The Different Forms of Flowers on Plants of the Same Species
by
Charles Darwin
Part 5 out of 6
we may assume that this was the case with our supposed varying plant; but that
it would have been beneficial to it to have been more regularly cross-
fertilised. We should bear in mind how important an advantage it has been proved
to be to many plants, though in different degrees and ways, to be cross-
fertilised. It might well happen that our supposed species did not vary in
function in the right manner, so as to become either dichogamous or completely
self-sterile, or in structure so as to ensure cross-fertilisation. If it had
thus varied, it would never have been rendered heterostyled, as this state would
then have been superfluous. But the parent-species of our several existing
heterostyled plants may have been, and probably were (judging from their present
constitution) in some degree self-sterile; and this would have made regular
cross-fertilisation still more desirable.
Now let us take a highly varying species with most or all of the anthers
exserted in some individuals, and in others seated low down in the corolla; with
the stigma also varying in position in like manner. Insects which visited such
flowers would have different parts of their bodies dusted with pollen, and it
would be a mere chance whether this were left on the stigma of the next flower
which was visited. If all the anthers could have been placed on the same level
in all the plants, then abundant pollen would have adhered to the same part of
the body of the insects which frequented the flowers, and would afterwards have
been deposited without loss on the stigma, if it likewise stood on the same
unvarying level in all the flowers. But as the stamens and pistils are supposed
to have already varied much in length and to be still varying, it might well
happen that they could be reduced much more easily through natural selection
into two sets of different lengths in different individuals, than all to the
same length and level in all the individuals. We know from innumerable
instances, in which the two sexes and the young of the same species differ, that
there is no difficulty in two or more sets of individuals being formed which
inherit different characters. In our particular case the law of compensation or
balancement (which is admitted by many botanists) would tend to cause the pistil
to be reduced in those individuals in which the stamens were greatly developed,
and to be increased in length in those which had their stamens but little
developed.
Now if in our varying species the longer stamens were to be nearly equalised in
length in a considerable body of individuals, with the pistil more or less
reduced; and in another body, the shorter stamens to be similarly equalised,
with the pistil more or less increased in length, cross-fertilisation would be
secured with little loss of pollen; and this change would be so highly
beneficial to the species, that there is no difficulty in believing that it
could be effected through natural selection. Our plant would then make a close
approach in structure to a heterostyled dimorphic species; or to a trimorphic
species, if the stamens were reduced to two lengths in the same flower in
correspondence with that of the pistils in the other two forms. But we have not
as yet even touched on the chief difficulty in understanding how heterostyled
species could have originated. A completely self-sterile plant or a dichogamous
one can fertilise and be fertilised by any other individual of the same species;
whereas the essential character of a heterostyled plant is that an individual of
one form cannot fully fertilise or be fertilised by an individual of the same
form, but only by one belonging to another form.
H. Muller has suggested that ordinary or homostyled plants may have been
rendered heterostyled merely through the effects of habit. (6/5. 'Die
Befruchtung der Blumen' page 352.) Whenever pollen from one set of anthers is
habitually applied to a pistil of particular length in a varying species, he
believes that at last the possibility of fertilisation in any other manner will
be nearly or completely lost. He was led to this view by observing that Diptera
frequently carried pollen from the long-styled flowers of Hottonia to the stigma
of the same form, and that this illegitimate union was not nearly so sterile as
the corresponding union in other heterostyled species. But this conclusion is
directly opposed by some other cases, for instance by that of Linum
grandiflorum; for here the long-styled form is utterly barren with its own-form
pollen, although from the position of the anthers this pollen is invariably
applied to the stigma. It is obvious that with heterostyled dimorphic plants the
two female and the two male organs differ in power; for if the same kind of
pollen be placed on the stigmas of the two forms, and again if the two kinds of
pollen be placed on the stigmas of the same form, the results are in each case
widely different. Nor can we see how this differentiation of the two female and
two male organs could have been effected merely through each kind of pollen
being habitually placed on one of the two stigmas.
Another view seems at first sight probable, namely, that an incapacity to be
fertilised in certain ways has been specially acquired by heterostyled plants.
