
pollen, taken from separate plants, so as to prevent any possible ill effects
from close inter-breeding. Many other flowers were also produced, which, as
before stated, must have received plenty of their own pollen; yet from all these
flowers, borne by the seventeen long-styled plants, only three capsules were
produced. One of these included no seed, and the other two together gave only
five good seeds. It is probable that this miserable product of two half-fertile
capsules from the seventeen plants, each of which must have produced at least
fifty or sixty flowers, resulted from their fertilisation with pollen from the
short-styled plants by the aid of Thrips; for I made a great mistake in keeping
the two forms under the same net, with their branches often interlocking; and it
is surprising that a greater number of flowers were not accidentally fertilised.
Twelve short-styled flowers were in this instance castrated, and afterwards
fertilised legitimately with pollen from the long-styled form; and they produced
seven fine capsules. These included on an average 7.6 seeds, but of apparently
good seed only 4.3 per capsule. At three separate times nearly a hundred flowers
were fertilised illegitimately with their own-form pollen, taken from separate
plants; and numerous other flowers were produced, many of which must have
received their own pollen. From all these flowers on the seventeen short-styled
plants only fifteen capsules were produced, of which only eleven contained any
good seed, on an average 4.2 per capsule. As remarked in the case of the long-
styled plants, some even of these capsules were perhaps the product of a little
pollen accidentally fallen from the adjoining flowers of the other form on to
the stigmas, or transported by Thrips. Nevertheless the short-styled plants seem
to be slightly more fertile with their own pollen than the long-styled, in the
proportion of fifteen capsules to three; nor can this difference be accounted
for by the short-styled stigmas being more liable to receive their own pollen
than the long-styled, for the reverse is the case. The greater self-fertility of
the short-styled flowers was likewise shown in 1861 by the plants in my flower-
garden, which were left to themselves, and were but sparingly visited by
insects.
On account of the probability of some of the flowers on the plants of both
forms, which were covered under the same net, having been legitimately
fertilised in an accidental manner, the relative fertility of the two legitimate
and two illegitimate unions cannot be compared with certainty; but judging from
the number of good seeds per capsule, the difference was at least in the ratio
of 100 to 7, and probably much greater.
Hildebrand tested my results, but only on a single short-styled plant, by
fertilising many flowers with their own-form pollen; and these did not produce
any seed. This confirms my suspicion that some of the few capsules produced by
the foregoing seventeen short-styled plants were the product of accidental
legitimate fertilisation. Other flowers on the same plant were fertilised by
Hildebrand with pollen from the long-styled form, and all produced fruit. (3/2.
'Botanische Zeitung' January 1, 1864 page 2.)
The absolute sterility (judging from the experiments of 1861) of the long-styled
plants with their own-form pollen led me to examine into its apparent cause; and
the results are so curious that they are worth giving in detail. The experiments
were tried on plants grown in pots and brought successively into the house.
FIRST.
Pollen from a short-styled plant was placed on the five stigmas of a long-styled
flower, and these, after thirty hours, were found deeply penetrated by a
multitude of pollen-tubes, far too numerous to be counted; the stigmas had also
become discoloured and twisted. I repeated this experiment on another flower,
and in eighteen hours the stigmas were penetrated by a multitude of long pollen-
tubes. This is what might have been expected, as the union is a legitimate one.
The converse experiment was likewise tried, and pollen from a long-styled flower
was placed on the stigmas of a short-styled flower, and in twenty-four hours the
stigmas were discoloured, twisted, and penetrated by numerous pollen-tubes; and
this, again, is what might have been expected, as the union was a legitimate
one.
SECONDLY.
Pollen from a long-styled flower was placed on all five stigmas of a long-styled
flower on a separate plant: after nineteen hours the stigmas were dissected, and
only a single pollen-grain had emitted a tube, and this was a very short one. To
make sure that the pollen was good, I took in this case, and in most of the
other cases, pollen either from the same anther or from the same flower, and
proved it to be good by placing it on the stigma of a short-styled plant, and
found numerous pollen-tubes emitted.
THIRDLY.
Repeated last experiment, and placed own-form pollen on all five stigmas of a
long-styled flower; after nineteen hours and a half, not one single grain had
emitted its tube.
FOURTHLY.
Repeated the experiment, with the same result after twenty-four hours.
FIFTHLY.
Repeated last experiment, and, after leaving pollen on for nineteen hours, put
on an additional quantity of own-form pollen on all five stigmas. After an
interval of three days, the stigmas were examined, and, instead of being
discoloured and twisted, they were straight and fresh-coloured. Only one grain
had emitted a quite short tube, which was drawn out of the stigmatic tissue
without being ruptured.
The following experiments are more striking:--
SIXTHLY.
I placed own-form pollen on three of the stigmas of a long-styled flower, and
pollen from a short-styled flower on the other two stigmas. After twenty-two
hours these two stigmas were discoloured, slightly twisted, and penetrated by
the tubes of numerous pollen-grains: the other three stigmas, covered with their
own-form pollen, were fresh, and all the pollen-grains were loose; but I did not
dissect the whole stigma.
SEVENTHLY.
Experiment repeated in the same manner, with the same result.
EIGHTHLY.
Experiment repeated, but the stigmas were carefully examined after an interval
of only five hours and a half. The two stigmas with pollen from a short-styled
flower were penetrated by innumerable tubes, which were as yet short, and the
stigmas themselves were not at all discoloured. The three stigmas covered with
their own-form pollen were not penetrated by a single pollen-tube.
NINTHLY.
Put pollen of a short-styled flower on a single long-styled stigma, and own-form
pollen on the other four stigmas; after twenty-four hours the one stigma was
somewhat discoloured and twisted, and penetrated by many long tubes: the other
four stigmas were quite straight and fresh; but on dissecting them I found that
three pollen-grains had protruded very short tubes into the tissue.
TENTHLY.
Repeated the experiment, with the same result after twenty-four hours, excepting
that only two own-form grains had penetrated the stigmatic tissue with their
tubes to a very short depth. The one stigma, which was deeply penetrated by a
multitude of tubes from the short-styled pollen, presented a conspicuous
difference in being much curled, half-shrivelled, and discoloured, in comparison
with the other four straight and bright pink stigmas.
I could add other experiments; but those now given amply suffice to show that
the pollen-grains of a short-styled flower placed on the stigma of a long-styled
flower emit a multitude of tubes after an interval of from five to six hours,
and penetrate the tissue ultimately to a great depth; and that after twenty-four
hours the stigmas thus penetrated change colour, become twisted, and appear
half-withered. On the other hand, pollen-grains from a long-styled flower placed
on its own stigmas, do not emit their tubes after an interval of a day, or even
three days; or at most only three or four grains out of a multitude emit their
tubes, and these apparently never penetrate the stigmatic tissue deeply, and the
stigmas themselves do not soon become discoloured and twisted.
This seems to me a remarkable physiological fact. The pollen-grains of the two
forms are undistinguishable under the microscope; the stigmas differ only in
length, degree of divergence, and in the size, shade of colour, and
approximation of their papillae, these latter differences being variable and
apparently due merely to the degree of elongation of the stigma. Yet we plainly
see that the two kinds of pollen and the two stigmas are widely dissimilar in
their mutual reaction--the stigmas of each form being almost powerless on their
own pollen, but causing, through some mysterious influence, apparently by simple
contact (for I could detect no viscid secretion), the pollen-grains of the
opposite form to protrude their tubes. It may be said that the two pollens and
the two stigmas mutually recognise each other by some means. Taking fertility as
the criterion of distinctness, it is no exaggeration to say that the pollen of
the long-styled Linum grandiflorum (and conversely that of the other form) has
been brought to a degree of differentiation, with respect to its action on the
stigma of the same form, corresponding with that existing between the pollen and
stigma of species belonging to distinct genera.
Linum perenne.
This species is conspicuously heterostyled, as has been noticed by several
authors. The pistil in the long-styled form is nearly twice as long as that of
the short-styled. In the latter the stigmas are smaller and, diverging to a
greater degree, pass out low down between the filaments. I could detect no
difference in the two forms in the size of the stigmatic papillae. In the long-
styled form alone the stigmatic surfaces of the mature pistils twist round, so
as to face the circumference of the flower; but to this point I shall presently
return. Differently from what occurs in L. grandiflorum, the long-styled flowers
have stamens hardly more than half the length of those in the short-styled. The
size of the pollen-grains is rather variable; after some doubt, I have come to
the conclusion that there is no uniform difference between the grains in the two
forms. The long-stamens in the short-styled form project to some height above
the corolla, and their filaments are coloured blue apparently from exposure to
the light. The anthers of the longer stamens correspond in height with the lower
part of the stigmas of the long-styled flowers; and the anthers of the shorter
stamens of the latter correspond in the same manner in height with the stigmas
of the short-styled flowers.
I raised from seed twenty-six plants, of which twelve proved to be long-styled
and fourteen short-styled. They flowered well, but were not large plants. As I
did not expect them to flower so soon, I did not transplant them, and they
unfortunately grew with their branches closely interlocked. All the plants were
covered under the same net, excepting one of each form. Of the flowers on the
long-styled plants, twelve were illegitimately fertilised with their own-form
pollen, taken in every case from a separate plant; and not one set a seed-
capsule: twelve other flowers were legitimately fertilised with pollen from
short-styled flowers; and they set nine capsules, each including on an average 7
good seeds, ten being the maximum number ever produced. Of the flowers on the
short-styled plants, twelve were illegitimately fertilised with own-form pollen,
and they yielded one capsule, including only 3 good seeds; twelve other flowers
were legitimately fertilised with pollen from long-styled flowers, and these
produced nine capsules, but one was bad; the eight good capsules contained on an
average 8 good seeds each. Judging from the number of seeds per capsule, the
fertility of the two legitimate to that of the two illegitimate unions is as 100
to 20.
The numerous flowers on the eleven long-styled plants under the net, which were
not fertilised, produced only three capsules, including 8, 4, and 1 good seeds.
