The Different Forms of Flowers on Plants of the Same Species
by
Charles Darwin
Part 3 out of 6
Middle: Mid-styled.
Bottom: Short-styled.
The dotted lines with the arrows show the directions in which pollen must be
carried to each stigma to ensure full fertility.)
The pistil in each form differs from that in either of the other forms, and in
each there are two sets of stamens different in appearance and function. But one
set of stamens in each form corresponds with a set in one of the other two
forms. Altogether this one species includes three females or female organs and
three sets of male organs, all as distinct from one another as if they belonged
to different species; and if smaller functional differences are considered,
there are five distinct sets of males. Two of the three hermaphrodites must
coexist, and pollen must be carried by insects reciprocally from one to the
other, in order that either of the two should be fully fertile; but unless all
three forms coexist, two sets of stamens will be wasted, and the organisation of
the species, as a whole, will be incomplete. On the other hand, when all three
hermaphrodites coexist, and pollen is carried from one to the other, the scheme
is perfect; there is no waste of pollen and no false co-adaptation. In short,
nature has ordained a most complex marriage-arrangement, namely a triple union
between three hermaphrodites,--each hermaphrodite being in its female organ
quite distinct from the other two hermaphrodites and partially distinct in its
male organs, and each furnished with two sets of males.
The three forms may be conveniently called, from the unequal lengths of their
pistils, the LONG-STYLED, MID-STYLED, and SHORT-STYLED. The stamens also are of
unequal lengths, and these may be called the LONGEST, MID-LENGTH, and SHORTEST.
Two sets of stamens of different length are found in each form. The existence of
the three forms was first observed by Vaucher, and subsequently more carefully
by Wirtgen ; but these botanists, not being guided by any theory or even
suspicion of their functional differences, did not perceive some of the most
curious points of difference in their structure. (4/1. Vaucher 'Hist. Phys. des
Plantes d'Europe' tome 2 1841 page 371. Wirtgen "Ueber Lythrum salicaria und
dessen Formen" 'Verhand. des naturhist. Vereins fur preuss. Rheinl.' 5 Jahrgang
1848 S. 7.) I will first briefly describe the three forms by the aid of Figure
4.10, which shows the flowers, six times magnified, in their natural position,
with their petals and calyx on the near side removed.
LONG-STYLED FORM.
This form can be at once recognised by the length of the pistil, which is
(including the ovarium) fully one-third longer than that of the mid-styled, and
more than thrice as long as that of the short-styled form. It is so
disproportionately long, that it projects in the bud through the folded petals.
It stands out considerably beyond the mid-length stamens; its terminal portion
depends a little, but the stigma itself is slightly upturned. The globular
stigma is considerably larger than that of the other two forms, with the
papillae on its surface generally longer. The six mid-length stamens project
about two-thirds the length of the pistil, and correspond in length with the
pistil of the mid-styled form. Such correspondence in this and the two following
forms is generally very close; the difference, where there is any, being usually
in a slight excess of length in the stamens. The six shortest stamens lie
concealed within the calyx; their ends are turned up, and they are graduated in
length, so as to form a double row. The anthers of these stamens are smaller
than those of the mid-length ones. The pollen is of the same yellow colour in
both sets. H. Muller measured the pollen-grain in all three forms, and his
measurements are evidently more trustworthy than those which I formerly made, so
I will give them. (4/2. 'Die Befruchtung der Blumen' 1873 page 193.) The numbers
refer to divisions of the micrometer equalling 1/300 millimetres. The grains,
distended with water, from the mid-length stamens are 7 to 7 1/2, and those from
the shortest stamens 6 to 6 1/2 in diameter, or as 100 to 86. The capsules of
this form contain on an average 93 seeds: how this average was obtained will
presently be explained. As these seeds, when cleaned, seemed larger than those
from the mid-styled or short-styled forms, 100 of them were placed in a good
balance, and by the double method of weighing were found to equal 121 seeds of
the mid-styled or 142 of the short-styled; so that five long-styled seeds very
nearly equal six mid-styled or seven short-styled seeds.
MID-STYLED FORM.
The pistil occupies the position represented in Figure 4.10, with its extremity
considerably upturned, but to a variable degree; the stigma is seated between
the anthers of the longest and the shortest stamens. The six longest stamens
correspond in length with the pistil of the long-styled form; their filaments
are coloured bright pink; the anthers are dark-coloured, but from containing
bright-green pollen and from their early dehiscence they appear emerald-green.
Hence in general appearance these stamens are remarkably dissimilar from the
mid-length stamens of the long-styled form. The six shortest stamens are
enclosed within the calyx, and resemble in all respects the shortest stamens of
the long-styled form; both these sets correspond in length with the short pistil
of the short-styled form. The green pollen-grains of the longest stamens are 9
to 10 in diameter, whilst the yellow grains from the shortest stamens are only
6; or as 100 to 63. But the pollen-grains from different plants appeared to me,
in this case and others, to be in some degree variable in size. The capsules
contain on an average 130 seeds; but perhaps, as we shall see, this is rather
too high an average. The seeds themselves, as before remarked, are smaller than
those of the long-styled form.
SHORT-STYLED FORM.
The pistil is here very short, not one-third of the length of that of the long-
styled form. It is enclosed within the calyx, which, differently from that in
the other two forms, does not enclose any anthers. The end of the pistil is
generally bent upwards at right angles. The six longest stamens, with their pink
filaments and green pollen, resemble the corresponding stamens of the mid-styled
form. But according to H. Muller, their pollen-grains are a little larger,
namely 9 1/2 to 10 1/2, instead of 9 to 10 in diameter. The six mid-length
stamens, with their uncoloured filaments and yellow pollen, resemble in the size
of their pollen-grains and in all other respects the corresponding stamens of
the long-styled form. The difference in diameter between the grains from the two
sets of anthers in the short-styled form is as 100 to 73. The capsules contain
fewer seeds on an average than those of either of the preceding forms, namely
83.5; and the seeds are considerably smaller. In this latter respect, but not in
number, there is a gradation parallel to that in the length of the pistil, the
long-styled having the largest seeds, the mid-styled the next in size, and the
short-styled the smallest.
We thus see that this plant exists under three female forms, which differ in the
length and curvature of the style, in the size and state of the stigma, and in
the number and size of the seed. There are altogether thirty-six males or
stamens, and these can be divided into three sets of a dozen each, differing
from one another in length, curvature, and colour of the filaments--in the size
of the anthers, and especially in the colour and diameter of the pollen-grains.
Each form bears half-a-dozen of one kind of stamens and half-a-dozen of another
kind, but not all three kinds. The three kinds of stamens correspond in length
with the three pistils: the correspondence is always between half of the stamens
in two of the forms with the pistil of the third form. Table 4.a of the
diameters of the pollen-grains, after immersion in water, from both sets of
stamens in all three forms is copied from H. Muller; they are arranged in the
order of their size:--
TABLE 4.a. Lythrum salicaria. Diameters of pollen-grains after immersion in
water.
Column 1: Source of Pollen-grains.
Column 2: Minimum diameter.
Column 3: Maximum diameter.
Longest stamens of short-styled form : 9 1/2 : 10 1/2.
Longest stamens of mid-styled form : 9 : 10.
Mid-length stamens of long-styled form : 7 : 7 1/2.
Mid-length stamens of short-styled form : 7 : 7 1/2.
Shortest stamens of long-styled form : 6 : 6 1/2.
Shortest stamens of mid-styled form : 6 : 6.
We here see that the largest pollen-grains come from the longest stamens, and
the least (smallest) from the shortest; the extreme difference in diameter
between them being as 100 to 60.
The average number of seeds in the three forms was ascertained by counting them
in eight fine selected capsules taken from plants growing wild, and the result
was, as we have seen, for the long-styled (neglecting decimals) 93, mid-styled
130, and short-styled 83. I should not have trusted in these ratios had I not
possessed a number of plants in my garden which, owing to their youth, did not
yield the full complement of seed, but were of the same age and grew under the
same conditions, and were freely visited by bees. I took six fine capsules from
each, and found the average to be for the long-styled 80, for the mid-styled 97,
and for the short-styled 61. Lastly, legitimate unions effected by me between
the three forms gave, as may be seen in the following tables, for the long-
styled an average of 90 seeds, for the mid-styled 117, and for the short-styled
71. So that we have good concurrent evidence of a difference in the average
production of seed by the three forms. To show that the unions effected by me
often produced their full effect and may be trusted, I may state that one mid-
styled capsule yielded 151 good seeds, which is the same number as in the finest
wild capsule which I examined. Some artificially fertilised short- and long-
styled capsules produced a greater number of seeds than was ever observed by me
in wild plants of the same forms, but then I did not examine many of the latter.
This plant, I may add, offers a remarkable instance, how profoundly ignorant we
are of the life-conditions of a species. Naturally it grows "in wet ditches,
watery places, and especially on the banks of streams," and though it produces
so many minute seeds, it never spreads on the adjoining land; yet, when planted
in my garden, on clayey soil lying over chalk, and which is so dry that a rush
cannot be found, it thrives luxuriantly, grows to above 6 feet in height,
produces self-sown seedlings, and (which is a severer test) is as fertile as in
a state of nature. Nevertheless it would be almost a miracle to find this plant
growing spontaneously on such land as that in my garden.
According to Vaucher and Wirtgen, the three forms coexist in all parts of
Europe. Some friends gathered for me in North Wales a number of twigs from
separate plants growing near one another, and classified them. My son did the
same in Hampshire, and here is the result:--
TABLE 4.22. Lythrum salicaria. Classification according to form of flower.
Column 1: Place of origin.
Column 2: Long-styled.
Column 3: Mid-styled.
Column 4: Short-styled.
Column 5: Total.
North Wales : 95 : 97 : 72 : 264.
Hampshire : 53 : 38 : 38 : 129.
Total : 148 : 135 : 110 : 393.
If twice or thrice the number had been collected, the three forms would probably
have been found nearly equal; I infer this from considering the above figures,
and from my son telling me that if he had collected in another spot, he felt
sure that the mid-styled plants would have been in excess. I several times sowed
small parcels of seed, and raised all three forms; but I neglected to record the
parent-form, excepting in one instance, in which I raised from short-styled seed
twelve plants, of which only one turned out long-styled, four mid-styled, and
seven short-styled.
Two plants of each form were protected from the access of insects during two
successive years, and in the autumn they yielded very few capsules and presented
a remarkable contrast with the adjoining uncovered plants, which were densely
covered with capsules. In 1863 a protected long-styled plant produced only five
poor capsules; two mid-styled plants produced together the same number; and two
short-styled plants only a single one. These capsules contained very few seeds;
yet the plants were fully productive when artificially fertilised under the net.