We may suppose that our varying species was somewhat sterile (as is often the
case) with pollen from its own stamens, whether these were long or short; and
that such sterility was transferred to all the individuals with pistils and
stamens of the same length, so that these became incapable of intercrossing
freely; but that such sterility was eliminated in the case of the individuals
which differed in the length of their pistils and stamens. It is, however,
incredible that so peculiar a form of mutual infertility should have been
specially acquired unless it were highly beneficial to the species; and although
it may be beneficial to an individual plant to be sterile with its own pollen,
cross-fertilisation being thus ensured, how can it be any advantage to a plant
to be sterile with half its brethren, that is, with all the individuals
belonging to the same form? Moreover, if the sterility of the unions between
plants of the same form had been a special acquirement, we might have expected
that the long-styled form fertilised by the long-styled would have been sterile
in the same degree as the short-styled fertilised by the short-styled; but this
is hardly ever the case. On the contrary, there is sometimes the widest
difference in this respect, as between the two illegitimate unions of Pulmonaria
angustifolia and of Hottonia palustris.
It is a more probable view that the male and female organs in two sets of
individuals have been by some means specially adapted for reciprocal action; and
that the sterility between the individuals of the same set or form is an
incidental and purposeless result. The meaning of the term "incidental" may be
illustrated by the greater or less difficulty in grafting or budding together
two plants belonging to distinct species; for as this capacity is quite
immaterial to the welfare of either, it cannot have been specially acquired, and
must be the incidental result of differences in their vegetative systems. But
how the sexual elements of heterostyled plants came to differ from what they
were whilst the species was homostyled, and how they became co-adapted in two
sets of individuals, are very obscure points. We know that in the two forms of
our existing heterostyled plants the pistil always differs, and the stamens
generally differ in length; so does the stigma in structure, the anthers in
size, and the pollen-grains in diameter. It appears, therefore, at first sight
probable that organs which differ in such important respects could act on one
another only in some manner for which they had been specially adapted. The
probability of this view is supported by the curious rule that the greater the
difference in length between the pistils and stamens of the trimorphic species
of Lythrum and Oxalis, the products of which are united for reproduction, by so
much the greater is the infertility of the union. The same rule applies to the
two illegitimate unions of some dimorphic species, namely, Primula vulgaris and
Pulmonaria angustifolia; but it entirely fails in other cases, as with Hottonia
palustris and Linum grandiflorum. We shall, however, best perceive the
difficulty of understanding the nature and origin of the co-adaptation between
the reproductive organs of the two forms of heterostyled plants, by considering
the case of Linum grandiflorum: the two forms of this plant differ exclusively,
as far as we can see, in the length of their pistils; in the long-styled form,
the stamens equal the pistil in length, but their pollen has no more effect on
it than so much inorganic dust; whilst this pollen fully fertilises the short
pistil of the other form. Now, it is scarcely credible that a mere difference in
the length of the pistil can make a wide difference in its capacity for being
fertilised. We can believe this the less because with some plants, for instance,
Amsinckia spectabilis, the pistil varies greatly in length without affecting the
fertility of the individuals which are intercrossed. So again I observed that
the same plants of Primula veris and vulgaris differed to an extraordinary
degree in the length of their pistils during successive seasons; nevertheless
they yielded during these seasons exactly the same average number of seeds when
left to fertilise themselves spontaneously under a net.
We must therefore look to the appearance of inner or hidden constitutional
differences between the individuals of a varying species, of such a nature that
the male element of one set is enabled to act efficiently only on the female
element of another set. We need not doubt about the possibility of variations in
the constitution of the reproductive system of a plant, for we know that some
species vary so as to be completely self-sterile or completely self-fertile,
either in an apparently spontaneous manner or from slightly changed conditions
of life. Gartner also has shown that the individual plants of the same species
vary in their sexual powers in such a manner that one will unite with a distinct
species much more readily than another. (6/6. Gartner 'Bastarderzeugung im
Pflanzenreich' 1849 page 165.) But what the nature of the inner constitutional
differences may be between the sets or forms of the same varying species, or
between distinct species, is quite unknown. It seems therefore probable that the
species which have become heterostyled at first varied so that two or three sets
of individuals were formed differing in the length of their pistils and stamens
and in other co-adapted characters, and that almost simultaneously their
reproductive powers became modified in such a manner that the sexual elements in
one set were adapted to act on the sexual elements of another set; and
consequently that these elements in the same set or form incidentally became
ill-adapted for mutual interaction, as in the case of distinct species. I have
elsewhere shown that the sterility of species when first crossed and of their
hybrid offspring must also be looked at as merely an incidental result,
following from the special co-adaptation of the sexual elements of the same
species. (6/7. 'Origin of Species' 6th edition page 247; 'Variation of Animals
and Plants under Domestication' 2nd edition volume 2 page 169; 'The Effects of
Cross and Self-fertilisation' page 463. It may be well here to remark that,
judging from the remarkable power with which abruptly changed conditions of life
act on the reproductive system of most organisms, it is probable that the close
adaptation of the male to the female elements in the two forms of the same
heterostyled species, or in all the individuals of the same ordinary species,
could be acquired only under long-continued nearly uniform conditions of life.)