Whether these three capsules were the product of accidental legitimate
fertilisation, owing to the branches of the plants of the two forms
interlocking, I will not pretend to decide. The single long-styled plant which
was left uncovered, and grew close by the uncovered short-styled plant, produced
five good pods; but it was a poor and small plant.
The flowers borne on the thirteen short-styled plants under the net, which were
not fertilised, produced twelve capsules, containing on an average 5.6 seeds. As
some of these capsules were very fine, and as five were borne on one twig, I
suspect that some minute insect had accidentally got under the net and had
brought pollen from the other form to the flowers which produced this little
group of capsules. The one uncovered short-styled plant which grew close to the
uncovered long-styled plant yielded twelve capsules.
From these facts we have some reason to believe, as in the case of L.
grandiflorum, that the short-styled plants are in a slight degree more fertile
with their own pollen than are the long-styled plants. Anyhow we have the
clearest evidence, that the stigmas of each form require for full fertility that
pollen from the stamens of corresponding height belonging to the opposite form
should be brought to them.
Hildebrand, in the paper lately referred to, confirms my results. He placed a
short-styled plant in his house, and fertilised about 20 flowers with their own
pollen, and about 30 with pollen from another plant belonging to the same form,
and these 50 flowers did not set a single capsule. On the other hand he
fertilised about 30 flowers with pollen from the long-styled form, and these,
with the exception of two, yielded capsules, containing good seeds.
It is a singular fact, in contrast with what occurred in the case of L.
grandiflorum, that the pollen-grains of both forms of L. perenne, when placed on
their own-form stigmas, emitted their tubes, though this action did not lead to
the production of seeds. After an interval of eighteen hours, the tubes
penetrated the stigmatic tissue, but to what depth I did not ascertain. In this
case the impotence of the pollen-grains on their own stigmas must have been due
either to the tubes not reaching the ovules, or to their not acting properly
after reaching them.
The plants both of L. perenne and grandiflorum, grew, as already stated, with
their branches interlocked, and with scores of flowers of the two forms close
together; they were covered by a rather coarse net, through which the wind, when
high, passed; and such minute insects as Thrips could not, of course, be
excluded; yet we have seen that the utmost possible amount of accidental
fertilisation on seventeen long-styled plants in the one case, and on eleven
long-styled plants in the other, resulted in the production, in each case, of
three poor capsules; so that when the proper insects are excluded, the wind does
hardly anything in the way of carrying pollen from plant to plant. I allude to
this fact because botanists in speaking of the fertilisation of various flowers,
often refer to the wind or to insects as if the alternative were indifferent.
This view, according to my experience, is entirely erroneous. When the wind is
the agent in carrying pollen, either from one sex to the other, or from
hermaphrodite to hermaphrodite, we can recognise structure as manifestly adapted
to its action as to that of insects when these are the carriers. We see
adaptation to the wind in the incoherence of the pollen,--in the inordinate
quantity produced (as in the Coniferae, Spinage, etc.),--in the dangling anthers
well fitted to shake out the pollen,--in the absence or small size of the
perianth,--in the protrusion of the stigmas at the period of fertilisation,--in
the flowers being produced before they are hidden by the leaves,--and in the
stigmas being downy or plumose (as in the Gramineae, Docks, etc), so as to
secure the chance-blown grains. In plants which are fertilised by the wind, the
flowers do not secrete nectar, their pollen is too incoherent to be easily
collected by insects, they have not bright-coloured corollas to serve as guides,
and they are not, as far as I have seen, visited by insects. When insects are
the agents of fertilisation (and this is incomparably the more frequent case
with hermaphrodite plants), the wind plays no part, but we see an endless number
of adaptations to ensure the safe transport of the pollen by the living workers.
These adaptations are most easily recognised in irregular flowers; but they are
present in regular flowers, of which those of Linum offer a good instance, as I
will now endeavour to show.
I have already alluded to the rotation of each separate stigma in the long-
styled form of Linum perenne. In both forms of the other heterostyled species
and in the homostyled species of Linum which I have seen, the stigmatic surfaces
face the centre of the flower, with the furrowed backs of the stigmas, to which
the styles are attached, facing outwards. This is the case with the stigmas of
the long-styled flowers of L. perenne whilst in bud. But by the time the flowers
have expanded, the five stigmas twist round so as to face the circumference,
owing to the torsion of that part of the style which lies beneath the stigma. I
should state that the five stigmas do not always turn round completely, two or
three sometimes facing only obliquely outwards. My observations were made during
October; and it is not improbable that earlier in the season the torsion would
have been more complete; for after two or three cold and wet days the movement
was very imperfectly performed. The flowers should be examined shortly after
their expansion, as their duration is brief; as soon as they begin to wither,
the styles become spirally twisted all together, the original position of the
parts being thus lost.
He who will compare the structure of the whole flower in both forms of L.
perenne and grandiflorum, and, as I may add, of L. flavum, will not doubt about
the meaning of this torsion of the styles in the one form alone of L. perenne,
as well as the meaning of the divergence of the stigmas in the short-styled form
of all three species. It is absolutely necessary as we know, that insects should
carry pollen from the flowers of the one form reciprocally to those of the
other. Insects are attracted by five drops of nectar, secreted exteriorly at the
base of the stamens, so that to reach these drops they must insert their
proboscides outside the ring of broad filaments, between them and the petals. In
the short-styled form of the above three species, the stigmas face the axis of
the flower; and had the styles retained their original upright and central
position, not only would the stigmas have presented their backs to the insects
which sucked the flowers, but their front and fertile surfaces would have been
separated from the entering insects by the ring of broad filaments, and would
never have received any pollen. As it is, the styles diverge and pass out
between the filaments. After this movement the short stigmas lie within the tube
of the corolla; and their papillous surfaces being now turned upwards are
necessarily brushed by every entering insect, and thus receive the required
pollen.
In the long-styled form of L. grandiflorum, the almost parallel or slightly
diverging anthers and stigmas project a little above the tube of the somewhat
concave flower; and they stand directly over the open space leading to the drops
of nectar. Consequently when insects visit the flowers of either form (for the
stamens in this species occupy the same position in both forms), they will get
their foreheads or proboscides well dusted with the coherent pollen. As soon as
they visit the flowers of the long-styled form they will necessarily leave
pollen on the proper surface of the elongated stigmas; and when they visit the
short-styled flowers, they will leave pollen on the upturned stigmatic surfaces.
Thus the stigmas of both forms will receive indifferently the pollen of both
forms; but we know that the pollen alone of the opposite form causes
fertilisation.
(Figure 3.5. Long-styled form of L. perenne var. Austriacum in its early
condition before the stigmas have rotated. The petals and calyx have been
removed on the near side. (3/3. I neglected to get drawings made from fresh
flowers of the two forms. But Mr. Fitch has made the above sketch of a long-
styled flower from dried specimens and from published engravings. His well-known
skill ensures accuracy in the proportional size of the parts.)
In the case of L. perenne, affairs are arranged more perfectly; for the stamens
in the two forms stand at different heights, so that pollen from the anthers of
the longer stamens will adhere to one part of an insect's body, and will
afterwards be brushed off by the rough stigmas of the longer pistils; whilst
pollen from the anthers of the shorter stamens will adhere to a different part
of the insect's body, and will afterwards be brushed off by the stigmas of the
shorter pistils; and this is what is required for the legitimate fertilisation
of both forms. The corolla of L. perenne is more expanded than that of L.
grandiflorum, and the stigmas of the long-styled form do not diverge greatly
from one another; nor do the stamens of either form. Hence insects, especially
rather small ones, will not insert their proboscides between the stigmas of the
long-styled form, nor between the anthers of either form (Figure 3.5), but will
strike against them, at nearly right angles, with the backs of their head or
thorax. Now, in the long-styled flowers, if each stigma did not rotate on its
axis, insects in visiting them would strike their heads against the backs of the
stigmas; as it is, they strike against that surface which is covered with
papillae, with their heads already charged with pollen from the stamens of
corresponding height borne by the flowers of the other form, and legitimate
fertilisation is thus ensured.
Thus we can understand the meaning of the torsion of the styles in the long-
styled flowers alone, as well as their divergence in the short-styled flowers.
One other point is worth notice. In botanical works many flowers are said to be
fertilised in the bud. This statement generally rests, as far as I can discover,
on the anthers opening in the bud; no evidence being adduced that the stigma is
at this period mature, or that it is not subsequently acted on by pollen brought
from other flowers. In the case of Cephalanthera grandiflora I have shown that
precocious and partial self-fertilisation, with subsequent full fertilisation,
is the regular course of events. (3/4. 'Fertilisation of Orchids' page 108; 2nd
edition 1877 page 84.) The belief that the flowers of many plants are fertilised
in the bud, that is, are perpetually self-fertilised, is a most effectual bar to
understanding their real structure. I am, however, far from wishing to assert
that some flowers, during certain seasons, are not fertilised in the bud; for I
have reason to believe that this is the case. A good observer, resting his
belief on the usual kind of evidence, states that in Linum Austriacum (which is
heterostyled, and is considered by Planchon as a variety of L. perenne) the
anthers open the evening before the expansion of the flowers, and that the
stigmas are then almost always fertilised. (3/5. H. Lecoq 'Etudes sur la Geogr.
Bot.' 1856 tome 5 page 325.) Now we know positively that, so far from Linum
perenne being fertilised by its own pollen in the bud, its own pollen is as
powerless on the stigma as so much inorganic dust.
Linum flavum.