In a state of nature the flowers are incessantly visited for their nectar by
hive- and other bees, various Diptera and Lepidoptera. (4/3. H. Muller gives a
list of the species 'Die Befruchtung der Blumen' page 196. It appears that one
bee, the Cilissa melanura, almost confines its visits to this plant.) The nectar
is secreted all round the base of the ovarium; but a passage is formed along the
upper and inner side of the flower by the lateral deflection (not represented in
the diagram) of the basal portions of the filaments; so that insects invariably
alight on the projecting stamens and pistil, and insert their proboscides along
the upper and inner margin of the corolla. We can now see why the ends of the
stamens with their anthers, and the ends of the pistils with their stigmas, are
a little upturned, so that they may be brushed by the lower hairy surfaces of
the insects' bodies. The shortest stamens which lie enclosed within the calyx of
the long- and mid-styled forms can be touched only by the proboscis and narrow
chin of a bee; hence they have their ends more upturned, and they are graduated
in length, so as to fall into a narrow file, sure to be raked by the thin
intruding proboscis. The anthers of the longer stamens stand laterally farther
apart and are more nearly on the same level, for they have to brush against the
whole breadth of the insect's body. In very many other flowers the pistil, or
the stamens, or both, are rectangularly bent to one side of the flower. This
bending may be permanent, as with Lythrum and many others, or may be effected,
as in Dictamnus fraxinella and others, by a temporary movement, which occurs in
the case of the stamens when the anthers dehisce, and in the case of the pistil
when the stigma is mature; but these two movements do not always take place
simultaneously in the same flower. Now I have found no exception to the rule,
that when the stamens and pistil are bent, they bend to that side of the flower
which secretes nectar, even though there be a rudimentary nectary of large size
on the opposite side, as in some species of Corydalis. When nectar is secreted
on all sides, they bend to that side where the structure of the flower allows
the easiest access to it, as in Lythrum, various Papilionaceae, and others. The
rule consequently is, that when the pistils and stamens are curved or bent, the
stigma and anthers are thus brought into the pathway leading to the nectary.
There are a few cases which seem to be exceptions to this rule, but they are not
so in truth; for instance, in the Gloriosa lily, the stigma of the grotesque and
rectangularly bent pistil is brought, not into any pathway from the outside
towards the nectar-secreting recesses of the flower, but into the circular route
which insects follow in proceeding from one nectary to the other. In
Scrophularia aquatica the pistil is bent downwards from the mouth of the
corolla, but it thus strikes the pollen-dusted breast of the wasps which
habitually visit these ill-scented flowers. In all these cases we see the
supreme dominating power of insects on the structure of flowers, especially of
those which have irregular corollas. Flowers which are fertilised by the wind
must of course be excepted; but I do not know of a single instance of an
irregular flower which is thus fertilised.
Another point deserves notice. In each of the three forms two sets of stamens
correspond in length with the pistils in the other two forms. When bees suck the
flowers, the anthers of the longest stamens, bearing the green pollen, are
rubbed against the abdomen and the inner sides of the hind legs, as is likewise
the stigma of the long-styled form. The anthers of the mid-length stamens and
the stigma of the mid-styled form are rubbed against the under side of the
thorax and between the front pair of legs. And, lastly, the anthers of the
shortest stamens and the stigma of the short-styled form are rubbed against the
proboscis and chin: for the bees in sucking the flowers insert only the front
part of their heads into the flower. On catching bees, I observed much green
pollen on the inner sides of the hind legs and on the abdomen, and much yellow
pollen on the under side of the thorax. There was also pollen on the chin, and,
it may be presumed, on the proboscis, but this was difficult to observe. I had,
however, independent proof that pollen is carried on the proboscis; for a small
branch of a protected short-styled plant (which produced spontaneously only two
capsules) was accidentally left during several days pressing against the net,
and bees were seen inserting their proboscides through the meshes, and in
consequence numerous capsules were formed on this one small branch. From these
several facts it follows that insects will generally carry the pollen of each
form from the stamens to the pistil of corresponding length; and we shall
presently see the importance of this adaptation. It must not, however, be
supposed that the bees do not get more or less dusted all over with the several
kinds of pollen; for this could be seen to occur with the green pollen from the
longest stamens. Moreover a case will presently be given of a long-styled plant
producing an abundance of capsules, though growing quite by itself, and the
flowers must have been fertilised by their own kinds of pollen; but these
capsules contained a very poor average of seed. Hence insects, and chiefly bees,
act both as general carriers of pollen, and as special carriers of the right
sort.
Wirtgen remarks on the variability of this plant in the branching of the stem,
in the length of the bracteae, size of the petals, and in several other
characters. (4/4. 'Verhand. des naturhist. Vereins fur Pr. Rheinl.' 5 Jahrgang
1848 pages 11, 13.) The plants which grew in my garden had their leaves, which
differed much in shape, arranged oppositely, alternately, or in whorls of three.
In this latter case the stems were hexagonal; those of the other plants being
quadrangular. But we are concerned chiefly, with the reproductive organs: the
upward bending of the pistil is variable, and especially in the short-styled
form, in which it is sometimes straight, sometimes slightly curved, but
generally bent at right angles. The stigma of the long-styled pistil frequently
has longer papillae or is rougher than that of the mid-styled, and the latter
than that of the short-styled; but this character, though fixed and uniform in
the two forms of Primula veris, etc., is here variable, for I have seen mid-
styled stigmas rougher than those of the long-styled. (4/5. The plants which I
observed grew in my garden, and probably varied rather more than those growing
in a state of nature. H. Muller has described the stigmas of all three forms
with great care, and he appears to have found the stigmatic papillae differing
constantly in length and structure in the three forms, being longest in the
long-styled form.) The degree to which the longest and mid-length stamens are
graduated in length and have their ends upturned is variable; sometimes all are
equally long. The colour of the green pollen in the longest stamens is variable,
being sometimes pale greenish-yellow; in one short-styled plant it was almost
white. The grains vary a little in size: I examined one short-styled plant with
the grains from the mid-length and shortest anthers of the same size. We here
see great variability in many important characters; and if any of these
variations were of service to the plant, or were correlated with useful
functional differences, the species is in that state in which natural selection
might readily do much for its modification.
ON THE POWER OF MUTUAL FERTILISATION BETWEEN THE THREE FORMS.
Nothing shows more clearly the extraordinary complexity of the reproductive
system of this plant, than the necessity of making eighteen distinct unions in
order to ascertain the relative fertilising power of the three forms. Thus the
long-styled form has to be fertilised with pollen from its own two kinds of
anthers, from the two in the mid-styled, and from the two in the short-styled
form. The same process has to be repeated with the mid-styled and short-styled
forms. It might have been thought sufficient to have tried on each stigma the
green pollen, for instance, from either the mid- or short-styled longest
stamens, and not from both; but the result proves that this would have been
insufficient, and that it was necessary to try all six kinds of pollen on each
stigma. As in fertilising flowers there will always be some failures, it would
have been advisable to have repeated each of the eighteen unions a score of
times; but the labour would have been too great; as it was, I made 223 unions,
i.e. on an average I fertilised above a dozen flowers in the eighteen different
methods. Each flower was castrated; the adjoining buds had to be removed, so
that the flowers might be safely marked with thread, wool, etc.; and after each
fertilisation the stigma was examined with a lens to see that there was
sufficient pollen on it. Plants of all three forms were protected during two
years by large nets on a framework; two plants were used during one or both
years, in order to avoid any individual peculiarity in a particular plant. As
soon as the flowers had withered, the nets were removed; and in the autumn the
capsules were daily inspected and gathered, the ripe seeds being counted under
the microscope. I have given these details that confidence may be placed in the
following tables, and as some excuse for two blunders which, I believe, were
made. These blunders are referred to, with their probable cause, in two
footnotes to the tables. The erroneous numbers, however, are entered in the
tables, that it may not be supposed that I have in any one instance tampered
with the results.
A few words explanatory of the three tables must be given. Each is devoted to
one of the three forms, and is divided into six compartments. The two upper ones
in each table show the number of good seeds resulting from the application to
the stigma of pollen from the two sets of stamens which correspond in length
with the pistil of that form, and which are borne by the other two forms. Such
unions are of a legitimate nature. The two next lower compartments show the
result of the application of pollen from the two sets of stamens, not
corresponding in length with the pistil, and which are borne by the other two
forms. These unions are illegitimate. The two lowest compartments show the
result of the application of each form's own two kinds of pollen from the two
sets of stamens belonging to the same form, and which do not equal the pistil in
length. These unions are likewise illegitimate. The term own-form pollen here
used does not mean pollen from the flower to be fertilised--for this was never
used--but from another flower on the same plant, or more commonly from a
distinct plant of the same form. The figure "0" means that no capsule was
produced, or if a capsule was produced that it contained no good seed. In some
part of each row of figures in each compartment, a short horizontal line may be
seen; the unions above this line were made in 1862, and below it in 1863. It is
of importance to observe this, as it shows that the same general result was
obtained during two successive years; but more especially because 1863 was a
very hot and dry season, and the plants had occasionally to be watered. This did
not prevent the full complement of seed being produced from the more fertile
unions; but it rendered the less fertile ones even more sterile than they
otherwise would have been. I have seen striking instances of this fact in making
illegitimate and legitimate unions with Primula; and it is well known that the
conditions of life must be highly favourable to give any chance of success in
producing hybrids between species which are crossed with difficulty.
TABLE 4.23. Lythrum salicaria, long-styled form.
TABLE 4.23.1. Legitimate union.
13 flowers fertilised by the longest stamens of the mid-styled. These stamens
equal in length the pistil of the long-styled.
Product of good seed in each capsule.
36 53
81 0
0 0
0 0
0 0
- 0
45
41
38 percent of these flowers yielded capsules. Each capsule contained, on an
average, 51.2 seeds.
TABLE 4.23.2. Legitimate union.
13 flowers fertilised by the longest stamens of the short-styled. These stamens
equal in length the pistil of the long-styled.
Product of good seed in each capsule.
159 104
43 119
96 poor seed. 96
103 99
0 131
0 116
-
114
84 percent of these flowers yielded capsules. Each capsule contained, on an
average, 107.3 seeds.
TABLE 4.23.3. Illegitimate union.
14 flowers fertilised by the shortest stamens of the mid-styled.
3 0
0 0
0 0
0 0
0 0
- 0
0 0
0
Too sterile for any average.
TABLE 4.23.4. Illegitimate union.
12 flowers fertilised by the mid-length stamens of the short-styled.
20 0
0 0
0 0
0 0
- 0
0 0
0
Too sterile for any average.
TABLE 4.23.5. Illegitimate union.
15 flowers fertilised by own-form mid-length stamens.
2 -
10 0
23 0
0 0
0 0
0 0
0 0
0 0
Too sterile for any average.
TABLE 4.23.6. Illegitimate union.
15 flowers fertilised by own-form shortest stamens.
4 -
8 0
4 0
0 0
0 0
0 0
0 0
0 0
Too sterile for any average.
Besides the above experiments, I fertilised a considerable number of long-styled
flowers with pollen, taken by a camel's-hair brush, from both the mid-length and
shortest stamens of their own form: only 5 capsules were produced, and these
yielded on an average 14.5 seeds. In 1863 I tried a much better experiment: a
long-styled plant was grown by itself, miles away from any other plant, so that
the flowers could have received only their own two kinds of pollen. The flowers
were incessantly visited by bees, and their stigmas must have received
successive applications of pollen on the most favourable days and at the most
favourable hours: all who have crossed plants know that this highly favours
fertilisation. This plant produced an abundant crop of capsules; I took by
chance 20 capsules, and these contained seeds in number as follows:--
20 20 35 21 19
26 24 12 23 10
7 30 27 29 13
20 12 29 19 35
This gives an average of 21.5 seeds per capsule. As we know that the long-styled
form, when standing near plants of the other two forms and fertilised by
insects, produces on an average 93 seeds per capsule, we see that this form,
fertilised by its own two pollens, yields only between one-fourth and one-fifth
of the full number of seed. I have spoken as if the plant had received both its
own kinds of pollen, and this is, of course, possible; but, from the enclosed
position of the shortest stamens, it is much more probable that the stigma
received exclusively pollen from the mid-length stamens; and this, as may be
seen in Table 4.23.5, is the more fertile of the two self-unions.
TABLE 4.24. Lythrum salicaria, mid-styled form.
TABLE 4.24.1. Legitimate union.