We can thus understand the striking parallelism, which has been shown to exist
between the effects of illegitimately uniting heterostyled plants and of
crossing distinct species. The great difference in the degree of sterility
between the various heterostyled species when illegitimately fertilised, and
between the two forms of the same species when similarly fertilised, harmonises
well with the view that the result is an incidental one which follows from
changes gradually effected in their reproductive systems, in order that the
sexual elements of the distinct forms should act perfectly on one another.
TRANSMISSION OF THE TWO FORMS BY HETEROSTYLED PLANTS.
The transmission of the two forms by heterostyled plants, with respect to which
many facts were given in the last chapter, may perhaps be found hereafter to
throw some light on their manner of development. Hildebrand observed that
seedlings from the long-styled form of Primula Sinensis when fertilised with
pollen from the same form were mostly long-styled, and many analogous cases have
since been observed by me. All the known cases are given in Tables 6.36 and
6.37.
TABLE 6.36. Nature of the offspring from illegitimately fertilised dimorphic
plants.
Column 1: Species and form.
Column 2: Number of long-styled offspring.
Column 3: Number of short-styled offspring.
Primula veris. Long-styled form, fertilised by own-form pollen during five
successive generations : 156 : 6.
Primula veris. Short-styled form, fertilised by own-form pollen : 5 : 9.
Primula vulgaris. Long-styled form, fertilised by own-form pollen during two
successive generations : 69 : 0.
Primula auricula. Short-styled form, fertilised by own-form pollen, is said to
produce during successive generations offspring in about the following
proportions : 25 : 75.
Primula Sinensis. Long-styled form, fertilised by own-form pollen during two
successive generations : 52 : 0.
Primula Sinensis. Long-styled form, fertilised by own-form pollen (Hildebrand) :
14 : 3.
Primula Sinensis. Short-styled form, fertilised by own-form pollen: 1 : 24.
Pulmonaria officinalis. Long-styled form, fertilised by own-form pollen : 11 :
0.
Polygonum fagopyrum. Long-styled form, fertilised by own-form pollen : 45 : 4.
Polygonum fagopyrum. Short-styled form, fertilised by own-form pollen : 13 : 20.
TABLE 6.37. Nature of the offspring from illegitimately fertilised trimorphic
plants.
Column 1: Species and form.
Column 2: Number of long-styled offspring.
Column 3: Number of mid-styled offspring.
Column 4: Number of short-styled offspring.
Lythrum salicaria. Long-styled form, fertilised by own-form pollen : 56 : 0 : 0.
Lythrum salicaria. Short-styled form, fertilised by own-form pollen : 1 : 0 : 8.
Lythrum salicaria. Short-styled form, fertilised by pollen from mid-length
stamens of long-styled form : 4 : 0 : 8.
Lythrum salicaria. Mid-styled form, fertilised by own-form pollen : 1 : 3 : 0.
Lythrum salicaria. Mid-styled form, fertilised by pollen from shortest stamens
of long-styled form : 17 : 8 : 0.
Lythrum salicaria. Mid-styled form, fertilised by pollen from longest stamens of
short-styled form : 14 : 8 : 18.
Oxalis rosea. Long-styled form, fertilised during several generations by own-
form pollen, produced offspring in the ratio of : 100 : 0 : 0.
Oxalis hedysaroides. Mid-styled form, fertilised by own-form pollen : 0 : 17 :
0.
We see in these two tables that the offspring from a form illegitimately
fertilised with pollen from another plant of the same form belong, with a few
exceptions, to the same form as their parents. For instance, out of 162
seedlings from long-styled plants of Primula veris fertilised during five
generations in this manner, 156 were long-styled and only 6 short-styled. Of 69
seedlings from P. vulgaris similarly raised all were long-styled. So it was with
56 seedlings from the long-styled form of the trimorphic Lythrum salicaria, and
with numerous seedlings from the long-styled form of Oxalis rosea. The offspring
from the short-styled forms of dimorphic plants, and from both the mid-styled
and short-styled forms of trimorphic plants, fertilised with their own-form
pollen, likewise tend to belong to the same form as their parents, but not in so
marked a manner as in the case of the long-styled form. There are three cases in
Table 6.37, in which a form of Lythrum was fertilised illegitimately with pollen
from another form; and in two of these cases all the offspring belonged to the
same two forms as their parents, whilst in the third case they belonged to all
three forms.