The pistil of the long-styled form of this species is nearly twice as long as
that of the short-styled; the stigmas are longer and the papillae coarser. In
the short-styled form the stigmas diverge and pass out between the filaments, as
in the previous species. The stamens in the two forms differ in length; and,
what is singular, the anthers of the longer stamens are not so long as those of
the other form; so that in the short-styled form both the stigmas and the
anthers are shorter than in the long-styled form. The pollen-grains of the two
forms do not differ in size. As this species is propagated by cuttings,
generally all the plants in the same garden belong to the same form. I have
inquired, but have never heard of its seeding in this country. Certainly my own
plants never produced a single seed as long as I possessed only one of the two
forms. After considerable search I procured both forms, but from want of time
only a few experiments were made. Two plants of the two forms were planted some
way apart in my garden, and were not covered by nets. Three flowers on the long-
styled plant were legitimately fertilised with pollen from the short-styled
plant, and one of them set a fine capsule. No other capsules were produced by
this plant. Three flowers on the short-styled plant were legitimately fertilised
with pollen from the long-styled, and all three produced capsules, containing
respectively no less than 8, 9, and 10 seeds. Three other flowers on this plant,
which had not been artificially fertilised, produced capsules containing 5, 1,
and 5 seeds; and it is quite possible that pollen may have been brought to them
by insects from the long-styled plant growing in the same garden. Nevertheless,
as they did not yield half the number of seeds compared with the other flowers
on the same plant which had been artificially and legitimately fertilised, and
as the short-styled plants of the two previous species apparently evince some
slight capacity for fertilisation with their own-form pollen, these three
capsules may have been the product of self-fertilisation.
Besides the three species now described, the yellow-flowered L. corymbiferum is
certainly heterostyled, as is, according to Planchon, L. salsoloides. (3/6.
Hooker's 'London Journal of Botany' 1848 volume 7 page 174.) This botanist is
the only one who seems to have inferred that heterostylism might have some
important functional bearing. Dr. Alefeld, who has made a special study of the
genus, says that about half of the sixty-five species known to him are
heterostyled. (3/7. 'Botanische Zeitung' September 18, 1863 page 281.) This is
the case with L. trigynum, which differs so much from the other species that it
has been formed by him into a distinct genus. (3/8. It is not improbable that
the allied genus, Hugonia, is heterostyled, for one species is said by Planchon
(Hooker's 'London Journal of Botany' 1848 volume 7 page 525) to be provided with
"staminibus exsertis;" another with "stylis staminibus longioribus," and another
has "stamina 5, majora, stylos longe superantia.") According to the same author,
none of the species which inhabit America and the Cape of Good Hope are
heterostyled.
I have examined only three homostyled species, namely, L. usitatissimum,
angustifolium, and catharticum. I raised 111 plants of a variety of the first-
named species, and these, when protected under a net, all produced plenty of
seed. The flowers, according to H. Muller, are frequented by bees and moths.
(3/9. 'Die Befruchtung der Blumen' etc. page 168.) With respect to L.
catharticum, the same author shows that the flowers are so constructed that they
can freely fertilise themselves; but if visited by insects they might be cross-
fertilised. He has, however, only once seen the flowers thus visited during the
day; but it may be suspected that they are frequented during the night by small
moths for the sake of the five minute drops of nectar secreted. Lastly, L.
Lewisii is said by Planchon to bear on the same plant flowers with stamens and
pistils of the same height, and others with the pistils either longer or shorter
than the stamens. This case formerly appeared to me an extraordinary one; but I
am now inclined to believe that it is one merely of great variability. (3/10.
Planchon in Hooker's 'London Journal of Botany' 1848 volume 7 page 175. See on
this subject Asa Gray in 'American Journal of Science' volume 36 September 1863
page 284.)
PULMONARIA (BORAGINEAE).
Pulmonaria officinalis.
Hildebrand has published a full account of this heterostyled plant. (3/11.
'Botanische Zeitung' 1865 January 13 page 13.) The pistil of the long-styled
form is twice as long as that of the short-styled; and the stamens differ in a
corresponding, though converse, manner. There is no marked difference in the
shape or state of surface of the stigma in the two forms. The pollen-grains of
the short-styled form are to those of the long-styled as 9 to 7, or as 100 to
78, in length, and as 7 to 6 in breadth. They do not differ in the appearance of
their contents. The corolla of the one form differs in shape from that of the
other in nearly the same manner as in Primula; but besides this difference the
flowers of the short-styled are generally the larger of the two. Hildebrand
collected on the Siebengebirge, ten wild long-styled and ten short-styled
plants. The former bore 289 flowers, of which 186 (i.e. 64 per cent) had set
fruit, yielding 1.88 seed per fruit. The ten short-styled plants bore 373
flowers, of which 262 (i.e. 70 per cent) had set fruit, yielding 1.86 seed per
fruit. So that the short-styled plants produced many more flowers, and these set
a rather larger proportion of fruit, but the fruits themselves yielded a
slightly lower average number of seeds than did the long-styled plants. The
results of Hildebrand's experiments on the fertility of the two forms are given
in Table 3.19.
TABLE 3.19. Pulmonaria officinalis (from Hildebrand).
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Fruits produced.
Column 4: Average Number of Seeds per Fruit.
Long-styled by pollen of short-styled. Legitimate union :
14 : 10 : 1.30.
Long-styled 14 by own-pollen, and 16 by pollen of other plant of same form.
Illegitimate union :
30 : 0 : 0.
Short-styled by pollen of long-styled. Legitimate union:
16 : 14 : 1.57.
Short-styled 11 by own-pollen, 14 by pollen of other plant of same form.
Illegitimate union :
25 : 0 : 0.
In the summer of 1864, before I had heard of Hildebrand's experiments, I noticed
some long-styled plants of this species (named for me by Dr. Hooker) growing by
themselves in a garden in Surrey; and to my surprise about half the flowers had
set fruit, several of which contained 2, and one contained even 3 seeds. These
seeds were sown in my garden and eleven seedlings thus raised, all of which
proved long-styled, in accordance with the usual rule in such cases. Two years
afterwards the plants were left uncovered, no other plant of the same genus
growing in my garden, and the flowers were visited by many bees. They set an
abundance of seeds: for instance, I gathered from a single plant rather less
than half of the seeds which it had produced, and they numbered 47. Therefore
this illegitimately fertilised plant must have produced about 100 seeds; that
is, thrice as many as one of the wild long-styled plants collected on the
Siebengebirge by Hildebrand, and which, no doubt, had been legitimately
fertilised. In the following year one of my plants was covered by a net, and
even under these unfavourable conditions it produced spontaneously a few seeds.
It should be observed that as the flowers stand either almost horizontally or
hang considerably downwards, pollen from the short stamens would be likely to
fall on the stigma. We thus see that the English long-styled plants when
illegitimately fertilised were highly fertile, whilst the German plants
similarly treated by Hildebrand were completely sterile. How to account for this
wide discordance in our results I know not. Hildebrand cultivated his plants in
pots and kept them for a time in the house, whilst mine were grown out of doors;
and he thinks that this difference of treatment may have caused the difference
in our results. But this does not appear to me nearly a sufficient cause,
although his plants were slightly less productive than the wild ones growing on
the Siebengbirge. My plants exhibited no tendency to become equal-styled, so as
to lose their proper long-styled character, as not rarely happens under
cultivation with several heterostyled species of Primula; but it would appear
that they had been greatly affected in function, either by long-continued
cultivation or by some other cause. We shall see in a future chapter that
heterostyled plants illegitimately fertilised during several successive
generations sometimes become more self-fertile; and this may have been the case
with my stock of the present species of Pulmonaria; but in this case we must
assume that the long-styled plants were at first sufficiently fertile to yield
some seed, instead of being absolutely self-sterile like the German plants.
Pulmonaria angustifolia.
(FIGURE 3.6. Pulmonaria angustifolia.
Left: Long-styled form.
Right: Short-styled form.)
Seedlings of this plant, raised from plants growing wild in the Isle of Wight,
were named for me by Dr. Hooker. It is so closely allied to the last species,
differing chiefly in the shape and spotting of the leaves, that the two have
been considered by several eminent botanists--for instance, Bentham--as mere
varieties. But, as we shall presently see, good evidence can be assigned for
ranking them as distinct. Owing to the doubts on this head, I tried whether the
two would mutually fertilise one another. Twelve short-styled flowers of P.
angustifolia were legitimately fertilised with pollen from long-styled plants of
P. officinalis (which, as we have just seen, are moderately self-fertile), but
they did not produce a single fruit. Thirty-six long-styled flowers of P.
angustifolia were also illegitimately fertilised during two seasons with pollen
from the long-styled P. officinalis, but all these flowers dropped off
unimpregnated. Had the plants been mere varieties of the same species these
illegitimate crosses would probably have yielded some seeds, judging from my
success in illegitimately fertilising the long-styled flowers of P. officinalis;
and the twelve legitimate crosses, instead of yielding no fruit, would almost
certainly have yielded a considerable number, namely, about nine, judging from
the results given in Table 3.20. Therefore P. officinalis and angustifolia
appear to be good and distinct species, in conformity with other important
functional differences between them, immediately to be described.
TABLE 3.20. Pulmonaria angustifolia.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Fruits produced.
Column 4: Average Number of Seeds per Fruit.
Long-styled by pollen of short-styled. Legitimate union :
18 : 9 : 2.11.
Long-styled by own-form pollen. Illegitimate union :
18 : 0 : 0.
Short-styled by pollen of long-styled. Legitimate union:
18 : 15 : 2.60.
Short-styled by own-form pollen. Illegitimate union :
12 : 7 : 1.86.
The long-styled and short-styled flowers of P. angustifolia differ from one
another in structure in nearly the same manner as those of P. officinalis. But
in Figure 3.6 a slight bulging of the corolla in the long-styled form, where the
anthers are seated, has been overlooked. My son William, who examined a large
number of wild plants in the Isle of Wight, observed that the corolla, though
variable in size, was generally larger in the long-styled flowers than in the
short-styled; and certainly the largest corollas of all were found on the long-
styled plants, and the smallest on the short-styled. Exactly the reverse occurs,
according to Hildebrand, with P. officinalis. Both the pistils and stamens of P.
angustifolia vary much in length; so that in the short-styled form the distance
between the stigma and the anthers varied from 119 to 65 divisions of the
micrometer, and in the long-styled from 115 to 112. From an average of seven
measurements of each form the distance between these organs in the long-styled
is to the same distance in the short-styled form as 100 to 69; so that the
stigma in the one form does not stand on a level with the anthers in the other.