12 flowers fertilised by the mid-length stamens of the long-styled. These
stamens equal in length the pistil of the mid-styled.
Product of good seed in each capsule.
138 122
149 50
147 151
109 119
133 138
144 0
-
92 percent of these flowers (probably 100 per cent) yielded capsules. Each
capsule contained, on an average, 127.3 seeds.
TABLE 4.24.2. Legitimate union.
12 flowers fertilised by the mid-length stamens of the short-styled. These
stamens equal in length the pistil of the mid-styled.
Product of good seed in each capsule.
112 109
130 143
143 124
100 145
33 12
- 141
104
100 percent of these flowers yielded capsules. Each capsule contained, on an
average, 108.0 seeds; or, excluding capsules with less than 20 seeds, the
average is 116.7 seeds.
TABLE 4.24.3. Illegitimate union.
13 flowers fertilised by the shortest stamens of the long-styled.
83 12
0 19
0 85 seeds small and poor.
- 0
44 0
44 0
45 0
54 percent of these flowers yielded capsules. Each capsule contained, on an
average, 47.4 seeds; or, excluding capsules with less than 20 seeds, the average
is 60.2 seeds.
TABLE 4.24.4. Illegitimate union.
15 flowers fertilised by the longest stamens of the short-styled.
130 86
115 113
14 29
6 17
2 113
9 79
- 128
132 0
93 percent of these flowers yielded capsules. Each capsule contained, on an
average, 69.5 seeds; or, excluding capsules with less than 20 seeds, the average
is 102.8 seeds.
TABLE 4.24.5. Illegitimate union.
12 flowers fertilised by own-form longest stamens.
92 0
9 0
63 0
- 0
136?* 0
0 0
0
(4/6. * I have hardly a doubt that this result of 136 seeds in Table 4.24.5 was
due to a gross error. The flowers to be fertilised by their own longest stamens
were first marked by "white thread," and those by the mid-length stamens of the
long-styled form by "white silk;" a flower fertilised in the later manner would
have yielded about 136 seeds, and it may be observed that one such pod is
missing, namely at the bottom of Table 4.24.1. Therefore I have hardly any doubt
that I fertilised a flower marked with "white thread" as if it had been marked
with "white silk." With respect to the capsule which yielded 92 seeds, in the
same column with that which yielded 136, I do not know what to think. I
endeavoured to prevent pollen dropping from an upper to a lower flower, and I
tried to remember to wipe the pincers carefully after each fertilisation; but in
making eighteen different unions, sometimes on windy days, and pestered by bees
and flies buzzing about, some few errors could hardly be avoided. One day I had
to keep a third man by me all the time to prevent the bees visiting the
uncovered plants, for in a few seconds' time they might have done irreparable
mischief. It was also extremely difficult to exclude minute Diptera from the
net. In 1862 I made the great mistake of placing a mid-styled and long-styled
under the same huge net: in 1863 I avoided this error.)
Excluding the capsule with 136 seeds, 25 percent of the flowers yielded
capsules, and each capsule contained, on an average, 54.6 seeds; or, excluding
capsules with less than 20 seeds, the average is 77.5.
TABLE 4.24.6. Illegitimate union.
12 flowers fertilised by own-form shortest stamens.
0 0
0 0
0 0
- 0
0 0
0 0
0
Not one flower yielded a capsule.
Besides the experiments in Table 4.24, I fertilised a considerable number of
mid-styled flowers with pollen, taken by a camel's-hair brush, from both the
longest and shortest stamens of their own form: only 5 capsules were produced,
and these yielded on an average 11.0 seeds.
TABLE 4.25. Lythrum salicaria, short-styled form.
TABLE 4.25.1. Legitimate union.
12 flowers fertilised by the shortest stamens of the long-styled. These stamens
equal in length the pistil of the short-styled.
69 56
61 88
88 112
66 111
0 62
0 100
-
83 percent of the flowers yielded capsules. Each capsule contained, on an
average, 81.3 seeds.
TABLE 4.25.2. Legitimate union.
13 flowers fertilised by the shortest stamens of the mid-styled. These stamens
equal in length the pistil of the short-styled.
93 69
77 69
48 53
43 9
0 0
0 0
- 0
61 percent of the flowers yielded capsules. Each capsule contained, on an
average, 64.6 seeds.
TABLE 4.25.3. Illegitimate union.
10 flowers fertilised by the mid-length stamens of the long-styled.
0 14
0 0
0 0
0 0
- 0
23
Too sterile for any average.
TABLE 4.25.4. Illegitimate union.
10 flowers fertilised by the longest stamens of the mid-styled.
0 0
0 0
0 0
0 0
- 0
0
Too sterile for any average.
TABLE 4.25.5. Illegitimate union.
10 flowers fertilised by own-form longest stamens.
0 0
0 0
0 0
- 0
0 0
0
Too sterile for any average.
TABLE 4.25.6. Illegitimate union.
10 flowers fertilised by own-form mid-length stamens.
64?* 0
0 0
0 0
- 0
21 0
9
(4/7. *I suspect that by mistake I fertilised this flower in Table 4.25.6 with
pollen from the shortest stamens of the long-styled form, and it would then have
yielded about 64 seeds. Flowers to be thus fertilised were marked with black
silk; those with pollen from the mid-length stamens of the short-styled with
black thread; and thus probably the mistake arose.)
Too sterile for any average.
Besides the experiments in the table, I fertilised a number of flowers without
particular care with their own two kinds of pollen, but they did not produce a
single capsule.
SUMMARY OF THE RESULTS.
LONG-STYLED FORM.
Twenty-six flowers fertilised legitimately by the stamens of corresponding
length, borne by the mid-and short-styled forms, yielded 61.5 per cent of
capsules, which contained on an average 89.7 seeds.
Twenty-six long-styled flowers fertilised illegitimately by the other stamens of
the mid-and short-styled forms yielded only two very poor capsules.
Thirty long-styled flowers fertilised illegitimately by their own-form two sets
of stamens yielded only eight very poor capsules; but long-styled flowers
fertilised by bees with pollen from their own stamens produced numerous capsules
containing on an average 21.5 seeds.
MID-STYLED FORM.
Twenty-four flowers legitimately fertilised by the stamens of corresponding
length, borne by the long and short-styled forms, yielded 96 (probably 100) per
cent of capsules, which contained (excluding one capsule with 12 seeds) on an
average 117.2 seeds.
Fifteen mid-styled flowers fertilised illegitimately by the longest stamens of
the short-styled form yielded 93 per cent of capsules, which (excluding four
capsules with less than 20 seeds) contained on an average 102.8 seeds.
Thirteen mid-styled flowers fertilised illegitimately by the mid-length stamens
of the long-styled form yielded 54 per cent of capsules, which (excluding one
with 19 seeds) contained on an average 60.2 seeds.
Twelve mid-styled flowers fertilised illegitimately by their own-form longest
stamens yielded 25 per cent of capsules, which (excluding one with 9 seeds)
contained on an average 77.5 seeds.
Twelve mid-styled flowers fertilised illegitimately by their own-form shortest
stamens yielded not a single capsule.
SHORT-STYLED FORM.
Twenty-five flowers fertilised legitimately by the stamens of corresponding
length, borne by the long and mid-styled forms, yielded 72 per cent of capsules,
which (excluding one capsule with only 9 seeds) contained on an average 70.8
seeds.
Twenty short-styled flowers fertilised illegitimately by the other stamens of
the long and mid-styled forms yielded only two very poor capsules.
Twenty short-styled flowers fertilised illegitimately by their own stamens
yielded only two poor (or perhaps three) capsules.
If we take all six legitimate unions together, and all twelve illegitimate
unions together, we get the following results:
TABLE 4.26.
Column 1: Nature of union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Average Number of Seeds per Capsule.
Column 5: Average Number of Seeds per Flower fertilised.
The six legitimate unions : 75 : 56 : 96.29 : 71.89.
The twelve illegitimate unions : 146 : 36 : 44.72 : 11.03.
Therefore the fertility of the legitimate unions to that of the illegitimate, as
judged by the proportion of the fertilised flowers which yielded capsules, is as
100 to 33; and judged by the average number of seeds per capsule, as 100 to 46.
From this summary and the several foregoing tables we see that it is only pollen
from the longest stamens which can fully fertilise the longest pistil; only that
from the mid-length stamens, the mid-length pistil; and only that from the
shortest stamens, the shortest pistil. And now we can comprehend the meaning of
the almost exact correspondence in length between the pistil in each form and a
set of six stamens in two of the other forms; for the stigma of each form is
thus rubbed against that part of the insect's body which becomes charged with
the proper pollen. It is also evident that the stigma of each form, fertilised
in three different ways with pollen from the longest, mid-length, and shortest
stamens, is acted on very differently, and conversely that the pollen from the
twelve longest, twelve mid-length, and twelve shortest stamens acts very
differently on each of the three stigmas; so that there are three sets of female
and of male organs. Moreover, in most cases the six stamens of each set differ
somewhat in their fertilising power from the six corresponding ones in one of
the other forms. We may further draw the remarkable conclusion that the greater
the inequality in length between the pistil and the set of stamens, the pollen
of which is employed for its fertilisation, by so much is the sterility of the
union increased. There are no exceptions to this rule. To understand what
follows the reader should look to Tables 4.23, 4.24 and 4.25, and to the diagram
Figure 4.10. In the long-styled form the short stamens obviously differ in
length from the pistil to a greater degree than do the mid-length stamens; and
the capsules produced by the use of pollen from the shortest stamens contain
fewer seeds than those produced by the pollen from the mid-length stamens. The
same result follows with the long-styled form, from the use of the pollen of
shortest stamens of the mid-styled form and of the mid-length stamens of the
short-styled form. The same rule also holds good with the mid-styled and short-
styled forms, when illegitimately fertilised with pollen from the stamens more
or less unequal in length to their pistils. Certainly the difference in
sterility in these several cases is slight; but, as far as we are enabled to
judge, it always increases with the increasing inequality of length between the
pistil and the stamens which are used in each case.
The correspondence in length between the pistil in each form and a set of
stamens in the other two forms, is probably the direct result of adaptation, as
it is of high service to the species by leading to full and legitimate
fertilisation. But the rule of the increased sterility of the illegitimate
unions according to the greater inequality in length between the pistils and
stamens employed for the union can be of no service. With some heterostyled
dimorphic plants the difference of fertility between the two illegitimate unions
appears at first sight to be related to the facility of self-fertilisation; so
that when from the position of the parts the liability in one form to self-
fertilisation is greater than in the other, a union of this kind has been
checked by having been rendered the more sterile of the two. But this
explanation does not apply to Lythrum; thus the stigma of the long-styled form
is more liable to be illegitimately fertilised with pollen from its own mid-
length stamens, or with pollen from the mid-length stamens of the short-styled
form, than by its own shortest stamens or those of the mid-styled form; yet the
two former unions, which it might have been expected would have been guarded
against by increased sterility, are much less likely to be effected. The same
relation holds good even in a more striking manner with the mid-styled form, and
with the short-styled form as far as the extreme sterility of all its
illegitimate unions allows of any comparison. We are led, therefore, to conclude
that the rule of increased sterility in accordance with increased inequality in
length between the pistils and stamens, is a purposeless result, incidental on
those changes through which the species has passed in acquiring certain
characters fitted to ensure the legitimate fertilisation of the three forms.