The cases hitherto given relate to illegitimate unions, but Hildebrand, Fritz
Muller, and myself found that a very large proportion, or all of the offspring,
from a legitimate union between any two forms of the trimorphic species of
Oxalis belonged to the same two forms. A similar rule therefore holds good with
unions which are fully fertile, as with those of an illegitimate nature which
are more or less sterile. When some of the seedlings from a heterostyled plant
belong to a different form from that of its parents, Hildebrand accounts for the
fact by reversion. For instance, the long-styled parent-plant of Primula veris,
from which the 162 illegitimate seedlings in Table 6.36 were derived in the
course of five generations, was itself no doubt derived from the union of a
long-styled and a short-styled parent; and the 6 short-styled seedlings may be
attributed to reversion to their short-styled progenitor. But it is a surprising
fact in this case, and in other similar ones, that the number of the offspring
which thus reverted was not larger. The fact is rendered still more strange in
the particular instance of P. veris, for there was no reversion until four or
five generations of long-styled plants had been raised. It may be seen in both
tables that the long-styled form transmits its form much more faithfully than
does the short-styled, when both are fertilised with their own-form pollen; and
why this should be so it is difficult to conjecture, unless it be that the
aboriginal parent-form of most heterostyled species possessed a pistil which
exceeded its own stamens considerably in length. (6/8. It may be suspected that
this was the case with Primula, judging from the length of the pistil in several
allied genera (see Mr. J. Scott 'Journal of the Linnean Society Botany' volume 8
1864 page 85). Herr Breitenbach found many specimens of Primula elatior growing
in a state of nature with some flowers on the same plant long-styled, others
short-styled and others equal-styled; and the long-styled form greatly
preponderated in number; there being 61 of this form to 9 of the short-styled
and 15 of the equal-styled.) I will only add that in a state of nature any
single plant of a trimorphic species no doubt produces all three forms; and this
may be accounted for either by its several flowers being separately fertilised
by both the other forms, as Hildebrand supposes; or by pollen from both the
other forms being deposited by insects on the stigma of the same flower.
EQUAL-STYLED VARIETIES.
The tendency of the dimorphic species of Primula to produce equal-styled
varieties deserves notice. Cases of this kind have been observed, as shown in
the last chapter, in no less than six species, namely, P. veris, vulgaris,
Sinensis, auricula, farinosa, and elatior. In the case of P. veris, the stamens
resemble in length, position and size of their pollen-grains the stamens of the
short-styled form; whilst the pistil closely resembles that of the long-styled,
but as it varies much in length, one proper to the short-styled form appears to
have been elongated and to have assumed at the same time the functions of a
long-styled pistil. Consequently the flowers are capable of spontaneous self-
fertilisation of a legitimate nature and yield a full complement of seed, or
even more than the number produced by ordinary flowers legitimately fertilised.
With P. Sinensis, on the other hand, the stamens resemble in all respects the
shorter ones proper to the long-styled form, whilst the pistil makes a near
approach to that of the short-styled, but as it varies in length, it would
appear as if a long-styled pistil had been reduced in length and modified in
function. The flowers in this case as in the last are capable of spontaneous
legitimate fertilisation, and are rather more productive than ordinary flowers
legitimately fertilised. With P. auricula and farinosa the stamens resemble
those of the short-styled form in length, but those of the long-styled in the
size of their pollen-grains; the pistil also resembles that of the long-styled,
so that although the stamens and pistil are of nearly equal length, and
consequently pollen is spontaneously deposited on the stigma, yet the flowers
are not legitimately fertilised and yield only a very moderate supply of seed.
We thus see, firstly, that equal-styled varieties have originated in various
ways, and, secondly, that the combination of the two forms in the same flower
differs in completeness. With P. elatior some of the flowers on the same plant
have become equal-styled, instead of all of them as in the other species.
Mr. Scott has suggested that the equal-styled varieties arise through reversion
to the former homostyled condition of the genus. This view is supported by the
remarkable fidelity with which the equal-styled variation is transmitted after
it has once appeared. I have shown in Chapter 13 of my 'Variation of Animals and
Plants under Domestication,' that any cause which disturbs the constitution
tends to induce reversion, and it is chiefly the cultivated species of Primula
which become equal-styled. Illegitimate fertilisation, which is an abnormal
process, is likewise an exciting cause; and with illegitimately descended long-
styled plants of P. Sinensis, I have observed the first appearance and
subsequent stages of this variation. With some other plants of P. Sinensis of
similar parentage the flowers appeared to have reverted to their original wild
condition. Again, some hybrids between P. veris and vulgaris were strictly
equal-styled, and others made a near approach to this structure. All these facts
support the view that this variation results, at least in part, from reversion
to the original state of the genus, before the species had become heterostyled.