The long-styled pistil is sometimes thrice as long as that of the short-styled;
but from an average of ten measurements of both, its length to that of the
short-styled was as 100 to 56. The stigma varies in being more or less, though
slightly, lobed. The anthers also vary much in length in both forms, but in a
greater degree in the long-styled than in the short-styled-form; many in the
former being from 80 to 63, and in the latter from 80 to 70 divisions of the
micrometer in length. From an average of seven measurements, the short-styled
anthers were to those from the long-styled as 100 to 91 in length. Lastly, the
pollen-grains from the long-styled flowers varied between 13 and 11.5 divisions
of the micrometer, and those from the short-styled between 15 and 13. The
average diameter of 25 grains from the latter, or short-styled form, was to that
of 20 grains from the long-styled as 100 to 91. We see, therefore, that the
pollen-grains from the smaller anthers of the shorter stamens in the long-styled
form are, as usual, of smaller size than those in the other form. But what is
remarkable, a larger proportion of the grains were small, shrivelled, and
worthless. This could be seen by merely comparing the contents of the anthers
from several distinct plants of each form. But in one instance my son found, by
counting, that out of 193 grains from a long-styled flower, 53 were bad, or 27
per cent; whilst out of 265 grains from a short-styled flower only 18 were bad,
or 7 per cent. From the condition of the pollen in the long-styled form, and
from the extreme variability of all the organs in both forms, we may perhaps
suspect that the plant is undergoing a change, and tending to become dioecious.
My son collected in the Isle of Wight on two occasions 202 plants, of which 125
were long-styled and 77 short-styled; so that the former were the more numerous.
On the other hand, out of 18 plants raised by me from seed, only 4 were long-
styled and 14 short-styled. The short-styled plants seemed to my son to produce
a greater number of flowers than the long-styled; and he came to this conclusion
before a similar statement had been published by Hildebrand with respect to P.
officinalis. My son gathered ten branches from ten different plants of both
forms, and found the number of flowers of the two forms to be as 100 to 89, 190
being short-styled and 169 long-styled. With P. officinalis the difference,
according to Hildebrand, is even greater, namely, as 100 flowers for the short-
styled to 77 for the long-styled plants. Table 3.20 shows the results of my
experiments.
We see in Table 3.20 that the fertility of the two legitimate unions to that of
the two illegitimate together is as 100 to 35, judged by the proportion of
flowers which produced fruit; and as 100 to 32, judged by the average number of
seeds per fruit. But the small number of fruit yielded by the 18 long-styled
flowers in the first line was probably accidental, and if so, the difference in
the proportion of legitimately and illegitimately fertilised flowers which yield
fruit is really greater than that represented by the ratio of 100 to 35. The 18
long-styled flowers illegitimately fertilised yielded no seeds,--not even a
vestige of one. Two long-styled plants which were placed under a net produced
138 flowers, besides those which were artificially fertilised, and none of these
set any fruit; nor did some plants of the same form which were protected during
the next summer. Two other long-styled plants were left uncovered (all the
short-styled plants having been previously covered up), and humble-bees, which
had their foreheads white with pollen, incessantly visited the flowers, so that
their stigmas must have received an abundance of pollen, yet these flowers did
not produce a single fruit. We may therefore conclude that the long-styled
plants are absolutely barren with their own-form pollen, though brought from a
distinct plant. In this respect they differ greatly from the long-styled English
plants of P. officinalis which were found by me to be moderately self-fertile;
but they agree in their behaviour with the German plants of P. officinalis
experimented on by Hildebrand.
Eighteen short-styled flowers legitimately fertilised yielded, as may be seen in
Table 3.20, 15 fruits, each having on an average 2.6 seeds. Four of these fruits
contained the highest possible number of seeds, namely 4, and four other fruits
contained each 3 seeds. The 12 illegitimately fertilised short-styled flowers
yielded 7 fruits, including on an average 1.86 seed; and one of these fruits
contained the maximum number of 4 seeds. This result is very surprising in
contrast with the absolute barrenness of the long-styled flowers when
illegitimately fertilised; and I was thus led to attend carefully to the degree
of self-fertility of the short-styled plants. A plant belonging to this form and
covered by a net bore 28 flowers besides those which had been artificially
fertilised, and of all these only two produced a fruit each including a single
seed. This high degree of self-sterility no doubt depended merely on the stigmas
not receiving any pollen, or not a sufficient quantity. For after carefully
covering all the long-styled plants in my garden, several short-styled plants
were left exposed to the visits of humble-bees, and their stigmas will thus have
received plenty of short-styled pollen; and now about half the flowers, thus
illegitimately fertilised, set fruit. I judge of this proportion partly from
estimation and partly from having examined three large branches, which had borne
31 flowers, and these produced 16 fruits. Of the fruits produced 233 were
collected (many being left ungathered), and these included on an average 1.82
seed. No less than 16 out of the 233 fruits included the highest possible number
of seeds, namely 4, and 31 included 3 seeds. So we see how highly fertile these
short-styled plants were when illegitimately fertilised with their own-form
pollen by the aid of bees.
The great difference in the fertility of the long and short-styled flowers, when
both are illegitimately fertilised, is a unique case, as far as I have observed
with heterostyled plants. The long-styled flowers when thus fertilised are
utterly barren, whilst about half of the short-styled ones produce capsules, and
these include a little above two-thirds of the number of seeds yielded by them
when legitimately fertilised. The sterility of the illegitimately fertilised
long-styled flowers is probably increased by the deteriorated condition of their
pollen; nevertheless this pollen was highly efficient when applied to the
stigmas of the short-styled flowers. With several species of Primula the short-
styled flowers are much more sterile than the long-styled, when both are
illegitimately fertilised; and it is a tempting view, as formerly remarked, that
this greater sterility of the short-styled flowers is a special adaptation to
check self-fertilisation, as their stigmas are eminently liable to receive their
own pollen. This view is even still more tempting in the case of the long-styled
form of Linum grandiflorum. On the other hand, with Pulmonaria angustifolia, it
is evident, from the corolla projecting obliquely upwards, that pollen is much
more likely to fall on, or to be carried by insects down to the stigma of the
short-styled than of the long-styled flowers; yet the short-styled instead of
being more sterile, as a protection against self-fertilisation, are far more
fertile than the long-styled, when both are illegitimately fertilised.
Pulmonaria azurea, according to Hildebrand, is not heterostyled. (3/12. 'Die
Geschlechter-Vertheilung bei den Pflanzen' 1867 page 37.)
[From an examination of dried flowers of Amsinckia spectabilis, sent me by
Professor Asa Gray, I formerly thought that this plant, a member of the
Boragineae, was heterostyled. The pistil varies to an extraordinary degree in
length, being in some specimens twice as long as in others, and the point of
insertion of the stamens likewise varies. But on raising many plants from seed,
I soon became convinced that the whole case was one of mere variability. The
first-formed flowers are apt to have stamens somewhat arrested in development,
with very little pollen in their anthers; and in such flowers the stigma
projects above the anthers, whilst generally it stands below and sometimes on a
level with them. I could detect no difference in the size of the pollen-grain or
in the structure of the stigma in the plants which differed most in the above
respects; and all of them, when protected from the access of insects, yielded
plenty of seeds. Again, from statements made by Vaucher, and from a hasty
inspection, I thought at first that the allied Anchusa arvensis and Echium
vulgare were heterostyled, but soon saw my error. From information given me, I
examined dried flowers of another member of the Boragineae, Arnebia
hispidissima, collected from several sites, and though the corolla, together
with the included organs, differed much in length, there was no sign of
heterostylism.]
Polygonum fagopyrum (Polygonaceae).
(FIGURE 3.7. Polygonum fagopyrum. (From H. Muller.)
Upper figure, the long-styled form; lower figure, the short-styled.
Some of the anthers have dehisced, others have not.)
Hildebrand has shown that this plant, the common Buck-wheat, is heterostyled.
(3/13. 'Die Geschlechter-Vertheilung' etc. 1867 page 34.) In the long-styled
form (Figure 3.7), the three stigmas project considerably above the eight short
stamens, and stand on a level with the anthers of the eight long stamens in the
short-styled form; and so it is conversely with the stigmas and stamens of this
latter form. I could perceive no difference in the structure of the stigmas in
the two forms. The pollen-grains of the short-styled form are to those of the
long-styled as 100 to 82 in diameter. This plant is therefore without doubt
heterostyled.
I experimented only in an imperfect manner on the relative fertility of the two
forms. Short-styled flowers were dragged several times over two heads of flowers
on long-styled plants, protected under a net, which were thus legitimately,
though not fully, fertilised. They produced 22 seeds, or 11 per flower-head.
Three flower-heads on long-styled plants received pollen in the same manner from
other long-styled plants, and were thus illegitimately fertilised. They produced
14 seeds, or only 4.66 per flower-head.
Two flower-heads on short-styled plants received pollen in like manner from
long-styled flowers, and were thus legitimately fertilised. They produced 8
seeds, or 4 per flower-head.
Four heads on short-styled plants similarly received pollen from other short-
styled plants, and were thus illegitimately fertilised. They produced 9 seeds,
or 2.25 per flower-head.
The results from fertilising the flower-heads in the above imperfect manner
cannot be fully trusted; but I may state that the four legitimately fertilised
flower-heads yielded on an average 7.50 seeds per head; whereas the seven
illegitimately fertilised heads yielded less than half the number, or on an
average only 3.28 seeds. The legitimately crossed seeds from the long-styled
flowers were finer than those from the illegitimately fertilised flowers on the
same plants, in the ratio of 100 to 82, as shown by the weights of an equal
number.