Another conclusion which may be drawn from Tables 4.23, 4.24, and 4.25, even
from a glance at them, is that the mid-styled form differs from both the others
in its much higher capacity for fertilisation in various ways. Not only did the
twenty-four flowers legitimately fertilised by the stamens of corresponding
lengths, all, or all but one, yield capsules rich in seed; but of the other four
illegitimate unions, that by the longest stamens of the short-styled form was
highly fertile, though less so than the two legitimate unions, and that by the
mid-length stamens of the long-styled form was fertile to a considerable degree;
the remaining two illegitimate unions, namely, with this form's own pollen, were
sterile, but in different degrees. So that the mid-styled form, when fertilised
in the six different possible methods, evinces five grades of fertility. By
comparing Tables 4.24.3 and 4.24.6 we may see that the action of the pollen from
the shortest stamens of the long-styled and mid-styled forms is widely
different; in the one case above half the fertilised flowers yielded capsules
containing a fair number of seeds; in the other case not one capsule was
produced. So, again, the green, large-grained pollen from the longest stamens of
the short-styled and mid-styled forms (in Tables 4.24.4 and 4.24.5) is widely
different. In both these cases the difference in action is so plain that it
cannot be mistaken, but it can be corroborated. If we look to Table 4.25 to the
legitimate action of the shortest stamens of the long- and mid-styled forms on
the short-styled form, we again see a similar but slighter difference, the
pollen of the shortest stamens of the mid-styled form yielding a smaller average
of seed during the two years of 1862 and 1863 than that from the shortest
stamens of the long-styled form. Again, if we look to Table 4.23, to the
legitimate action on the long-styled form of the green pollen of the two sets of
longest stamens, we shall find exactly the same result, namely, that the pollen
from the longest stamens of the mid-styled form yielded during both years fewer
seeds than that from the longest stamens of the short-styled form. Hence it is
certain that the two kinds of pollen produced by the mid-styled form are less
potent than the two similar kinds of pollen produced by the corresponding
stamens of the other two forms.
In close connection with the lesser potency of the two kinds of pollen of the
mid-styled form is the fact that, according to H. Muller, the grains of both are
a little less in diameter than the corresponding grains produced by the other
two forms. Thus the grains from the longest stamens of the mid-styled form are 9
to 10, whilst those from the corresponding stamens of the short-styled form are
9 1/2 to 10 1/2 in diameter. So, again, the grains from the shortest stamens of
the mid-styled are 6, whilst those from the corresponding stamens of the long-
styled are 6 to 6 1/2 in diameter. It would thus appear as if the male organs of
the mid-styled form, though not as yet rudimentary, were tending in this
direction. On the other hand, the female organs of this form are in an eminently
efficient state, for the naturally fertilised capsules yielded a considerably
larger average number of seeds than those of the other two forms--almost every
flower which was artificially fertilised in a legitimate manner produced a
capsule--and most of the illegitimate unions were highly productive. The mid-
styled form thus appears to be highly feminine in nature; and although, as just
remarked, it is impossible to consider its two well-developed sets of stamens
which produce an abundance of pollen as being in a rudimentary condition, yet we
can hardly avoid connecting as balanced the higher efficiency of the female
organs in this form with the lesser efficiency and lesser size of its two kinds
of pollen-grains. The whole case appears to me a very curious one.
It may be observed in Tables 4.23 to 4.25 that some of the illegitimate unions
yielded during neither year a single seed; but, judging from the long-styled
plants, it is probable, if such unions were to be effected repeatedly by the aid
of insects under the most favourable conditions, some few seeds would be
produced in every case. Anyhow, it is certain that in all twelve illegitimate
unions the pollen-tubes penetrated the stigma in the course of eighteen hours.
At first I thought that two kinds of pollen placed together on the same stigma
would perhaps yield more seed than one kind by itself; but we have seen that
this is not so with each form's own two kinds of pollen; nor is it probable in
any case, as I occasionally got, by the use of a single kind of pollen, fully as
many seeds as a capsule naturally fertilised ever produces. Moreover the pollen
from a single anther is far more than sufficient to fertilise fully a stigma;
hence, in this as with so many other plants, more than twelve times as much of
each kind of pollen is produced as is necessary to ensure the full fertilisation
of each form. From the dusted condition of the bodies of the bees which I caught
on the flowers, it is probable that pollen of various kinds is often deposited
on all three stigmas; but from the facts already given with respect to the two
forms of Primula, there can hardly be a doubt that pollen from the stamens of
corresponding length placed on a stigma would be prepotent over any other kind
of pollen and obliterate its effects,--even if the latter had been placed on the
stigma some hours previously.
Finally, it has now been shown that Lythrum salicaria presents the extraordinary
case of the same species bearing three females, different in structure and
function, and three or even five sets (if minor differences are considered) of
males; each set consisting of half-a-dozen, which likewise differ from one
another in structure and function.
[Lythrum Graefferi.
I have examined numerous dried flowers of this species, each from a separate
plant, sent me from Kew. Like L. salicaria, it is trimorphic, and the three
forms apparently occur in about equal numbers. In the long-styled form the
pistil projects about one-third of the length of the calyx beyond its mouth, and
is therefore relatively much shorter than in L. salicaria; the globose and
hirsute stigma is larger than that of the other two forms; the six mid-length
stamens, which are graduated in length, have their anthers standing close above
and close beneath the mouth of the calyx; the six shortest stamens rise rather
above the middle of the calyx. In the mid-styled form the stigma projects just
above the mouth of the calyx, and stands almost on a level with the mid-length
stamens of the long and short-styled forms; its own longest stamens project well
above the mouth of the calyx, and stand a little above the level of the stigma
of the long-styled form. In short, without entering on further details, there is
a close general correspondence in structure between this species and L.
salicaria, but with some differences in the proportional lengths of the parts.
The fact of each of the three pistils having two sets of stamens of
corresponding lengths, borne by the two other forms, comes out conspicuously. In
the mid-styled form the pollen-grains from the longest stamens are nearly double
the diameter of those from the shortest stamens; so that there is a greater
difference in this respect than in L. salicaria. In the long-styled form, also,
the difference in diameter between the pollen-grains of the mid-length and
shortest stamens is greater than in L. salicaria. These comparisons, however,
must be received with caution, as they were made on specimens soaked in water
after having been long kept dry.
Lythrum thymifolia.
This form, according to Vaucher, is dimorphic, like Primula, and therefore
presents only two forms. (4/8. 'Hist. Phys. des Plantes d'Europe' tome 2 1841
pages 369, 371.) I received two dried flowers from Kew, which consisted of the
two forms; in one the stigma projected far beyond the calyx, in the other it was
included within the calyx; in this latter form the style was only one-fourth of
the length of that in the other form. There are only six stamens; these are
somewhat graduated in length, and their anthers in the short-styled form stand a
little above the stigma, but yet by no means equal in length the pistil of the
long-styled form. In the latter the stamens are rather shorter than those in the
other form. The six stamens alternate with the petals, and therefore correspond
homologically with the longest stamens of L. salicaria and L. Graefferi.
Lythrum hyssopifolia.
This species is said by Vaucher, but I believe erroneously, to be dimorphic. I
have examined dried flowers from twenty-two separate plants from various
localities, sent to me by Mr. Hewett C. Watson, Professor Babington, and others.
These were all essentially alike, so that the species cannot be heterostyled.
The pistil varies somewhat in length, but when unusually long, the stamens are
likewise generally long; in the bud the stamens are short; and Vaucher was
perhaps thus deceived. There are from six to nine stamens, graduated in length.
The three stamens, which vary in being either present or absent, correspond with
the six shorter stamens of L. salicaria and with the six which are always absent
in L. thymifolia. The stigma is included within the calyx, and stands in the
midst of the anthers, and would generally be fertilised by them; but as the
stigma and anthers are upturned, and as, according to Vaucher, there is a
passage left in the upper side of the flower to the nectary, there can hardly be
a doubt that the flowers are visited by insects, and would occasionally be
cross-fertilised by them, as surely as the flowers of the short-styled L.
salicaria, the pistil of which and the corresponding stamens in the other two
forms closely resemble those of L. hyssopifolia. According to Vaucher and Lecoq,
this species, which is an annual, generally grows almost solitarily (4/9.
'Geograph. Bot. de l'Europe' tome 6 1857 page 157.), whereas the three preceding
species are social; and this fact alone would almost have convinced me that L.
hyssopifolia was not heterostyled, as such plants cannot habitually live
isolated any better than one sex of a dioecious species.
We thus see that within this genus some species are heterostyled and trimorphic;
one apparently heterostyled and dimorphic, and one homostyled.
Nesaea verticillata.
I raised a number of plants from seed sent me by Professor Asa Gray, and they
presented three forms. These differed from one another in the proportional
lengths of their organs of fructification and in all respects, in very nearly
the same way as the three forms of Lythrum Graefferi. The green pollen-grains
from the longest stamens, measured along their longer axis and not distended
with water, were 13/7000 of an inch in length; those from the mid-length stamens
9 to 10/7000, and those from the shortest stamens 8 to 9/7000 of an inch. So
that the largest pollen-grains are to the smallest in diameter as 100 to 65.
This plant inhabits swampy ground in the United States. According to Fritz
Muller, a species of this genus in St. Catharina, in Southern Brazil, is
homostyled. (4/10. 'Botanische Zeitung' 1868 page 112.)
Lagerstroemia Indica.
This plant, a member of the Lythraceae, may perhaps be heterostyled, or may
formerly have been so. It is remarkable from the extreme variability of its
stamens. On a plant, growing in my hothouse, the flowers included from nineteen
to twenty-nine short stamens with yellow pollen, which correspond in position
with the shortest stamens of Lythrum; and from one to five (the latter number
being the commonest) very long stamens, with thick flesh-coloured filaments and
green pollen, corresponding in position with the longest stamens of Lythrum. In
one flower, two of the long stamens produced green, while a third produced
yellow pollen, although the filaments of all three were thick and flesh-
coloured. In an anther of another flower, one cell contained green and the other
yellow pollen. The green and yellow pollen-grains from the stamens of different
length are of the same size. The pistil is a little bowed upwards, with the
stigma seated between the anthers of the short and long stamens, so that this
plant was mid-styled. Eight flowers were fertilised with green pollen, and six
with yellow pollen, but not one set fruit. This latter fact by no means proves
that the plant is heterostyled, as it may belong to the class of self-sterile
species. Another plant growing in the Botanic Gardens at Calcutta, as Mr. J.
Scott informs me, was long-styled, and it was equally sterile with its own
pollen; whilst a long-styled plant of L. reginae, though growing by itself,
produced fruit. I examined dried flowers from two plants of L. parviflora, both
of which were long-styled, and they differed from L. Indica in having eight long
stamens with thick filaments, and a crowd of shorter stamens. Thus the evidence
whether L. Indica is heterostyled is curiously conflicting: the unequal number
of the short and long stamens, their extreme variability, and especially the
fact of their pollen-grains not differing in size, are strongly opposed to this
belief; on the other hand, the difference in length of the pistils in two of the
plants, their sterility with their own pollen, and the difference in length and
structure of the two sets of stamens in the same flower, and in the colour of
their pollen, favour the belief. We know that when plants of any kind revert to
a former condition, they are apt to be highly variable, and the two halves of
the same organ sometimes differ much, as in the case of the above-described
anther of the Lagerstroemia; we may therefore suspect that this species was once
heterostyled, and that it still retains traces of its former state, together
with a tendency to revert more completely to it. It deserves notice, as bearing
on the nature of Lagerstroemia, that in Lythrum hyssopifolia, which is a
homostyled species, some of the shorter stamens vary in being either present or
absent; and that these same stamens are altogether absent in L. thymifolia. In
another genus of the Lythraceae, namely Cuphea, three species raised by me from
seed certainly were homostyled; nevertheless their stamens consisted of two
sets, differing in length and in the colour and thickness of their filaments,
but not in the size or colour of their pollen-grains; so that they thus far
resembled the stamens of Lagerstroemia. I found that Cuphea purpurea was highly
fertile with its own pollen when artificially aided, but sterile when insects
were excluded. (4/11. Mr. Spence informs me that in several species of the genus
Mollia (Tiliaceae) which he collected in South America, the stamens of the five
outer cohorts have purplish filaments and green pollen, whilst the stamens of
the five inner cohorts have yellow pollen. He therefore suspected that these
species might prove to be heterostyled and trimorphic: but he did not notice the
length of the pistils. In the allied Luhea the outer purplish stamens are
destitute of anthers. I procured some specimens of Mollia lepidota and speciosa
from Kew, but could not make out that their pistils differed in length in
different plants; and in all those which I examined the stigma stood close
beneath the uppermost anthers. The numerous stamens are graduated in length, and
the pollen-grains from the longest and shortest ones did not present any marked
difference in diameter. Therefore these species do not appear to be
heterostyled.)]