On the other hand, some considerations indicate, as previously remarked, that
the aboriginal parent-form of Primula had a pistil which exceeded the stamens in
length. The fertility of the equal-styled varieties has been somewhat modified,
being sometimes greater and sometimes less than that of a legitimate union.
Another view, however, may be taken with respect to the origin of the equal-
styled varieties, and their appearance may be compared with that of
hermaphrodites amongst animals which properly have their sexes separated; for
the two sexes are combined in a monstrous hermaphrodite in a somewhat similar
manner as the two sexual forms are combined in the same flower of an equal-
styled variety of a heterostyled species.
FINAL REMARKS.
The existence of plants which have been rendered heterostyled is a highly
remarkable phenomenon, as the two or three forms of the same undoubted species
differ not only in important points of structure, but in the nature of their
reproductive powers. As far as structure is concerned, the two sexes of many
animals and of some plants differ to an extreme degree; and in both kingdoms the
same species may consist of males, females, and hermaphrodites. Certain
hermaphrodite cirripedes are aided in their reproduction by a whole cluster of
what I have called complemental males, which differ wonderfully from the
ordinary hermaphrodite form. With ants we have males and females, and two or
three castes of sterile females or workers. With Termites there are, as Fritz
Muller has shown, both winged and wingless males and females, besides the
workers. But in none of these cases is there any reason to believe that the
several males or several females of the same species differ in their sexual
powers, except in the atrophied condition of the reproductive organs in the
workers of social insects. Many hermaphrodite animals must unite for
reproduction, but the necessity of such union apparently depends solely on their
structure. On the other hand, with heterostyled dimorphic species there are two
females and two sets of males, and with trimorphic species three females and
three sets of males, which differ essentially in their sexual powers. We shall,
perhaps, best perceive the complex and extraordinary nature of the marriage
arrangements of a trimorphic plant by the following illustration. Let us suppose
that the individuals of the same species of ant always lived in triple
communities; and that in one of these, a large-sized female (differing also in
other characters) lived with six middle-sized and six small-sized males; in the
second community a middle-sized female lived with six large- and six small-sized
males; and in the third, a small-sized female lived with six large- and six
middle-sized males. Each of these three females, though enabled to unite with
any male, would be nearly sterile with her own two sets of males, and likewise
with two other sets of males of the same size with her own which lived in the
other two communities; but she would be fully fertile when paired with a male of
her own size. Hence the thirty-six males, distributed by half-dozens in the
three communities, would be divided into three sets of a dozen each; and these
sets, as well as the three females, would differ from one another in their
reproductive powers in exactly the same manner as do the distinct species of the
same genus. But it is a still more remarkable fact that young ants raised from
any one of the three female ants, illegitimately fertilised by a male of a
different size would resemble in a whole series of relations the hybrid
offspring from a cross between two distinct species of ants. They would be
dwarfed in stature, and more or less, or even utterly barren. Naturalists are so
much accustomed to behold great diversities of structure associated with the two
sexes, that they feel no surprise at almost any amount of difference; but
differences in sexual nature have been thought to be the very touchstone of
specific distinction. We now see that such sexual differences--the greater or
less power of fertilising and being fertilised--may characterise the co-existing
individuals of the same species, in the same manner as they characterise and
have kept separate those groups of individuals, produced during the lapse of
ages, which we rank and denominate as distinct species.
CHAPTER VII.
POLYGAMOUS, DIOECIOUS, AND GYNO-DIOECIOUS PLANTS.
The conversion in various ways of hermaphrodite into dioecious plants.
Heterostyled plants rendered dioecious.
Rubiaceae.
Verbenaceae.
Polygamous and sub-dioecious plants.
Euonymus.
Fragaria.
The two sub-forms of both sexes of Rhamnus and Epigaea.
Ilex.
Gyno-dioecious plants.
Thymus, difference in fertility of the hermaphrodite and female individuals.
Satureia.
Manner in which the two forms probably originated.
Scabiosa and other gyno-dioecious plants.
Difference in the size of the corolla in the forms of polygamous, dioecious, and
gyno-dioecious plants.
There are several groups of plants in which all the species are dioecious, and
these exhibit no rudiments in the one sex of the organs proper to the other.
About the origin of such plants nothing is known. It is possible that they may
be descended from ancient lowly organised forms, which had from the first their
sexes separated; so that they have never existed as hermaphrodites. There are,
however, many other groups of species and single ones, which from being allied
on all sides to hermaphrodites, and from exhibiting in the female flowers plain
rudiments of male organs, and conversely in the male flowers rudiments of female
organs, we may feel sure are descended from plants which formerly had the two
sexes combined in the same flower. It is a curious and obscure problem how and
why such hermaphrodites have been rendered bisexual.