About a dozen plants, including both forms, were protected under nets, and early
in the season they produced spontaneously hardly any seeds, though at this
period the artificially fertilised flowers produced an abundance; but it is a
remarkable fact that later in the season, during September, both forms became
highly self-fertile. They did not, however, produce so many seeds as some
neighbouring uncovered plants which were visited by insects. Therefore the
flowers of neither form when left to fertilise themselves late in the season
without the aid of insects, are nearly so sterile as most other heterostyled
plants. A large number of insects, namely 41 kinds as observed by H. Muller,
visit the flowers for the sake of the eight drops of nectar. (3/14. 'Die
Befruchtung' etc. page 175 and 'Nature' January 1, 1874 page 166.) He infers
from the structure of the flowers that insects would be apt to fertilise them
both illegitimately as well as legitimately; but he is mistaken in supposing
that the long-styled flowers cannot spontaneously fertilise themselves.
Differently to what occurs in the other genera hitherto noticed, Polygonum,
though a very large genus, contains, as far as is at present known, only a
single heterostyled species, namely the present one. H. Muller in his
interesting description of several other species shows that P. bistorta is so
strongly proterandrous (the anthers generally falling off before the stigmas are
mature) that the flowers must be cross-fertilised by the many insects which
visit them. Other species bear much less conspicuous flowers which secrete
little or no nectar, and consequently are rarely visited by insects; these are
adapted for self-fertilisation, though still capable of cross-fertilisation.
According to Delpino, the Polygonaceae are generally fertilised by the wind,
instead of by insects as in the present genus.
[Leucosmia Burnettiana (Thymeliae).
As Professor Asa Gray has expressed his belief that this species and L.
acuminata, as well as some species in the allied genus Drymispermum, are
dimorphic or heterostyled (3/15. 'American Journal of Science' 1865 page 101 and
Seemann's 'Journal of Botany' volume 3 1865 page 305.), I procured from Kew,
through the kindness of Dr. Hooker, two dried flowers of the former species, an
inhabitant of the Friendly Islands in the Pacific. The pistil of the long-styled
form is to that of the short-styled as 100 to 86 in length; the stigma projects
just above the throat of the corolla, and is surrounded by five anthers, the
tips of which reach up almost to its base; and lower down, within the tubular
corolla, five other and rather smaller anthers are seated. In the short-styled
form, the stigma stands some way down the tube of the corolla, nearly on a level
with the lower anthers of the other form: it differs remarkably from the stigma
of the long-styled form, in being more papillose, and in being longer in the
ratio of 100 to 60. The anthers of the upper stamens in the short-styled form
are supported on free filaments, and project above the throat of the corolla,
whilst the anthers of the lower stamens are seated in the throat on a level with
the upper stamens of the other form. The diameters of a considerable number of
grains from both sets of anthers in both forms were measured, but they did not
differ in any trustworthy degree. The mean diameter of twenty-two grains from
the short-styled flower was to that of twenty-four grains from the long-styled,
as 100 to 99. The anthers of the upper stamens in the short-styled form appeared
to be poorly developed, and contained a considerable number of shrivelled grains
which were omitted in striking the above average. Notwithstanding the fact of
the pollen-grains from the two forms not differing in diameter in any
appreciable degree, there can hardly be a doubt from the great difference in the
two forms in the length of the pistil, and especially of the stigma, together
with its more papillose condition in the short-styled form, that the present
species is truly heterostyled. This case resembles that of Linum grandiflorum,
in which the sole difference between the two forms consists in the length of the
pistils and stigmas. From the great length of the tubular corolla of Leucosmia,
it is clear that the flowers are cross-fertilised by large Lepidoptera or by
honey-sucking birds, and the position of the stamens in two whorls one beneath
the other, which is a character that I have not seen in any other heterostyled
dimorphic plant, probably serves to smear the inserted organ thoroughly with
pollen.
Menyanthes trifoliata (Gentianeae).
This plant inhabits marshes: my son William gathered 247 flowers from so many
distinct plants, and of these 110 were long-styled, and 137 short-styled. The
pistil of the long-styled form is in length to that of the short-styled in the
ratio of about 3 to 2. The stigma of the former, as my son observed, is
decidedly larger than that of the short-styled; but in both forms it varies much
in size. The stamens of the short-styled are almost double the length of those
of the long-styled; so that their anthers stand rather above the level of the
stigma of the long-styled form. The anthers also vary much in size, but seem
often to be of larger size in the short-styled flowers. My son made with the
camera many drawings of the pollen-grains, and those from the short-styled
flowers were in diameter in nearly the ratio of 100 to 84 to those from the
long-styled flowers. I know nothing about the capacity for fertilisation in the
two forms; but short-styled plants, living by themselves in the gardens at Kew,
have produced an abundance of capsules, yet the seeds have never germinated; and
this looks as if the short-styled form was sterile with its own pollen.
Limnanthemum Indicum (Gentianeae).
This plant is mentioned by Mr. Thwaites in his Enumeration of the Plants of
Ceylon as presenting two forms; and he was so kind as to send me specimens
preserved in spirits. The pistil of the long-styled form is nearly thrice as
long (i.e. as 14 to 5) as that of the short-styled, and is very much thinner in
the ratio of about 3 to 5. The foliaceous stigma is more expanded, and twice as
large as that of the short-styled form. In the latter the stamens are about
twice as long as those of the long-styled, and their anthers are larger in the
ratio of 100 to 70. The pollen-grains, after having been long kept in spirits,
were of the same shape and size in both forms. The ovules, according to Mr.
Thwaites, are equally numerous (namely from 70 to 80) in the two forms.
Villarsia [sp.?] (Gentianeae).
Fritz Muller sent me from South Brazil dried flowers of this aquatic plant,
which is closely allied to Limnanthemum. In the long-styled form the stigma
stands some way above the anthers, and the whole pistil, together with the
ovary, is in length to that of the short-styled form as about 3 to 2. In the
latter form the anthers stand above the stigma, and the style is very short and
thick; but the pistil varies a good deal in length, the stigma being either on a
level with the tips of the sepals or considerably beneath them. The foliaceous
stigma in the long-styled form is larger, with the expansions running farther
down the style, than in the other form. One of the most remarkable differences
between the two forms is that the anthers of the longer stamens in the short-
styled flowers are conspicuously longer than those of the shorter stamens in the
long-styled flowers. In the former the sub-triangular pollen-grains are larger;
the ratio between their breadth (measured from one angle to the middle of the
opposite side) and that of the grains from the long-styled flowers being about
100 to 75. Fritz Muller also informs me that the pollen of the short-styled
flowers has a bluish tint, whilst that of the long-styled is yellow. When we
treat of Lythrum salicaria we shall find a strongly marked contrast in the
colour of the pollen in two of the forms.
The three genera, Menyanthes, Limnanthemum, and Villarsia, now described,
constitute a well-marked sub-tribe of the Gentianeae. All the species, as far as
at present known, are heterostyled, and all inhabit aquatic or sub-aquatic
stations.
Forsythia suspensa (Oleaceae).
Professor Asa Gray states that the plants of this species growing in the Botanic
Gardens at Cambridge, U.S., are short-styled, but that Siebold and Zuccarini
describe the long-styled form, and give figures of two forms; so that there can
be little doubt, as he remarks, about the plant being dimorphic. (3/16. 'The
American Naturalist' July 1873 page 422.) I therefore applied to Dr. Hooker, who
sent me a dried flower from Japan, another from China, and another from the
Botanic Gardens at Kew. The first proved to be long-styled, and the other two
short-styled. In the long-styled form, the pistil is in length to that of the
short-styled as 100 to 38, the lobes of the stigma being a little longer (as 10
to 9), but narrower and less divergent. This last character, however, may be
only a temporary one. There seems to be no difference in the papillose condition
of the two stigmas. In the short-styled form, the stamens are in length to those
of the long-styled as 100 to 66, but the anthers are shorter in the ratio of 87
to 100; and this is unusual, for when there is any difference in size between
the anthers of the two forms, those from the longer stamens of the short-styled
are generally the longest. The pollen-grains from the short-styled flowers are
certainly larger, but only in a slight degree, than those from the long-styled,
namely, as 100 to 94 in diameter. The short-styled form, which grows in the
Gardens at Kew, has never there produced fruit.
Forsythia viridissima appears likewise to be heterostyled; for Professor Asa
Gray says that although the long-styled form alone grows in the gardens at
Cambridge, U.S., the published figures of this species belong to the short-
styled form.
Cordia [sp.?] (Cordiaceae).
Fritz Muller sent me dried specimens of this shrub, which he believes to be
heterostyled; and I have not much doubt that this is the case, though the usual
characteristic differences are not well pronounced in the two forms. Linum
grandiflorum shows us that a plant may be heterostyled in function in the
highest degree, and yet the two forms may have stamens of equal length, and
pollen-grains of equal size. In the present species of Cordia, the stamens of
both forms are of nearly equal length, those of the short-styled being rather
the longest; and the anthers of both are seated in the mouth of the corolla. Nor
could I detect any difference in the size of the pollen-grains, when dry or
after being soaked in water. The stigmas of the long-styled form stand clear
above the anthers, and the whole pistil is longer than that of the short-styled,
in about the ratio of 3 to 2.
The stigmas of the short-styled form are seated beneath the anthers, and they
are considerably shorter than those of the long-styled form. This latter
difference is the most important one of any between the two forms.
Gilia (Ipomopsis) pulchella vel aggregata (Polemoniaceae).
Professor Asa Gray remarks with respect to this plant: "the tendency to
dimorphism, of which there are traces, or perhaps rather incipient
manifestations in various portions of the genus, is most marked in G.
aggregata." (3/17. 'Proceedings of the American Academy of Arts and Sciences.'
June 14, 1870 page 275.) He sent me some dried flowers, and I procured others
from Kew. They differ greatly in size, some being nearly twice as long as others
(namely as 30 to 17), so that it was not possible to compare, except by
calculation, the absolute length of the organs from different plants. Moreover,
the relative position of the stigmas and anthers is variable: in some long-
styled flowers the stigmas and anthers were exserted only just beyond the throat
of the corolla; whilst in others they were exserted as much as 4/10 of an inch.
I suspect also that the pistil goes on growing for some time after the anthers
have dehisced. Nevertheless it is possible to class the flowers under two forms.