Oxalis (Geraniaceae).
(Figure 4.11. Oxalis speciosa (with the petals removed).
Left: Long-styled.
Centre: Mid-styled.
Right: Short-styled.
S, S, S, stigmas. The dotted lines with arrows show which pollen must be carried
to the stigmas for legitimate fertilisation.)
In 1863 Mr. Roland Trimen wrote to me from the Cape of Good Hope that he had
there found species of Oxalis which presented three forms; and of these he
enclosed drawings and dried specimens. Of one species he collected 43 flowers
from distinct plants, and they consisted of 10 long-styled, 12 mid-styled, and
21 short-styled. Of another species he collected 13 flowers, consisting of 3
long-styled, 7 mid-styled, and 3 short-styled. In 1866 Professor Hildebrand
proved by an examination of the specimens in several herbaria that 20 species
are certainly heterostyled and trimorphic, and 51 others almost certainly so.
(4/12. 'Monatsber. der Akad. der Wiss. Berlin' 1866 pages 352, 372. He gives
drawings of the three forms at page 42 of his 'Geschlechter-Vertheilung' etc.
1867.) He also made some interesting observations on living plants belonging to
one form alone; for at that time he did not possess the three forms of any
living species. During the years 1864 to 1868 I occasionally experimented on
Oxalis speciosa, but until now have never found time to publish the results. In
1871 Hildebrand published an admirable paper in which he shows in the case of
two species of Oxalis, that the sexual relations of the three forms are nearly
the same as in Lythrum salicaria. (4/13. 'Botanische Zeitung' 1871 pages 416 and
432.) I will now give an abstract of his observations, and afterwards of my own
less complete ones. I may premise that in all the species seen by me, the
stigmas of the five straight pistils of the long-styled form stand on a level
with the anthers of the longest stamens in the two other forms. In the mid-
styled form, the stigmas pass out between the filaments of the longest stamens
(as in the short-styled form of Linum); and they stand rather nearer to the
upper anthers than to the lower ones. In the short-styled form, the stigmas also
pass out between the filaments nearly on a level with the tips of the sepals.
The anthers in this latter form and in the mid-styled rise to the same height as
the corresponding stigmas in the other two forms.
Oxalis Valdiviana.
This species, an inhabitant of the west coast of South America, bears yellow
flowers. Hildebrand states that the stigmas of the three forms do not differ in
any marked manner, but that the pistil of the short-styled form alone is
destitute of hairs. The diameters of the pollen-grains are as follows:--
Table 4.b. Oxalis Valdiviana. Diameters of pollen-grains in divisions of the
micrometer.
Column 1: Source of Pollen-grains.
Column 2: Minimum diameter.
Column 3: Maximum diameter.
From the:
Longest stamens of short-styled form : 8 to 9.
Mid-length stamens of short-styled form : 7 to 8.
Longest stamens of mid-styled form : 8.
Shortest stamens of mid-styled form : 8.
Mid-length stamens of long-styled form : 7.
Shortest stamens of long-styled form : 6.
Therefore the extreme difference in diameter is as 8.5 to 6, or as 100 to 71.
The results of Hildebrand's experiments are given in Table 4.27, drawn up in
accordance with my usual plan.
Table 4.27. Oxalis Valdiviana (from Hildebrand).
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union :
28 : 28 : 11.9.
Long-styled by pollen of longest stamens of mid-styled. Legitimate union :
21 : 21 : 12.0.
Long-styled by pollen of own and own-form mid-length stamens. Illegitimate union
:
40 : 2 : 5.5.
Long-styled by pollen of own and own-form shortest stamens. Illegitimate union :
26 : 0 : 0.
Long-styled by pollen of shortest stamens of short-styled. Illegitimate union :
16 : 1 : 1.
Long-styled by pollen of shortest stamens of mid-styled. Illegitimate union :
9 : 0 : 0.
Mid-styled by pollen of mid-length stamens of long-styled. Legitimate union :
38 : 38 : 11.3.
Mid-styled by pollen of mid-length stamens of short-styled. Legitimate union :
23 : 23 : 10.4.
Mid-styled by pollen of own and own-form longest stamens. Illegitimate union :
52 : 0 : 0.
Mid-styled by pollen of own and own-form shortest stamens. Illegitimate union :
30 : 1 : 6.
Mid-styled by pollen of shortest stamens of long-styled. Illegitimate union :
16 : 0 : 0.
Mid-styled by pollen of longest stamens of short-styled. Illegitimate union :
16 : 2 : 2.5.
Short-styled by pollen of shortest stamens of long-styled. Legitimate union:
18 : 18 : 11.0.
Short-styled by pollen of shortest stamens of mid-styled. Legitimate union:
10 : 10 : 11.3.
Short-styled by pollen of own and own-form longest stamens. Illegitimate union :
21 : 0 : 0.
Short-styled by pollen of own and own-form mid-length stamens. Illegitimate
union :
22 : 0 : 0.
Short-styled by pollen of longest stamens of mid-styled. Illegitimate union:
4 : 0 : 0.
Short-styled by pollen of mid-length stamens of long-styled. Illegitimate union:
3 : 0 : 0.
We here have the remarkable result that every one of 138 legitimately fertilised
flowers on the three forms yielded capsules, containing on an average 11.33
seeds. Whilst of the 255 illegitimately fertilised flowers, only 6 yielded
capsules, which contained 3.83 seeds on an average. Therefore the fertility of
the six legitimate to that of the twelve illegitimate unions, as judged by the
proportion of flowers that yielded capsules, is as 100 to 2, and as judged by
the average number of seeds per capsule as 100 to 34. It may be added that some
plants which were protected by nets did not spontaneously produce any fruit; nor
did one which was left uncovered by itself and was visited by bees. On the other
hand, scarcely a single flower on some uncovered plants of the three forms
growing near together failed to produce fruit.
Oxalis Regnelli.
This species bears white flowers and inhabits Southern Brazil. Hildebrand says
that the stigma of the long-styled form is somewhat larger than that of the mid-
styled, and this than that of the short-styled. The pistil of the latter is
clothed with a few hairs, whilst it is very hairy in the other two forms. The
diameter of the pollen-grains from both sets of the longest stamens equals 9
divisions of the micrometer,--that from the mid-length stamens of the long-
styled form between 8 and 9, and of the short-styled 8,--and that from the
shortest stamens of both sets 7. So that the extreme difference in diameter is
as 9 to 7 or as 100 to 78. The experiments made by Hildebrand, which are not so
numerous as in the last case, are given in Table 4.28 in the same manner as
before.
TABLE 4.28. Oxalis Regnelli (from Hildebrand).
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Average Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union :
6 : 6 : 10.1.
Long-styled by pollen of longest stamens of mid-styled. Legitimate union :
5 : 5 : 10.6.
Long-styled by pollen of own mid-length stamens. Illegitimate union :
4 : 0 : 0.
Long-styled by pollen of own shortest stamens. Illegitimate union :
1 : 0 : 0.
Mid-styled by pollen of mid-length stamens of short-styled. Legitimate union :
9 : 9 : 10.4.
Mid-styled by pollen of mid-length stamens of long-styled. Legitimate union :
10 : 10 : 10.1.
Mid-styled by pollen of own longest stamens. Illegitimate union :
9 : 0 : 0.
Mid-styled by pollen of own shortest stamens. Illegitimate union :
2 : 0 : 0.
Mid-styled by pollen of longest stamens of short-styled. Illegitimate union :
1 : 0 : 0.
Short-styled by pollen of shortest stamens of mid-styled. Legitimate union:
9 : 9 : 10.6.
Short-styled by pollen of shortest stamens of long-styled. Legitimate union:
2 : 2 : 9.5.
Short-styled by pollen of own mid-length stamens. Illegitimate union :
12 : 0 : 0.
Short-styled by pollen of own longest stamens. Illegitimate union :
9 : 0 : 0.
Short-styled by pollen of mid-length stamens of long-styled. Illegitimate union:
1 : 0 : 0.
The results are nearly the same as in the last case, but more striking; for 41
flowers belonging to the three forms fertilised legitimately all yielded
capsules, containing on an average 10.31 seeds; whilst 39 flowers fertilised
illegitimately did not yield a single capsule or seed. Therefore the fertility
of the six legitimate to that of the several illegitimate unions, as judged both
by the proportion of flowers which yielded capsules and by the average number of
contained seeds, is as 100 to 0.
Oxalis speciosa.
This species, which bears pink flowers, was introduced from the Cape of Good
Hope. A sketch of the reproductive organs of the three forms (Figure 4.11) has
already been given. The stigma of the long-styled form (with the papillae on its
surface included) is twice as large as that of the short-styled, and that of the
mid-styled intermediate in size. The pollen-grains from the stamens in the three
forms are in their longer diameters as follows:--
Table 4.c. Oxalis speciosa. Diameters of pollen-grains in divisions of the
micrometer.
Column 1: Source of Pollen-grains.
Column 2: Minimum diameter.
Column 3: Maximum diameter.
From the:
Longest stamens of short-styled form : 15 to 16.
Mid-length stamens of short-styled form : 12 to 13.
Longest stamens of mid-styled form : 16.
Shortest stamens of mid-styled form : 11 to 12.
Mid-length stamens of long-styled form : 14.
Shortest stamens of long-styled form : 12.
Therefore the extreme difference in diameter is as 16 to 11, or as 100 to 69;
but as the measurements were taken at different times, they are probably only
approximately accurate. The results of my experiments in fertilising the three
forms are given in Table 4.29.
Table 4.29. Oxalis speciosa.
Column 1: Nature of the Union.
Column 2: Number of Flowers fertilised.
Column 3: Number of Capsules produced.
Column 4: Average Number of Seeds per Capsule.
Long-styled by pollen of longest stamens of short-styled. Legitimate union :
19 : 15 : 57.4.
Long-styled by pollen of longest stamens of mid-styled. Legitimate union :
4 : 3 : 59.0.
Long-styled by pollen of own-form mid-length stamens. Illegitimate union :
9 : 2 : 42.5.
Long-styled by pollen of own-form shortest stamens. Illegitimate union :
11 : 0 : 0.
Long-styled by pollen of shortest stamens of mid-styled. Illegitimate union :
4 : 0 : 0.
Long-styled by pollen of mid-length stamens of short-styled. Illegitimate union
:
12 : 5 : 30.0.
Mid-styled by pollen of mid-length stamens of long-styled. Legitimate union :
3 : 3 : 63.6.
Mid-styled by pollen of mid-length stamens of short-styled. Legitimate union :
4 : 4 : 56.3.