If in some individuals of a species the stamens alone were to abort, females and
hermaphrodites would be left existing, of which many instances occur; and if the
female organs of the hermaphrodite were afterwards to abort, the result would be
a dioecious plant. Conversely, if we imagine the female organs alone to abort in
some individuals, males and hermaphrodites would be left; and the hermaphrodites
might afterwards be converted into females.
In other cases, as in that of the common Ash-tree mentioned in the Introduction,
the stamens are rudimentary in some individuals, the pistils in others, others
again remaining as hermaphrodites. Here the modification of the two sets of
organs appears to have occurred simultaneously, as far as we can judge from
their equal state of abortion. If the hermaphrodites were supplanted by the
individuals having separated sexes, and if these latter were equalised in
number, a strictly dioecious species would be formed.
There is much difficulty in understanding why hermaphrodite plants should ever
have been rendered dioecious. There would be no such conversion, unless pollen
was already carried regularly by insects or by the wind from one individual to
the other; for otherwise every step towards dioeciousness would lead towards
sterility. As we must assume that cross-fertilisation was assured before an
hermaphrodite could be changed into a dioecious plant, we may conclude that the
conversion has not been effected for the sake of gaining the great benefits
which follow from cross-fertilisation. We can, however, see that if a species
were subjected to unfavourable conditions from severe competition with other
plants, or from any other cause, the production of the male and female elements
and the maturation of the ovules by the same individual, might prove too great a
strain on its powers, and the separation of the sexes would then be highly
beneficial. This, however, would be effected only under the contingency of a
reduced number of seeds, produced by the females alone, being sufficient to keep
up the stock.
There is another way of looking at the subject which partially removes a
difficulty that appears at first sight insuperable, namely, that during the
conversion of an hermaphrodite into a dioecious plant, the male organs must
abort in some individuals and the female organs in others. Yet as all are
exposed to the same conditions, it might have been expected that those which
varied would tend to vary in the same manner. As a general rule only a few
individuals of a species vary simultaneously in the same manner; and there is no
improbability in the assumption that some few individuals might produce larger
seeds than the average, better stocked with nourishment. If the production of
such seeds were highly beneficial to a species, and on this head there can be
little doubt, the variety with the large seeds would tend to increase. (7/1. See
the facts given in 'The Effects of Cross and Self-fertilisation' page 353.) But
in accordance with the law of compensation we might expect that the individuals
which produced such seeds would, if living under severe conditions, tend to
produce less and less pollen, so that their anthers would be reduced in size and
might ultimately become rudimentary. This view occurred to me owing to a
statement by Sir J.E. Smith that there are female and hermaphrodite plants of
Serratula tinctoria, and that the seeds of the former are larger than those of
the hermaphrodite form. (7/2. 'Transactions of the Linnean Society' volume 8
page 600.) It may also be worth while to recall the case of the mid-styled form
of Lythrum salicaria, which produces a larger number of seeds than the other
forms, and has somewhat smaller pollen-grains which have less fertilising power
than those of the corresponding stamens in the other two forms; but whether the
larger number of seeds is the indirect cause of the diminished power of the
pollen, or vice versa, I know not. As soon as the anthers in a certain number of
individuals became reduced in size in the manner just suggested or from any
other cause, the other individuals would have to produce a larger supply of
pollen; and such increased development would tend to reduce the female organs
through the law of compensation, so as ultimately to leave them in a rudimentary
condition; and the species would then become dioecious.
Instead of the first change occurring in the female organs we may suppose that
the male ones first varied, so that some individuals produced a larger supply of
pollen. This would be beneficial under certain circumstances, such as a change
in the nature of the insects which visited the flowers, or in their becoming
more anemophilous, for such plants require an enormous quantity of pollen. The
increased action of the male organs would tend to affect through compensation
the female organs of the same flower; and the final result would be that the
species would consist of males and hermaphrodites. But it is of no use
considering this case and other analogous ones, for, as stated in the
Introduction, the coexistence of male and hermaphrodite plants is excessively
rare.
It is no valid objection to the foregoing views that changes of such a nature
would be effected with extreme slowness, for we shall presently see good reason
to believe that various hermaphrodite plants have become or are becoming
dioecious by many and excessively small steps. In the case of polygamous
species, which exist as males, females and hermaphrodites, the latter would have
to be supplanted before the species could become strictly dioecious; but the
extinction of the hermaphrodite form would probably not be difficult, as a
complete separation of the sexes appears often to be in some way beneficial. The
males and females would also have to be equalised in number, or produced in some
fitting proportion for the effectual fertilisation of the females.