In some of the long-styled, the length of pistil to that of the short-styled was
as 100 to 82; but this result was gained by reducing the size of the corollas to
the same scale. In another pair of flowers the difference in length between the
pistils of the two forms was certainly greater, but they were not actually
measured. In the short-styled flowers whether large or small, the stigma is
seated low down within the tube of the corolla. The papillae on the long-styled
stigma are longer than those on the short-styled, in the ratio of 100 to 40. The
filaments in some of the short-styled flowers were, to those of the long-styled,
as 100 to 25 in length, the free, or unattached portion being alone measured;
but this ratio cannot be trusted, owing to the great variability of the stamens.
The mean diameter of eleven pollen-grains from long-styled flowers, and of
twelve from the short-styled, was exactly the same. It follows from these
several statements, that the difference in length and state of surface of the
stigmas in the flowers is the sole reliable evidence that this species is
heterostyled; for it would be rash to trust to the difference in the length of
the pistils, seeing how variable they are. I should have left the case
altogether doubtful, had it not been for the observations on the following
species; and these leave little doubt on my mind that the present plant is truly
heterostyled. Professor Gray informs me that in another species, G.
coronopifolia, belonging to the same section of the genus, he can see no sign of
dimorphism.
Gilia (Leptosiphon) micrantha.
A few flowers sent me from Kew had been somewhat injured, so that I cannot say
anything positively with respect to the position and relative length of the
organs in the two forms. But their stigmas differed almost exactly in the same
manner as in the last species; the papillae on the long-styled stigma being
longer than those on the short-styled, in the ratio of 100 to 42. My son
measured nine pollen-grains from the long-styled, and the same number from the
short-styled form; and the mean diameter of the former was to that of the latter
as 100 to 81. Considering this difference, as well as that between the stigmas
of the two forms, there can be no doubt that this species is heterostyled. So
probably is Gilia nudicaulis, which likewise belongs to the Leptosiphon section
of the genus, for I hear from Professor Asa Gray that in some individuals the
style is very long, with the stigma more or less exserted, whilst in others it
is deeply included within the tube; the anthers being always seated in the
throat of the corolla.
Phlox subulata (Polemoniaceae).
Professor Asa Gray informs me that the greater number of the species in this
genus have a long pistil, with the stigma more or less exserted; whilst several
other species, especially the annuals, have a short pistil seated low down
within the tube of the corolla. In all the species the anthers are arranged one
below the other, the uppermost just protruding from the throat of the corolla.
In Phlox subulata alone he has "seen both long and short styles; and here the
short-styled plant has (irrespective of this character) been described as a
distinct species (P. nivalis, P. Hentzii), and is apt to have a pair of ovules
in each cell, while the long-styled P. subulata rarely shows more than one."
(3/18. 'Proceedings of the American Academy of Arts and Sciences' June 14, 1870
page 248.) Some dried flowers of both forms were sent me by him, and I received
others from Kew, but I have failed to make out whether the species is
heterostyled. In two flowers of nearly equal size, the pistil of the long-styled
form was twice as long as that of the short-styled; but in other cases the
difference was not nearly so great. The stigma of the long-styled pistil stands
nearly in the throat of the corolla; whilst in the short-styled it is placed low
down--sometimes very low down in the tube, for it varies greatly in position.
The stigma is more papillose, and of greater length (in one instance in the
ratio of 100 to 67), in the short-styled flowers than in the long-styled. My son
measured twenty pollen-grains from a short-styled flower, and nine from a long-
styled, and the former were in diameter to the latter as 100 to 93; and this
difference accords with the belief that the plant is heterostyled. But the
grains from the short-styled varied much in diameter. He afterwards measured ten
grains from a distinct long-styled flower, and ten from another plant of the
same form, and these grains differed in diameter in the ratio of 100 to 90. The
mean diameter of these two lots of twenty grains was to that of twelve grains
from another short-styled flower as 100 to 75: here, then, the grains from the
short-styled form were considerably smaller than those from the long-styled,
which is the reverse of what occurred in the former instance, and of what is the
general rule with heterostyled plants. The whole case is perplexing in the
highest degree, and will not be understood until experiments are tried on living
plants. The greater length, and more papillose condition of the stigma in the
short-styled than in the long-styled flowers, looks as if the plant was
heterostyled; for we know that with some species--for instance, Leucosmia and
certain Rubiaceae--the stigma is longer and more papillose in the short-styled
form, though the reverse of this holds good in Gilia, a member of the same
family with Phlox. The similar position of the anthers in the two forms is
somewhat opposed to the present species being heterostyled; as is the great
difference in the length of the pistil in several short-styled flowers. But the
extraordinary variability in diameter of the pollen-grains, and the fact that in
one set of flowers the grains from the long-styled flowers were larger than
those from the short-styled, is strongly opposed to the belief that Phlox
subulata is heterostyled. Possibly this species was once heterostyled, but is
now becoming sub-dioecious; the short-styled plants having been rendered more
feminine in nature. This would account for their ovaries usually containing more
ovules, and for the variable condition of their pollen-grains. Whether the long-
styled plants are now changing their nature, as would appear to be the case from
the variability of their pollen-grains, and are becoming more masculine, I will
not pretend to conjecture; they might remain as hermaphrodites, for the
coexistence of hermaphrodite and female plants of the same species is by no
means a rare event.
Erythroxylum [sp.?] (Erythroxylidae).
(FIGURE 3.8. Erythroxylon [sp.?]
Left: Long-styled form.
Right: Short-styled form.
From a sketch by Fritz Muller, magnified five times.)
Fritz Muller sent me from South Brazil dried flowers of this tree, together with
the drawings (Figure 3.8.), which show the two forms, magnified about five
times, with the petals removed. In the long-styled form the stigmas project
above the anthers, and the styles are nearly twice as long as those of the
short-styled form, in which the stigmas stand beneath the anthers. The stigmas
in many, but not in all the short-styled flowers are larger than those in the
long-styled. The anthers of the short-styled flowers stand on a level with the
stigmas of the other form; but the stamens are longer by only one-fourth or one-
fifth of their own length than those of the long-styled. Consequently the
anthers of the latter do not stand on a level with, but rather above the stigmas
of the other form. Differently from what occurs in the following closely allied
genus, Sethia, the stamens are of nearly equal length in the flowers of the same
form. The pollen-grains of the short-styled flowers, measured in their dry
state, are a little larger than those from the long-styled flowers in about the
ratio of 100 to 93. (3/19. F. Muller remarks in his letter to me that the
flowers, of which he carefully examined many specimens, are curiously variable
in the number of their parts: 5 sepals and petals, 10 stamens and 3 pistils are
the prevailing numbers; but the sepals and petals often vary from 5 to 7; the
stamens from 10 to 14, and the pistils from 3 to 4.)
Sethia acuminata (Erythroxylidae).
Mr. Thwaites pointed out several years ago that this plant exists under two
forms, which he designated as forma stylosa et staminea; and the flowers sent to
me by him are clearly heterostyled. (3/20. 'Enumeratio Plantarum Zeylaniae' 1864
page 54.) In the long-styled form the pistil is nearly twice as long, and the
stamens half as long as the corresponding organs in the short-styled form. The
stigmas of the long-styled seem rather smaller than those of the short-styled.
All the stamens in the short-styled flowers are of nearly equal length, whereas
in long-styled they differ in length, being alternately a little longer and
shorter; and this difference in the stamens of the two forms is probably
related, as we shall hereafter see in the case of the short-styled flowers of
Lythrum salicaria, to the manner in which insects can best transport pollen from
the long-styled flowers to the stigmas of the short-styled. The pollen-grains
from the short-styled flowers, though variable in size, are to those of the
long-styled, as far as I could make out, as 100 to 83 in their longer diameter.
Sethia obtusifolia is heterostyled like S. acuminata.
Cratoxylon formosum (Hypericineae).
Mr. Thiselton Dyer remarks that this tree, an inhabitant of Malacca and Borneo,
appears to be heterostyled. (3/21. 'Journal of Botany' London 1872 page 26.) He
sent me dried flowers, and the difference between the two forms is conspicuous.
In the short-styled form the pistils are in length to those of the short-styled
as 100 to 40, with their globular stigmas about twice as thick. These stand just
above the numerous anthers and a little beneath the tips of the petals. In the
short-styled form the anthers project high above the pistils, the stigmas of
which diverge between the three bundles of stamens, and stand only a little
above the tips of the sepals. The stamens in this form are to those of the long-
styled as 100 to 86 in length; and therefore they do not differ so much in
length as do the pistils. Ten pollen-grains from each form were measured, and
those from the short-styled were to those from the long-styled as 100 to 86 in
diameter. This plant, therefore, is in all respects a well-characterised
heterostyled species.
Aegiphila elata (Verbenaceae).
Mr. Bentham was so kind as to send me dried flowers of this species and of Ae.
mollis, both inhabitants of South America. The two forms differ conspicuously,
as the deeply bifid stigma of the one, and the anthers of the other project far
above the mouth of the corolla. In the long-styled form of the present species,
the style is twice and a half as long as that of the short-styled. The divergent
stigmas of the two forms do not differ much in length, nor as far as I could
perceive in their papillae. In the long-styled flowers the filaments adhere to
the corolla close up to the anthers, which are enclosed some way down within the
tube. In the short-styled flowers the filaments are free above the point where
the anthers are seated in the other form, and they project from the corolla to
an equal height with that of the stigmas in the long-styled flowers. It is often
difficult to measure with accuracy pollen-grains, which have long been dried and
then soaked in water; but they here manifestly differed greatly in size. Those
from the short-styled flowers were to those from the long-styled in diameter in
about the ratio of 100 to 62. The two forms of Ae. mollis present a like
difference in the length of their pistils and stamens.
Aegiphila obdurata.