Mid-styled by mixed pollen from both own-form longest and shortest stamens.
Illegitimate union :
9 : 2 : 19.
Mid-styled by pollen of longest stamens of short-styled. Illegitimate union :
12 : 1 : 8.
Short-styled by pollen of shortest stamens of mid-styled. Legitimate union:
3 : 2 : 67.
Short-styled by pollen of shortest stamens of long-styled. Legitimate union:
3 : 3 : 54.3.
Short-styled by pollen of own-form longest stamens. Illegitimate union:
5 : 1 : 8.
Short-styled by pollen of own-form mid-length stamens. Illegitimate union :
3 : 0 : 0.
Short-styled by both pollens mixed together, of own-form longest and mid-length
stamens. Illegitimate union:
13 : 0 : 0.
Short-styled by pollen of longest stamens of mid-styled. Illegitimate union :
7 : 0 : 0.
Short-styled by pollen of mid-length stamens of long-styled. Illegitimate union:
10 : 1 : 54.
We here see that thirty-six flowers on the three forms legitimately fertilised
yielded 30 capsules, these containing on an average 58.36 seeds. Ninety-five
flowers illegitimately fertilised yielded 12 capsules, containing on an average
28.58 seeds. Therefore the fertility of the six legitimate to that of the twelve
illegitimate unions, as judged by the proportion of flowers which yielded
capsules, is as 100 to 15, and judged by the average number of seeds per capsule
as 100 to 49. This plant, in comparison with the two South American species
previously described, produces many more seeds, and the illegitimately
fertilised flowers are not quite so sterile.
Oxalis rosea.
Hildebrand possessed in a living state only the long-styled form of this
trimorphic Chilian species. (4/14. 'Monatsber. der Akad. der Wiss. Berlin' 1866
page 372.) The pollen-grains from the two sets of anthers differ in diameter as
9 to 7.5, or as 100 to 83. He has further shown that there is an analogous
difference between the grains from the two sets of anthers of the same flower in
five other species of Oxalis, besides those already described. The present
species differs remarkably from the long-styled form of the three species
previously experimented on, in a much larger proportion of the flowers setting
capsules when fertilised with their own-form pollen. Hildebrand fertilised 60
flowers with pollen from the mid-length stamens (of either the same or another
flower), and they yielded no less than 55 capsules, or 92 per cent. These
capsules contained on an average 5.62 seeds; but we have no means of judging how
near an approach this average makes to that from flowers legitimately
fertilised. He also fertilised 45 flowers with pollen from the shortest stamens,
and these yielded only 17 capsules, or 31 per cent, containing on an average
only 2.65 seeds. We thus see that about thrice as many flowers, when fertilised
with pollen from the mid-length stamens, produced capsules, and these contained
twice as many seeds, as did the flowers fertilised with pollen from the shortest
stamens. It thus appears (and we find some evidence of the same fact with O.
speciosa), that the same rule holds good with Oxalis as with Lythrum salicaria;
namely, that in any two unions, the greater the inequality in length between the
pistils and stamens, or, which is the same thing, the greater the distance of
the stigma from the anthers, the pollen of which is used for fertilisation, the
less fertile is the union,--whether judged by the proportion of flowers which
set capsules, or by the average number of seeds per capsule. The rule cannot be
explained in this case any more than in that of Lythrum, by supposing that
wherever there is greater liability to self-fertilisation, this is checked by
the union being rendered more sterile; for exactly the reverse occurs, the
liability to self-fertilisation being greatest in the unions between the pistils
and stamens which approach each other the nearest, and these are the more
fertile. I may add that I also possessed some long-styled plants of this
species: one was covered by a net, and it set spontaneously a few capsules,
though extremely few compared with those produced by a plant growing by itself,
but exposed to the visits of bees.
With most of the species of Oxalis the short-styled form seems to be the most
sterile of the three forms, when these are illegitimately fertilised; and I will
add two other cases to those already given. I fertilised 29 short-styled flowers
of O. compressa with pollen from their own two sets of stamens (the pollen-
grains of which differ in diameter as 100 and 83), and not one produced a
capsule. I formerly cultivated during several years the short-styled form of a
species purchased under the name of O. Bowii (but I have some doubts whether it
was rightly named), and fertilised many flowers with their own two kinds of
pollen, which differ in diameter in the usual manner, but never got a single
seed. On the other hand, Hildebrand says that the short-styled form of O.
Deppei, growing by itself, yields plenty of seed; but it is not positively known
that this species is heterostyled; and the pollen-grains from the two sets of
anthers do not differ in diameter.
Some facts communicated to me by Fritz Muller afford excellent evidence of the
utter sterility of one of the forms of certain trimorphic species of Oxalis,
when growing isolated. He has seen in St. Catharina, in Brazil, a large field of
young sugar-cane, many acres in extent, covered with the red blossoms of one
form alone, and these did not produce a single seed. His own land is covered
with the short-styled form of a white-flowered trimorphic species, and this is
equally sterile; but when the three forms were planted near together in his
garden they seeded freely. With two other trimorphic species he finds that
isolated plants are always sterile.
Fritz Muller formerly believed that a species of Oxalis, which is so abundant in
St. Catharina that it borders the roads for miles, was dimorphic instead of
trimorphic. Although the pistils and stamens vary greatly in length, as was
evident in some specimens sent to me, yet the plants can be divided into two
sets, according to the lengths of these organs. A large proportion of the
anthers are of a white colour and quite destitute of pollen; others which are
pale yellow contain many bad with some good grains; and others again which are
bright yellow have apparently sound pollen; but he has never succeeded in
finding any fruit on this species. The stamens in some of the flowers are
partially converted into petals. Fritz Muller after reading my description,
hereafter to be given, of the illegitimate offspring of various heterostyled
species, suspects that these plants of Oxalis may be the variable and sterile
offspring of a single form of some trimorphic species, perhaps accidentally
introduced into the district, which has since been propagated asexually. It is
probable that this kind of propagation would be much aided by there being no
expenditure in the production of seed.
Oxalis (Biophytum) sensitiva.
This plant is ranked by many botanists as a distinct genus. Mr. Thwaites sent me
a number of flowers preserved in spirits from Ceylon, and they are clearly
trimorphic. The style of the long-styled form is clothed with many scattered
hairs, both simple and glandular; such hairs are much fewer on the style of the
mid-styled, and quite absent from that of the short-styled form; so that this
plant resembles in this respect O. Valdiviana and Regnelli. Calling the length
of the two lobes of the stigma of the long-styled form 100, that of the mid-
styled is 141, and that of the short-styled 164. In all other cases, in which
the stigma in this genus differs in size in the three forms, the difference is
of a reversed nature, the stigma of the long-styled being the largest, and that
of the short-styled the smallest. The diameter of the pollen-grains from the
longest stamens being represented by 100, those from the mid-length stamens are
91, and those from the shortest stamens 84 in diameter. This plant is
remarkable, as we shall see in the last chapter of this volume, by producing
long-styled, mid-styled, and short-styled cleistogamic flowers.
HOMOSTYLED SPECIES OF OXALIS.
Although the majority of the species in the large genus Oxalis seem to be
trimorphic, some are homostyled, that is, exist under a single form; for
instance the common O. acetosella, and according to Hildebrand two other widely
distributed European species, O. stricta and corniculata. Fritz Muller also
informs me that a similarly constituted species is found in St. Catharina, and
that it is quite fertile with its own pollen when insects are excluded. The
stigmas of O. stricta and of another homostyled species, namely O.
tropaeoloides, commonly stand on a level with the upper anthers, and both these
species are likewise quite fertile when insects are excluded.
With respect to O. acetosella, Hildebrand says that in all the many specimens
examined by him the pistil exceeded the longer stamens in length. I procured 108
flowers from the same number of plants growing in three distant parts of
England; of these 86 had their stigmas projecting considerably above, whilst 22
had them nearly on a level with the upper anthers. In one lot of 17 flowers from
the same wood, the stigmas in every flower projected fully as much above the
upper anthers as these stood above the lower anthers. So that these plants might
fairly be compared with the long-styled form of a heterostyled species; and I at
first thought that O. acetosella was trimorphic. But the case is one merely of
great variability. The pollen-grains from the two sets of anthers, as observed
by Hildebrand and myself, do not differ in diameter. I fertilised twelve flowers
on several plants with pollen from a distinct plant, choosing those with pistils
of a different length; and 10 of these (i.e. 83 per cent) produced capsules,
which contained on an average 7.9 seeds. Fourteen flowers were fertilised with
their own pollen, and 11 of these (i.e. 79 per cent) yielded capsules,
containing a larger average of seed, namely 9.2. These plants, therefore, in
function show not the least sign of being heterostyled. I may add that 18
flowers protected by a net were left to fertilise themselves, and only 10 of
these (i.e. 55 per cent) yielded capsules, which contained on an average only
6.3 seeds. So that the access of insects, or artificial aid in placing pollen on
the stigma, increases the fertility of the flowers; and I found that this
applied especially to those having shorter pistils. It should be remembered that
the flowers hang downwards, so that those with short pistils would be the least
likely to receive their own pollen, unless they were aided in some manner.
Finally, as Hildebrand has remarked, there is no evidence that any of the
heterostyled species of Oxalis are tending towards a dioecious condition, as
Zuccarini and Lindley inferred from the differences in the reproductive organs
of the three forms, the meaning of which they did not understand.
PONTEDERIA [SP.?] (PONTEDERIACEAE).
Fritz Muller found this aquatic plant, which is allied to the Liliaceae, growing
in the greatest profusion on the banks of a river in Southern Brazil. (4/15.
"Ueber den Trimorphismus der Pontederien" 'Jenaische Zeitschrift' etc. Band 6
1871 page 74.) But only two forms were found, the flowers of which include three
long and three short stamens. The pistil of the long-styled form, in two dried
flowers which were sent me, was in length as 100 to 32, and its stigma as 100 to
80, compared with the same organs in the short-styled form. The long-styled
stigma projects considerably above the upper anthers of the same flower, and
stands on a level with the upper ones of the short-styled form. In the latter
the stigma is seated beneath both its own sets of anthers, and is on a level
with the anthers of the shorter stamens in the long-styled form. The anthers of
the longer stamens of the short-styled form are to those of the shorter stamens
of the long-styled form as 100 to 88 in length. The pollen-grains distended with
water from the longer stamens of the short-styled form are to those from the
shorter stamens of the same form as 100 to 87 in diameter, as deduced from ten
measurements of each kind. We thus see that the organs in these two forms differ
from one another and are arranged in an analogous manner, as in the long and
short-styled forms of the trimorphic species of Lythrum and Oxalis. Moreover,
the longer stamens of the long-styled form of Pontederia, and the shorter ones
of the short-styled form are placed in a proper position for fertilising the
stigma of a mid-styled form. But Fritz Muller, although he examined a vast
number of plants, could never find one belonging to the mid-styled form. The
older flowers of the long-styled and short-styled plants had set plenty of
apparently good fruit; and this might have been expected, as they could
legitimately fertilise one another. Although he could not find the mid-styled
form of this species, he possessed plants of another species growing in his
garden, and all these were mid-styled; and in this case the pollen-grains from
the anthers of the longer stamens were to those from the shorter stamens of the
same flower as 100 to 86 in diameter, as deduced from ten measurements of each
kind. These mid-styled plants growing by themselves never produced a single
fruit.