There are, no doubt, many unknown laws which govern the suppression of the male
or female organs in hermaphrodite plants, quite independently of any tendency in
them to become monoecious, dioecious, or polygamous. We see this in those
hermaphrodites which from the rudiments still present manifestly once possessed
more stamens or pistils than they now do,--even twice as many, as a whole
verticil has often been suppressed. Robert Brown remarks that "the order of
reduction or abortion of the stamina in any natural family may with some
confidence be predicted," by observing in other members of the family, in which
their number is complete, the order of the dehiscence of the anthers (7/3.
'Transactions of the Linnean Society' volume 12 page 98 or 'Miscellaneous Works'
volume 2 pages 278-81.); for the lesser permanence of an organ is generally
connected with its lesser perfection, and he judges of perfection by priority of
development. He also states that whenever there is a separation of the sexes in
an hermaphrodite plant, which bears flowers on a simple spike, it is the females
which expand first; and this he likewise attributes to the female sex being the
more perfect of the two, but why the female should be thus valued he does not
explain.
Plants under cultivation or changed conditions of life frequently become
sterile; and the male organs are much oftener affected than the female, though
the latter alone are sometimes affected. The sterility of the stamens is
generally accompanied by a reduction in their size; and we may feel sure, from a
wide-spread analogy, that both the male and female organs would become
rudimentary in the course of many generations if they failed altogether to
perform their proper functions. According to Gartner, if the anthers on a plant
are contabescent (and when this occurs it is always at a very early period of
growth) the female organs are sometimes precociously developed. (7/4. 'Beitrage
zur Kenntniss' etc. page 117 et seq. The whole subject of the sterility of
plants from various causes has been discussed in my 'Variation of Animals and
Plants under Domestication' chapter 18 2nd edition volume 2 pages 146-56.) I
mention this case as it appears to be one of compensation. So again is the well-
known fact, that plants which increase largely by stolons or other such means
are often utterly barren, with a large proportion of their pollen-grains in a
worthless condition.
Hildebrand has shown that with hermaphrodite plants which are strongly
proterandrous, the stamens in the flowers which open first sometimes abort; and
this seems to follow from their being useless, as no pistils are then ready to
be fertilised. Conversely the pistils in the flowers which open last sometimes
abort; as when they are ready for fertilisation all the pollen has been shed. He
further shows by means of a series of gradations amongst the Compositae, that a
tendency from the causes just specified to produce either male or female
florets, sometimes spreads to all the florets on the same head, and sometimes
even to the whole plant (7/5. 'Ueber die Geschlechtsverhaltnisse bei den
Compositen' 1869 page 89.); and in this latter case the species becomes
dioecious. In those rare instances mentioned in the Introduction, in which some
of the individuals of both monoecious and hermaphrodite plants are
proterandrous, others being proterogynous, their conversion into a dioecious
condition would probably be much facilitated, as they already consist of two
bodies of individuals, differing to a certain extent in their reproductive
functions.
Dimorphic heterostyled plants offer still more strongly marked facilities for
becoming dioecious; for they likewise consist of two bodies of individuals in
approximately equal numbers, and what probably is more important, both the male
and female organs differ in the two forms, not only in structure but in
function, in nearly the same manner as do the reproductive organs of two
distinct species belonging to the same genus. Now if two species are subjected
to changed conditions, though of the same nature, it is notorious that they are
often affected very differently; therefore the male organs, for instance, in one
form of a heterostyled plant might be affected by those unknown causes which
induce abortion, differently from the homologous but functionally different
organs in the other form; and so conversely with the female organs. Thus the
great difficulty before alluded to is much lessened in understanding how any
cause whatever could lead to the simultaneous reduction and ultimate suppression
of the male organs in half the individuals of a species, and of the female
organs in the other half, whilst all were subjected to exactly the same
conditions of life.
That such reduction or suppression has occurred in some heterostyled plants is
almost certain. The Rubiaceae contain more heterostyled genera than any other
family, and from their wide distribution we may infer that many of them became
heterostyled at a remote period, so that there will have been ample time for
some of the species to have been since rendered dioecious. Asa Gray informs me
that Coprosma is dioecious, and that it is closely allied through Nertera to
Mitchella, which as we know is a heterostyled dimorphic species. In the male
flowers of Coprosma the stamens are exserted, and in the female flowers the
stigmas; so that, judging from the affinities of the above three genera, it
seems probable that an ancient short-styled form bearing long stamens with large
anthers and large pollen-grains (as in the case of several Rubiaceous genera)
has been converted into the male Coprosma; and that an ancient long-styled form
with short stamens, small anthers and small pollen-grains has been converted
into the female form. But according to Mr. Meehan, Mitchella itself is dioecious
in some districts; for he says that one form has small sessile anthers without a
trace of pollen, the pistil being perfect; while in another form the stamens are
perfect and the pistil rudimentary. (7/6. 'Proceedings of the Academy of
Sciences of Philadelphia' July 28, 1868 page 183.) He adds that plants may be
observed in the autumn bearing an abundant crop of berries, and others without a
single one. Should these statements be confirmed, Mitchella will be proved to be
heterostyled in one district and dioecious in another.