Flowers of this bush were sent me from St. Catharina in Brazil, by Fritz Muller,
and were named for me at Kew. They appeared at first sight grandly heterostyled,
as the stigma of the long-styled form projects far out of the corolla, whilst
the anthers are seated halfway down within the tube; whereas in the short-styled
form the anthers project from the corolla and the stigma is enclosed in the tube
at nearly the same level with the anthers of the other form. The pistil of the
long-styled is to that of the short-styled as 100 to 60 in length, and the
stigmas, taken by themselves, as 100 to 55. Nevertheless, this plant cannot be
heterostyled. The anthers in the long-styled form are brown, tough, and fleshy,
and less than half the length of those in the short-styled form, strictly as 44
to 100; and what is much more important, they were in a rudimentary condition in
the two flowers examined by me, and did not contain a single grain of pollen. In
the short-styled form, the divided stigma, which as we have seen is much
shortened, is thicker and more fleshy than the stigma of the long-styled, and is
covered with small irregular projections, formed of rather large cells. It had
the appearance of having suffered from hyperthrophy, and is probably incapable
of fertilisation. If this be so the plant is dioecious, and judging from the two
species previously described, it probably was once heterostyled, and has since
been rendered dioecious by the pistil in the one form, and the stamens in the
other having become functionless and reduced in size. It is, however, possible
that the flowers may be in the same state as those of the common thyme and of
several other Labiatae, in which females and hermaphrodites regularly co-exist.
Fritz Muller, who thought that the present plant was heterostyled, as I did at
first, informs me that he found bushes in several places growing quite isolated,
and that these were completely sterile; whilst two plants growing close together
were covered with fruit. This fact agrees better with the belief that the
species is dioecious than that it consists of hermaphrodites and females; for if
any one of the isolated plants had been an hermaphrodite, it would probably have
produced some fruit.]
RUBIACEAE.
This great natural family contains a much larger number of heterostyled genera
than any other one, as yet known.
Mitchella repens.
Professor Asa Gray sent me several living plants collected when out of flower,
and nearly half of these proved long-styled, and the other half short-styled.
The white flowers, which are fragrant and which secrete plenty of nectar, always
grow in pairs with their ovaries united, so that the two together produce "a
berry-like double drupe." (3/22. A. Gray 'Manual of the Botany of the United
States' 1856 page 172.) In my first series of experiments (1864) I did not
suppose that this curious arrangement of the flowers would have any influence on
their fertility; and in several instances only one of the two flowers in a pair
was fertilised; and a large proportion or all of these failed to produce
berries. In the ensuing year both flowers of each pair were invariably
fertilised in the same manner; and the latter experiments alone serve to show
the proportion of flowers which yield berries, when legitimately and
illegitimately fertilised; but for calculating the average number of seeds per
berry I have used those produced during both seasons.
In the long-styled flowers the stigma projects just above the bearded throat of
the corolla, and the anthers are seated some way down the tube. In the short-
styled flowers those organs occupy reversed positions. In this latter form the
fresh pollen-grains are a little larger and more opaque than those of the long-
styled form. The results of my experiments are given in Table 3.21.
TABLE 3.21. Mitchella repens.
Column 1: Nature of the Union.
Column 2: Number of Pairs of Flowers fertilised during the second season.
Column 3: Number of Drupes produced during the second season.
Column 4: Average Number of good Seeds per Drupe in all the Drupes during the
two Seasons.
Long-styled by pollen of short-styled. Legitimate union :
9 : 8 : 4.6.
Long-styled by own-form pollen. Illegitimate union :
8 : 3 : 2.2.
Short-styled by pollen of long-styled. Legitimate union:
8 : 7 : 4.1.
Short-styled by own-form pollen. Illegitimate union :
9 : 0 : 2.0.
The two legitimate unions together :
17 : 15 : 4.4.
The two illegitimate unions together :
17 : 3 : 2.1.
It follows from this table that 88 per cent of the paired flowers of both forms,
when legitimately fertilised, yielded double berries, nineteen of which
contained on an average 4.4 seeds, with a maximum in one of 8 seeds. Of the
illegitimately fertilised paired flowers only 18 per cent yielded berries, six
of which contained on an average only 2.1 seeds, with a maximum in one of 4
seeds. Thus the two legitimate unions are more fertile than the two
illegitimate, according to the proportion of flowers which yielded berries, in
the ratio of 100 to 20; and according to the average number of contained seeds
as 100 to 47.
Three long-styled and three short-styled plants were protected under separate
nets, and they produced altogether only 8 berries, containing on an average only
1.5 seed. Some additional berries were produced which contained no seeds. The
plants thus treated were therefore excessively sterile, and their slight degree
of fertility may be attributed in part to the action of the many individuals of
Thrips which haunted the flowers. Mr. J. Scott informs me that a single plant
(probably a long-styled one), growing in the Botanic Gardens at Edinburgh, which
no doubt was freely visited by insects, produced plenty of berries, but how many
of them contained seeds was not observed.
Borreria, nov. sp. near valerianoides (Rubiaceae).
Fritz Muller sent me seeds of this plant, which is extremely abundant in St.
Catharina, in South Brazil; and ten plants were raised, consisting of five long-
styled and five short-styled. The pistil of the long-styled flowers projects
just beyond the mouth of the corolla, and is thrice as long as that of the
short-styled, and the divergent stigmas are likewise rather larger. The anthers
in the long-styled form stand low down within the corolla, and are quite hidden.
In the short-styled flowers the anthers project just above the mouth of the
corolla, and the stigma stands low down within the tube. Considering the great
difference in the length of the pistils in the two forms, it is remarkable that
the pollen-grains differ very little in size, and Fritz Muller was struck with
the same fact. In a dry state the grains from the short-styled flowers could
just be perceived to be larger than those from the long-styled, and when both
were swollen by immersion in water, the former were to the latter in diameter in
the ratio of 100 to 92. In the long-styled flowers beaded hairs almost fill up
the mouth of the corolla and project above it; they therefore stand above the
anthers and beneath the stigma. In the short-styled flowers a similar brush of
hairs is situated low down within the tubular corolla, above the stigma and
beneath the anthers. The presence of these beaded hairs in both forms, though
occupying such different positions, shows that they are probably of considerable
functional importance. They would serve to guard the stigma of each form from
its own pollen; but in accordance with Professor Kerner's view their chief use
probably is to prevent the copious nectar being stolen by small crawling
insects, which could not render any service to the species by carrying pollen
from one form to the other. (3/23. 'Die Schutzmittel der Bluthen gegen
unberufene Gaste' 1876 page 37.)
The flowers are so small and so crowded together that I was not willing to
expend time in fertilising them separately; but I dragged repeatedly heads of
short-styled flowers over three long-styled flower-heads, which were thus
legitimately fertilised; and they produced many dozen fruits, each containing
two good seeds. I fertilised in the same manner three heads on the same long-
styled plant with pollen from another long-styled plant, so that these were
fertilised illegitimately, and they did not yield a single seed. Nor did this
plant, which was of course protected by a net, bear spontaneously any seeds.
Nevertheless another long-styled plant, which was carefully protected, produced
spontaneously a very few seeds; so that the long-styled form is not always quite
sterile with its own pollen.
Faramea [sp.?] (Rubiaceae).
(FIGURE 3.9. Faramea [sp.?]
Left: Short-styled form.
Right: Long-styled form.
Outlines of flowers from dried specimens. Pollen-grains magnified 180 times, by
Fritz Muller.)
Fritz Muller has fully described the two forms of this remarkable plant, an
inhabitant of South Brazil. (3/24. 'Botanische Zeitung' September 10, 1869 page
606.) In the long-styled form the pistil projects above the corolla, and is
almost exactly twice as long as that of the short-styled, which is included
within the tube. The former is divided into two rather short and broad stigmas,
whilst the short-styled pistil is divided into two long, thin, sometimes much
curled stigmas. The stamens of each form correspond in height or length with the
pistils of the other form. The anthers of the short-styled form are a little
larger than those of the long-styled; and their pollen-grains are to those of
the other form as 100 to 67 in diameter. But the pollen-grains of the two forms
differ in a much more remarkable manner, of which no other instance is known;
those from the short-styled flowers being covered with sharp points; the smaller
ones from the long-styled being quite smooth. Fritz Muller remarks that this
difference between the pollen-grains of the two forms is evidently of service to
the plant; for the grains from the projecting stamens of the short-styled form,
if smooth, would have been liable to be blown away by the wind, and would thus
have been lost; but the little points on their surfaces cause them to cohere,
and at the same time favour their adhesion to the hairy bodies of insects, which
merely brush against the anthers of these stamens whilst visiting the flowers.
On the other hand, the smooth grains of the long-styled flowers are safely
included within the tube of the corolla, so that they cannot be blown away, but
are almost sure to adhere to the proboscis of an entering insect, which is
necessarily pressed close against the enclosed anthers.
It may be remembered that in the long-styled form of Linum perenne each separate
stigma rotates on its own axis, when the flower is mature, so as to turn its
papillose surface outwards. There can be no doubt that this movement, which is
confined to the long-styled form, is effected in order that the proper surface
of the stigma should receive pollen brought by insects from the other form. Now
with Faramea, as Fritz Muller shows, it is the stamens which rotate on their
axes in one of the two forms, namely, the short-styled, in order that their
pollen should be brushed off by insects and transported to the stigmas of the
other form. In the long-styled flowers the anthers of the short enclosed stamens
do not rotate on their axes, but dehisce on their inner sides, as is the common
rule with the Rubiaceae; and this is the best position for the adherence of the
pollen-grains to the proboscis of an entering insect. Fritz Muller therefore
infers that as the plant became heterostyled, and as the stamens of the short-
styled form increased in length, they gradually acquired the highly beneficial
power of rotating on their own axes. But he has further shown, by the careful
examination of many flowers, that this power has not as yet been perfected; and,
consequently, that a certain proportion of the pollen is rendered useless,
namely, that from the anthers which do not rotate properly. It thus appears that
the development of the plant has not as yet been completed; the stamens have
indeed acquired their proper length, but not their full and perfect power of
rotation. (3/25. Fritz Muller gives another instance of the want of absolute
perfection in the flowers of another member of the Rubiaceae, namely, Posoqueria
fragrans, which is adapted in a most wonderful manner for cross-fertilisation by
the agency of moths. (See 'Botanische Zeitung' 1866 Number 17.) In accordance
with the nocturnal habits of these insects, most of the flowers open only during
the night; but some open in the day, and the pollen of such flowers is robbed,
as Fritz Muller has often seen, by humble-bees and other insects, without any
benefit being thus conferred on the plant.)