Considering these several facts, there can hardly be a doubt that both these
species of Pontederia are heterostyled and trimorphic. This case is an
interesting one, for no other Monocotyledonous plant is known to be
heterostyled. Moreover, the flowers are irregular, and all other heterostyled
plants have almost symmetrical flowers. The two forms differ somewhat in the
colour of their corollas, that of the short-styled being of a darker blue,
whilst that of the long-styled tends towards violet, and no other such case is
known. Lastly, the three longer stamens alternate with the three shorter ones,
whereas in Lythrum and Oxalis the long and short stamens belong to distinct
whorls. With respect to the absence of the mid-styled form in the case of the
Pontederia which grows wild in Southern Brazil, this would probably follow if
only two forms had been originally introduced there; for, as we shall hereafter
see from the observations of Hildebrand, Fritz Muller and myself, when one form
of Oxalis is fertilised exclusively by either of the other two forms, the
offspring generally belong to the two parent-forms.
Fritz Muller has recently discovered, as he informs me, a third species of
Pontederia, with all three forms growing together in pools in the interior of S.
Brazil; so that no shadow of doubt can any longer remain about this genus
including trimorphic species. He sent me dried flowers of all three forms. In
the long-styled form the stigma stands a little above the tips of the petals,
and on a level with the anthers of the longest stamens in the other two forms.
The pistil is in length to that of the mid-styled as 100 to 56, and to that of
the short-styled as 100 to 16. Its summit is rectangularly bent upwards, and the
stigma is rather broader than that of the mid-styled, and broader in about the
ratio of 7 to 4 than that of the short-styled. In the mid-styled form, the
stigma is placed rather above the middle of the corolla, and nearly on a level
with the mid-length stamens in the other two forms; its summit is a little bent
upwards. In the short-styled form the pistil is, as we have seen, very short,
and differs from that in the other two forms in being straight. It stands rather
beneath the level of the anthers of the shortest stamens in the long-styled and
mid-styled forms. The three anthers of each set of stamens, more especially
those of the shortest stamens, are placed one beneath the other, and the ends of
the filaments are bowed a little upwards, so that the pollen from all the
anthers would be effectively brushed off by the proboscis of a visiting insect.
The relative diameters of the pollen-grains, after having been long soaked in
water, are given in Table 4.d, as measured by my son Francis.
TABLE 4.d. Pontederia. Diameters of pollen-grains, after having been long soaked
in water, in divisions of the micrometer.
Column 1: Source of Pollen-grains.
Column 2: diameter.
Long-styled form, mid-length stamens (Average of 20 measurements): 13.2.
Long-styled form, shortest stamens (10 measurements): 9.0.
Mid-styled form, longest stamens (15 measurements) : 16.4.
Mid-styled form, shortest stamens (20 measurements): 9.1.
Short-styled form, longest stamens (20 measurements): 14.6.
Short-styled form, mid-length stamens (20 measurements): 12.3.
We have here the usual rule of the grains from the longer stamens, the tubes of
which have to penetrate the longer pistil, being larger than those from the
stamens of less length. The extreme difference in diameter between the grains
from the longest stamens of the mid-styled form, and from the shortest stamens
of the long-styled, is as 16.4 to 9.0, or as 100 to 55; and this is the greatest
difference observed by me in any heterostyled plant. It is a singular fact that
the grains from the corresponding longest stamens in the two forms differ
considerably in diameter; as do those in a lesser degree from the corresponding
mid-length stamens in the two forms; whilst those from the corresponding
shortest stamens in the long- and mid-styled forms are almost exactly equal.
Their inequality in the two first cases depends on the grains in both sets of
anthers in the short-styled form being smaller than those from the corresponding
anthers in the other two forms; and here we have a case parallel with that of
the mid-styled form of Lythrum salicaria. In this latter plant the pollen-grains
of the mid-styled forms are of smaller size and have less fertilising power than
the corresponding ones in the other two forms; whilst the ovarium, however
fertilised, yields a greater number of seeds; so that the mid-styled form is
altogether more feminine in nature than the other two forms. In the case of
Pontederia, the ovarium includes only a single ovule, and what the meaning of
the difference in size between the pollen-grains from the corresponding sets of
anthers may be, I will not pretend to conjecture.
The clear evidence that the species just described is heterostyled and
trimorphic is the more valuable as there is some doubt with respect to P.
cordata, an inhabitant of the United States. Mr. Leggett suspects that it is
either dimorphic or trimorphic, for the pollen-grains of the longer stamens are
"more than twice the diameter or than eight times the mass of the grains of the
shorter stamens. Though minute, these smaller grains seem as perfect as the
larger ones." (4/16. 'Bulletin of the Torrey Botanical Club' 1875 volume 6 page
62.) On the other hand, he says that in all the mature flowers, "the style was
as long at least as the longer stamens;" "whilst in the young flowers it was
intermediate in length between the two sets of stamens;" and if this be so, the
species can hardly be heterostyled.
CHAPTER V.
ILLEGITIMATE OFFSPRING OF HETEROSTYLED PLANTS.
Illegitimate offspring from all three forms of Lythrum salicaria.
Their dwarfed stature and sterility, some utterly barren, some fertile.
Oxalis, transmission of form to the legitimate and illegitimate seedlings.
Primula Sinensis, Illegitimate offspring in some degree dwarfed and infertile.
Equal-styled varieties of P. Sinensis, auricula, farinosa, and elatior.
P. vulgaris, red-flowered variety, Illegitimate seedlings sterile.
P. veris, Illegitimate plants raised during several successive generations,
their dwarfed stature and sterility.
Equal-styled varieties of P. veris.
Transmission of form by Pulmonaria and Polygonum.
Concluding remarks.
Close parallelism between illegitimate fertilisation and hybridism.
We have hitherto treated of the fertility of the flowers of heterostyled plants,
when legitimately and illegitimately fertilised. The present chapter will be
devoted to the character of their offspring or seedlings. Those raised from
legitimately fertilised seeds will be here called LEGITIMATE SEEDLINGS or
PLANTS, and those from illegitimately fertilised seeds, ILLEGITIMATE SEEDLINGS
or PLANTS. They differ chiefly in their degree of fertility, and in their powers
of growth or vigour. I will begin with trimorphic plants, and I must remind the
reader that each of the three forms can be fertilised in six different ways; so
that all three together can be fertilised in eighteen different ways. For
instance, a long-styled form can be fertilised legitimately by the longest
stamens of the mid-styled and short-styled forms, and illegitimately by its own-
form and mid-length and shortest stamens, also by the mid-length stamens of the
mid-styled and by the shortest stamens of the short-styled form; so that the
long-styled can be fertilised legitimately in two ways and illegitimately in
four ways. The same holds good with respect to the mid-styled and short-styled
forms. Therefore with trimorphic species six of the eighteen unions yield
legitimate offspring, and twelve yield illegitimate offspring.
I will give the results of my experiments in detail, partly because the
observations are extremely troublesome, and will not probably soon be repeated--
thus, I was compelled to count under the microscope above 20,000 seeds of
Lythrum salicaria--but chiefly because light is thus indirectly thrown on the
important subject of hybridism.
Lythrum salicaria.
Of the twelve illegitimate unions two were completely barren, so that no seeds
were obtained, and of course no seedlings could be raised. Seedlings were,
however, raised from seven of the ten remaining illegitimate unions. Such
illegitimate seedlings when in flower were generally allowed to be freely and
legitimately fertilised, through the agency of bees, by other illegitimate
plants belonging to the two other forms growing close by. This is the fairest
plan, and was usually followed; but in several cases (which will always be
stated) illegitimate plants were fertilised with pollen taken from legitimate
plants belonging to the other two forms; and this, as might have been expected,
increased their fertility. Lythrum salicaria is much affected in its fertility
by the nature of the season; and to avoid error from this source, as far as
possible, my observations were continued during several years. Some few
experiments were tried in 1863. The summer of 1864 was too hot and dry, and,
though the plants were copiously watered, some few apparently suffered in their
fertility, whilst others were not in the least affected. The years 1865 and,
especially, 1866, were highly favourable. Only a few observations were made
during 1867. The results are arranged in classes according to the parentage of
the plants. In each case the average number of seeds per capsule is given,
generally taken from ten capsules, which, according to my experience, is a
nearly sufficient number. The maximum number of seeds in any one capsule is also
given; and this is a useful point of comparison with the normal standard--that
is, with the number of seeds produced by legitimate plants legitimately
fertilised. I will give likewise in each case the minimum number. When the
maximum and minimum differ greatly, if no remark is made on the subject, it may
be understood that the extremes are so closely connected by intermediate figures
that the average is a fair one. Large capsules were always selected for
counting, in order to avoid over-estimating the infertility of the several
illegitimate plants.
In order to judge of the degree of inferiority in fertility of the several
illegitimate plants, the following statement of the average and of the maximum
number of seeds produced by ordinary or legitimate plants, when legitimately
fertilised, some artificially and some naturally, will serve as a standard of
comparison, and may in each case be referred to. But I give under each
experiment the percentage of seeds produced by the illegitimate plants, in
comparison with the standard legitimate number of the same form. For instance,
ten capsules from the illegitimate long-styled plant (Number 10), which was
legitimately and naturally fertilised by other illegitimate plants, contained on
an average 44.2 seeds; whereas the capsules on legitimate long-styled plants,
legitimately and naturally fertilised by other legitimate plants, contained on
an average 93 seeds. Therefore this illegitimate plant yielded only 47 per cent
of the full and normal complement of seeds.
STANDARD NUMBER OF SEEDS PRODUCED BY LEGITIMATE PLANTS OF THE THREE FORMS, WHEN
LEGITIMATELY FERTILISED.
Long-styled form:
Average number of seeds in each capsule, 93;
Maximum number observed out of twenty-three capsules, 159.
Mid-styled form:
Average number of seeds, 130;
Maximum number observed out of thirty-one capsules, 151.
Short-styled form:
Average number of seeds, 83.5; but we may, for the sake of brevity, say 83;
Maximum number observed out of twenty-five capsules, 112.
CLASSES 1 AND 2. ILLEGITIMATE PLANTS RAISED FROM LONG-STYLED PARENTS FERTILISED
WITH POLLEN FROM THE MID-LENGTH OR THE SHORTEST STAMENS OF OTHER PLANTS OF THE
SAME FORM.
From this union I raised at different times three lots of illegitimate
seedlings, amounting altogether to 56 plants. I must premise that, from not
foreseeing the result, I did not keep a memorandum whether the eight plants of
the first lot were the product of the mid-length or shortest stamens of the same
form; but I have good reason to believe that they were the product of the
latter. These eight plants were much more dwarfed, and much more sterile than
those in the other two lots. The latter were raised from a long-styled plant
growing quite isolated, and fertilised by the agency of bees with its own
pollen; and it is almost certain, from the relative position of the organs of
fructification, that the stigma under these circumstances would receive pollen
from the mid-length stamens.
All the fifty-six plants in these three lots proved long-styled; now, if the
parent-plants had been legitimately fertilised by pollen from the longest
stamens of the mid-styled and short-styled forms, only about one-third of the
seedlings would have been long-styled, the other two-thirds being mid-styled and
short-styled. In some other trimorphic and dimorphic genera we shall find the
same curious fact, namely, that the long-styled form, fertilised illegitimately
by its own-form pollen, produces almost exclusively long-styled seedlings. (5/1.
Hildebrand first called attention to this fact in the case of Primula Sinensis
('Botanische Zeitung' January 1, 1864 page 5); but his results were not nearly
so uniform as mine.)
The eight plants of the first lot were of low stature: three which I measured
attained, when fully grown, the heights of only 28, 29, and 47 inches; whilst
legitimate plants growing close by were double this height, one being 77 inches.