The several points of difference in structure between the two forms of Faramea
are highly remarkable. Until within a recent period, if any one had been shown
two plants which differed in a uniform manner in the length of their stamens and
pistils,--in the form of their stigmas,--in the manner of dehiscence and
slightly in the size of their anthers,--and to an extraordinary degree in the
diameter and structure of their pollen-grains, he would have declared it
impossible that the two could have belonged to one and the same species.
[Suteria (species unnamed in the herbarium at Kew.) (Rubiaceae).
I owe to the kindness of Fritz Muller dried flowers of this plant from St.
Catharina, in Brazil. In the long-styled form the stigma stands in the mouth of
the corolla, above the anthers, which latter are enclosed within the tube, but
only a short way down. In the short-styled form the anthers are placed in the
mouth of the corolla above the stigma, which occupies the same position as the
anthers in the other form, being seated only a short way down the tube.
Therefore the pistil of the long-styled form does not exceed in length that of
the short-styled in nearly so great a degree as in many other Rubiaceae.
Nevertheless there is a considerable difference in the size of the pollen-grains
in the two forms; for, as Fritz Muller informs me, those of the short-styled are
to those of the long-styled as 100 to 75 in diameter.
Houstonia coerulea (Rubiaceae).
Professor Asa Gray has been so kind as to send me an abstract of some
observations made by Dr. Rothrock on this plant. The pistil is exserted in the
one form and the stamens in the other, as has long been observed. The stigmas of
the long-styled form are shorter, stouter, and far more hispid than in the other
form. The stigmatic hairs or papillae on the former are .04 millimetres, and on
the latter only .023 millimetres in length. In the short-styled form the anthers
are larger, and the pollen-grains, when distended with water, are to those from
the long-styled form as 100 to 72 in diameter.
Selected capsules from some long-styled plants growing in the Botanic Gardens at
Cambridge, U.S., near where plants of the other form grew, contained on an
average 13 seeds; but these plants must have been subjected to unfavourable
conditions, for some long-styled plants in a state of nature yielded an average
of 21.5 seeds per capsule. Some short-styled plants, which had been planted by
themselves in the Botanic Gardens, where it was not likely that they would have
been visited by insects that had previously visited long-styled plants, produced
capsules, eleven of which were wholly sterile, but one contained 4, and another
8 seeds. So that the short-styled form seems to be very sterile with its own
pollen. Professor Asa Gray informs me that the other North American species of
this genus are likewise heterostyled.
Oldenlandia [sp.?] (Rubiaceae).
Mr. J. Scott sent me from India dried flowers of a heterostyled species of this
genus, which is closely allied to the last. The pistil in the long-styled
flowers is longer by about a quarter of its length, and the stamens shorter in
about the same proportion, than the corresponding organs in the short-styled
flowers. In the latter the anthers are longer, and the divergent stigmas
decidedly longer and apparently thinner than in the long-styled form. Owing to
the state of the specimens, I could not decide whether the stigmatic papillae
were longer in the one form than in the other. The pollen-grains, distended with
water, from the short-styled flowers were to those from the long-styled as 100
to 78 in diameter, as deduced from the mean of ten measurements of each kind.
Hedyotis [sp.?] (Rubiaceae).
Fritz Muller sent me from St. Catharina, in Brazil, dried flowers of a small
delicate species, which grows on wet sand near the edges of fresh-water pools.
In the long-styled form the stigma projects above the corolla, and stands on a
level with the projecting anthers of the short-styled form; but in the latter
the stigmas stand rather beneath the level of the anthers in the other or long-
styled form, these being enclosed within the tube of the corolla. The pistil of
the long-styled form is nearly thrice as long as that of the short-styled, or,
speaking strictly, as 100 to 39; and the papillae on the stigma of the former
are broader, in the ratio of 4 to 3, but whether longer than those of the short-
styled, I could not decide. In the short-styled form, the anthers are rather
larger, and the pollen-grains are to those from the long-styled flowers, as 100
to 88 in diameter. Fritz Muller sent me a second, small-sized species, which is
likewise heterostyled.
Coccocypselum [sp.?] (Rubiaceae).
Fritz Muller also sent me dried flowers of this plant from St. Catharina, in
Brazil. The exserted stigma of the long-styled form stands a little above the
level of the exserted anthers of the short-styled form; and the enclosed stigma
of the latter also stands a little above the level of the enclosed anthers in
the long-styled form. The pistil of the long-styled is about twice as long as
that of the short-styled, with its two stigmas considerably longer, more
divergent, and more curled. Fritz Muller informs me that he could detect no
difference in the size of the pollen-grains in the two forms. Nevertheless,
there can be no doubt that this plant is heterostyled.
Lipostoma [sp.?] (Rubiaceae).
Dried flowers of this plant, which grows in small wet ditches in St. Catharina,
in Brazil, were likewise sent me by Fritz Muller. In the long-styled form the
exserted stigma stands rather above the level of the exserted anthers of the
other form; whilst in the short-styled form it stands on a level with the
anthers of the other form. So that the want of strict correspondence in height
between the stigmas and anthers in the two forms is reversed, compared with what
occurs in Hedyotis. The long-styled pistil is to that of the short-styled as 100
to 36 in length; and its divergent stigmas are longer by fully one-third of
their own length than those of the short-styled form. In the latter the anthers
are a little larger, and the pollen-grains are as 100 to 80 in diameter,
compared with those from the long-styled form.
Cinchona micrantha (Rubiaceae).
Dried specimens of both forms of this plant were sent me from Kew. (3/26. My
attention was called to this plant by a drawing copied from Howard's
'Quinologia' Table 3 given by Mr. Markham in his 'Travels in Peru' page 539.) In
the long-styled form the apex of the stigma stands just beneath the bases of the
hairy lobes of the corolla; whilst the summits of the anthers are seated about
halfway down the tube. The pistil is in length as 100 to 38 to that of the
short-styled form. In the latter the anthers occupy the same position as the
stigma of the other form, and they are considerably longer than those of the
long-styled form. As the summit of the stigma in the short-styled form stands
beneath the bases of the anthers, which are seated halfway down the corolla, the
style has been extremely shortened in this form, its length to that of the long-
styled being, in the specimens examined, only as 5.3 to 100! The stigma, also,
in the short-styled form is very much shorter than that in the long-styled, in
the ratio of 57 to 100. The pollen grains from the short-styled flowers, after
having been soaked in water, were rather larger--in about the ratio of 100 to
91--than those from the long-styled flowers, and they were more triangular, with
the angles more prominent. As all the grains from the short-styled flowers were
thus characterised, and as they had been left in water for three days, I am
convinced that this difference in shape in the two sets of grains cannot be
accounted for by unequal distension with water.
Besides the several Rubiaceous genera already mentioned, Fritz Muller informs me
that two or three species of Psychotria and Rudgea eriantha, natives of St.
Catharina, in Brazil, are heterostyled, as is Manettia bicolor. I may add that I
formerly fertilised with their own pollen several flowers on a plant of this
latter species in my hothouse, but they did not set a single fruit. From Wight
and Arnott's description, there seems to be little doubt that Knoxia in India is
heterostyled; and Asa Gray is convinced that this is the case with Diodia and
Spermacoce in the United States. Lastly, from Mr. W.W. Bailey's description, it
appears that the Mexican Bouvardia leiantha is heterostyled. (3/27. 'Bulletin of
the Torrey Bot. Club' 1876 page 106.)]
Altogether we now know of 17 heterostyled genera in the great family of the
Rubiaceae; though more information is necessary with respect to some of them,
more especially those mentioned in the last paragraph, before we can feel
absolutely safe. In the 'Genera Plantarum,' by Bentham and Hooker, the Rubiaceae
are divided into 25 tribes, containing 337 genera; and it deserves notice that
the genera now known to be heterostyled are not grouped in one or two of these
tribes, but are distributed in no less than eight of them. From this fact we may
infer that most of the genera have acquired their heterostyled structure
independently of one another; that is, they have not inherited this structure
from some one or even two or three progenitors in common. It further deserves
notice that in the homostyled genera, as I am informed by Professor Asa Gray,
the stamens are either exserted or are included within the tube of the corolla,
in a nearly constant manner; so that this character, which is not even of
specific value in the heterostyled species, is often of generic value in other
members of the family.
CHAPTER IV.
HETEROSTYLED TRIMORPHIC PLANTS.
Lythrum salicaria.
Description of the three forms.
Their power and complex manner of fertilising one another.
Eighteen different unions possible.
Mid-styled form eminently feminine in nature.
Lythrum Graefferi likewise trimorphic.
L. thymifolia dimorphic.
L. Hyssopifolia homostyled.
Nesaea verticillata trimorphic.
Lagerstroemia, nature doubtful.
Oxalis, trimorphic species of.
O. Valdiviana.
O. Regnelli, the illegitimate unions quite barren.
O. speciosa.
O. sensitiva.
Homostyled species of Oxalis.
Pontederia, the one monocotyledonous genus known to include heterostyled
species.
In the previous chapters various heterostyled dimorphic plants have been
described, and now we come to heterostyled trimorphic plants, or those which
present three forms. These have been observed in three families, and consist of
species of Lythrum and of the allied genus Nesaea, of Oxalis and Pontederia. In
their manner of fertilisation these plants offer a more remarkable case than can
be found in any other plant or animal.
Lythrum salicaria.
(FIGURE 4.10. Diagram of the flowers of the three forms of Lythrum salicaria, in
their natural position, with the petals and calyx removed on the near side:
enlarged six times.
Top: Long-styled.
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