They all betrayed in their general appearance a weak constitution; they flowered
rather later in the season, and at a later age than ordinary plants. Some did
not flower every year; and one plant, behaving in an unprecedented manner, did
not flower until three years old. In the two other lots none of the plants grew
quite to their full and proper height, as could at once be seen by comparing
them with the adjoining rows of legitimate plants. In several plants in all
three lots, many of the anthers were either shrivelled or contained brown and
tough, or pulpy matter, without any good pollen-grains, and they never shed
their contents; they were in the state designated by Gartner as contabescent,
which term I will for the future use. (5/2. 'Beitrage zur Kenntniss der
Befruchtung' 1844 page 116.) In one flower all the anthers were contabescent
excepting two which appeared to the naked eye sound; but under the microscope
about two-thirds of the pollen-grains were seen to be small and shrivelled. In
another plant, in which all the anthers appeared sound, many of the pollen-
grains were shrivelled and of unequal sizes. I counted the seeds produced by
seven plants (1 to 7) in the first lot of eight plants, probably the product of
parents fertilised by their own-form shortest stamens, and the seeds produced by
three plants in the other two lots, almost certainly the product of parents
fertilised by their own-form mid-length stamens.
[PLANT 1.
This long-styled plant was allowed during 1863 to be freely and legitimately
fertilised by an adjoining illegitimate mid-styled plant, but it did not yield a
single seed-capsule. It was then removed and planted in a remote place close to
a brother long-styled plant Number 2, so that it must have been freely though
illegitimately fertilised; under these circumstances it did not yield during
1864 and 1865 a single capsule. I should here state that a legitimate or
ordinary long-styled plant, when growing isolated, and freely though
illegitimately fertilised by insects with its own pollen, yielded an immense
number of capsules, which contained on an average 21.5 seeds.
PLANT 2.
This long-styled plant, after flowering during 1863 close to an illegitimate
mid-styled plant, produced less than twenty capsules, which contained on an
average between four and five seeds. When subsequently growing in company with
Number 1, by which it will have been illegitimately fertilised, it yielded in
1866 not a single capsule, but in 1865 it yielded twenty-two capsules: the best
of these, fifteen in number, were examined; eight contained no seed, and the
remaining seven contained on an average only three seeds, and these seeds were
so small and shrivelled that I doubt whether they would have germinated.
PLANTS 3 AND 4.
These two long-styled plants, after being freely and legitimately fertilised
during 1863 by the same illegitimate mid-styled plant as in the last case, were
as miserably sterile as Number 2.
PLANT 5.
This long-styled plant, after flowering in 1863 close to an illegitimate mid-
styled plant, yielded only four capsules, which altogether included only five
seeds. During 1864, 1865, and 1866, it was surrounded either by illegitimate or
legitimate plants of the other two forms; but it did not yield a single capsule.
It was a superfluous experiment, but I likewise artificially fertilised in a
legitimate manner twelve flowers; but not one of these produced a capsule; so
that this plant was almost absolutely barren.
PLANT 6.
This long-styled plant, after flowering during the favourable year of 1866,
surrounded by illegitimate plants of the other two forms, did not produce a
single capsule.
PLANT 7.
This long-styled plant was the most fertile of the eight plants of the first
lot. During 1865 it was surrounded by illegitimate plants of various parentage,
many of which were highly fertile, and must thus have been legitimately
fertilised. It produced a good many capsules, ten of which yielded an average of
36.1 seeds, with a maximum of 47 and a minimum of 22; so that this plant
produced 39 per cent of the full number of seeds. During 1864 it was surrounded
by legitimate and illegitimate plants of the other two forms; and nine capsules
(one poor one being rejected) yielded an average of 41.9 seeds, with a maximum
of 56 and a minimum of 28; so that, under these favourable circumstances, this
plant, the most fertile of the first lot, did not yield, when legitimately
fertilised, quite 45 per cent of the full complement of seeds.]
In the second lot of plants in the present class, descended from the long-styled
form, almost certainly fertilised with pollen from its own mid-length stamens,
the plants, as already stated, were not nearly so dwarfed or so sterile as in
the first lot. All produced plenty of capsules. I counted the number of seeds in
only three plants, namely Numbers 8, 9, and 10.
[PLANT 8.
This plant was allowed to be freely fertilised in 1864 by legitimate and
illegitimate plants of the other two forms, and ten capsules yielded on an
average 41.1 seeds, with a maximum of 73 and a minimum of 11. Hence this plant
produced only 44 per cent of the full complement of seeds.
PLANT 9.
This long-styled plant was allowed in 1865 to be freely fertilised by
illegitimate plants of the other two forms, most of which were moderately
fertile. Fifteen capsules yielded on an average 57.1 seeds, with a maximum of 86
and a minimum of 23. Hence the plant yielded 61 per cent of the full complement
of seeds.
PLANT 10.
This long-styled plant was freely fertilised at the same time and in the same
manner as the last. Ten capsules yielded an average of 44.2 seeds, with a
maximum of 69 and a minimum of 25; hence this plant yielded 47 per cent of the
full complement of seeds.]
The nineteen long-styled plants of the third lot, of the same parentage as the
last lot, were treated differently; for they flowered during 1867 by themselves
so that they must have been illegitimately fertilised by one another. It has
already been stated that a legitimate long-styled plant, growing by itself and
visited by insects, yielded an average of 21.5 seeds per capsule, with a maximum
of 35; but, to judge fairly of its fertility, it ought to have been observed
during successive seasons. We may also infer from analogy that, if several
legitimate long-styled plants were to fertilise one another, the average number
of seeds would be increased; but how much increased I do not know; hence I have
no perfectly fair standard of comparison by which to judge of the fertility of
the three following plants of the present lot, the seeds of which I counted.
[PLANT 11.
This long-styled plant produced a large crop of capsules, and in this respect
was one of the most fertile of the whole lot of nineteen plants. But the average
from ten capsules was only 35.9 seeds, with a maximum of 60 and a minimum of 8.
PLANT 12.
This long-styled plant produced very few capsules; and ten yielded an average of
only 15.4 seeds, with a maximum of 30 and a minimum of 4.
PLANT 13.
This plant offers an anomalous case; it flowered profusely, yet produced very
few capsules; but these contained numerous seeds. Ten capsules yielded an
average of 71.9 seeds, with a maximum of 95 and a minimum of 29. Considering
that this plant was illegitimate and illegitimately fertilised by its brother
long-styled seedlings, the average and the maximum are so remarkably high that I
cannot at all understand the case. We should remember that the average for a
legitimate plant legitimately fertilised is 93 seeds.]
CLASS 3. ILLEGITIMATE PLANTS RAISED FROM A SHORT-STYLED PARENT FERTILISED WITH
POLLEN FROM OWN-FORM MID-LENGTH STAMENS.
I raised from this union nine plants, of which eight were short-styled and one
long-styled; so that there seems to be a strong tendency in this form to
reproduce, when self-fertilised, the parent-form; but the tendency is not so
strong as with the long-styled. These nine plants never attained the full height
of legitimate plants growing close to them. The anthers were contabescent in
many of the flowers on several plants.
[PLANT 14.
This short-styled plant was allowed during 1865 to be freely and legitimately
fertilised by illegitimate plants descended from self-fertilised mid-, long- and
short-styled plants. Fifteen capsules yielded an average of 28.3 seeds, with a
maximum of 51 and a minimum of 11; hence this plant produced only 33 per cent of
the proper number of seeds. The seeds themselves were small and irregular in
shape. Although so sterile on the female side, none of the anthers were
contabescent.
PLANT 15.
This short-styled plant, treated like the last during the same year, yielded an
average, from fifteen capsules, of 27 seeds, with a maximum of 49 and a minimum
of 7. But two poor capsules may be rejected, and then the average rises to 32.6,
with the same maximum of 49 and a minimum of 20; so that this plant attained 38
per cent of the normal standard of fertility, and was rather more fertile than
the last, yet many of the anthers were contabescent.
PLANT 16.
This short-styled plant, treated like the two last, yielded from ten capsules an
average of 77.8 seeds, with a maximum of 97 and a minimum of 60; so that this
plant produced 94 per cent of the full number of seeds.
PLANT 17.
This, the one long-styled plant of the same parentage as the last three plants,
when freely and legitimately fertilised in the same manner as the last, yielded
an average from ten capsules of 76.3 rather poor seeds, with a maximum of 88 and
a minimum of 57. Hence this plant produced 82 per cent of the proper number of
seeds. Twelve flowers enclosed in a net were artificially and legitimately
fertilised with pollen from a legitimate short-styled plant; and nine capsules
yielded an average of 82.5 seeds, with a maximum of 98 and a minimum of 51; so
that its fertility was increased by the action of pollen from a legitimate
plant, but still did not reach the normal standard.]
CLASS 4. ILLEGITIMATE PLANTS RAISED FROM A MID-STYLED PARENT FERTILISED WITH
POLLEN FROM OWN-FORM LONGEST STAMENS.
After two trials, I succeeded in raising only four plants from this illegitimate
union. These proved to be three mid-styled and one long-styled; but from so
small a number we can hardly judge of the tendency in mid-styled plants when
self-fertilised to reproduce the same form. These four plants never attained
their full and normal height; the long-styled plant had several of its anthers
contabescent.
[PLANT 18.
This mid-styled plant, when freely and legitimately fertilised during 1865 by
illegitimate plants descended from self-fertilised long-, short-, and mid-styled
plants, yielded an average from ten capsules of 102.6 seeds, with a maximum of
131 and a minimum of 63: hence this plant did not produce quite 80 per cent of
the normal number of seeds. Twelve flowers were artificially and legitimately
fertilised with pollen from a legitimate long-styled plant, and yielded from
nine capsules an average of 116.1 seeds, which were finer than in the previous
case, with a maximum of 135 and a minimum of 75; so that, as with Plant 17,
pollen from a legitimate plant increased the fertility, but did not bring it up
to the full standard.
PLANT 19.
This mid-styled plant, fertilised in the same manner and at the same period as
the last, yielded an average from ten capsules of 73.4 seeds, with a maximum of
87 and a minimum of 64: hence this plant produced only 56 per cent of the full
number of seeds. Thirteen flowers were artificially and legitimately fertilised
with pollen from a legitimate long-styled plant, and yielded ten capsules with
an average of 95.6 seeds; so that the application of pollen from a legitimate
plant added, as in the two previous cases, to the fertility, but did not bring
it up to the proper standard.
PLANT 20.
This long-styled plant, of the same parentage with the two last mid-styled
plants, and freely fertilised in the same manner, yielded an average from ten
capsules of 69.6 seeds, with a maximum of 83 and a minimum of 52: hence this
plant produced 75 per cent of the full number of seeds.]
CLASS 5. ILLEGITIMATE PLANTS RAISED FROM A SHORT-STYLED PARENT FERTILISED WITH
POLLEN FROM THE MID-LENGTH STAMENS OF THE LONG-STYLED FORM.
In the four previous classes, plants raised from the three forms fertilised with
pollen from either the longer or shorter stamens of the same form, but generally
not from the same plant, have been described. Six other illegitimate unions are
possible, namely, between the three forms and the stamens in the other two forms
which do not correspond in height with their pistils. But I succeeded in raising
plants from only three of these six unions. From one of them, forming the
present Class 5, twelve plants were raised; these consisted of eight short-
styled, and four long-styled plants, with not one mid-styled. These twelve
plants never attained quite their full and proper height, but by no means
deserved to be called dwarfs. The anthers in some of the flowers were
contabescent. One plant was remarkable from all the longer stamens in every
flower and from many of the shorter ones having their anthers in this condition.
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