Asperula is likewise a Rubiaceous genus, and from the published description of
the two forms of A. scoparia, an inhabitant of Tasmania, I did not doubt that it
was heterostyled; but on examining some flowers sent me by Dr. Hooker they
proved to be dioecious. The male flowers have large anthers and a very small
ovarium, surmounted by a mere vestige of a stigma without any style; whilst the
female flowers possess a large ovarium, the anthers being rudimentary and
apparently quite destitute of pollen. Considering how many Rubiaceous genera are
heterostyled, it is a reasonable suspicion that this Asperula is descended from
a heterostyled progenitor; but we should be cautious on this head, for there is
no improbability in a homostyled Rubiaceous plant becoming dioecious. Moreover,
in an allied plant, Galium cruciatum, the female organs have been suppressed in
most of the lower flowers, whilst the upper ones remain hermaphrodite; and here
we have a modification of the sexual organs without any connection with
heterostylism.
Mr. Thwaites informs me that in Ceylon various Rubiaceous plants are
heterostyled; but in the case of Discospermum one of the two forms is always
barren, the ovary containing about two aborted ovules in each loculus; whilst in
the other form each loculus contains several perfect ovules; so that the species
appears to be strictly dioecious.
Most of the species of the South American genus Aegiphila, a member of the
Verbenaceae, apparently are heterostyled; and both Fritz Muller and myself
thought that this was the case with Ae. obdurata, so closely did its flowers
resemble those of the heterostyled species. But on examining the flowers, the
anthers of the long-styled form were found to be entirely destitute of pollen
and less than half the size of those in the other form, the pistil being
perfectly developed. On the other hand, in the short-styled form the stigmas are
reduced to half their proper length, having also an abnormal appearance; whilst
the stamens are perfect. This plant therefore is dioecious; and we may, I think,
conclude that a short-styled progenitor, bearing long stamens exserted beyond
the corolla, has been converted into the male; and a long-styled progenitor with
fully developed stigmas into the female.
From the number of bad pollen-grains in the small anthers of the short stamens
of the long-styled form of Pulmonaria angustifolia, we may suspect that this
form is tending to become female; but it does not appear that the other or
short-styled form is becoming more masculine. Certain appearances countenance
the belief that the reproductive system of Phlox subulata is likewise undergoing
a change of some kind.
I have now given the few cases known to me in which heterostyled plants appear
with some considerable degree of probability to have been rendered dioecious.
Nor ought we to expect to find many such cases, for the number of heterostyled
species is by no means large, at least in Europe, where they could hardly have
escaped notice. Therefore the number of dioecious species which owe their origin
to the transformation of heterostyled plants is probably not so large as might
have been anticipated from the facilities which they offer for such conversion.
In searching for cases like the foregoing ones, I have been led to examine some
dioecious or sub-dioecious plants, which are worth describing, chiefly as they
show by what fine gradations hermaphrodites may pass into polygamous or
dioecious species.
POLYGAMOUS, DIOECIOUS, AND SUB-DIOECIOUS PLANTS.
Euonymus Europaeus (CELASTRINEAE).
(Figure 7.12. Euonymus Europaeus
Left: Hermaphrodite or male.
Right: Female.)
The spindle-tree is described in all the botanical works which I have consulted
as an hermaphrodite. Asa Gray speaks of the flowers of the American species as
perfect, whilst those in the allied genus Celastrus are said to be "polygamo-
dioecious." If a number of bushes of our spindle-tree be examined, about half
will be found to have stamens equal in length to the pistil, with well-developed
anthers; the pistil being likewise to all appearance well developed. The other
half have a perfect pistil, with the stamens short, bearing rudimentary anthers
destitute of pollen; so that these bushes are females. All the flowers on the
same plant present the same structure. The female corolla is smaller than that
on the polleniferous bushes. The two forms are shown in Figure 7.12.
I did not at first doubt that this species existed under an hermaphrodite and
female form; but we shall presently see that some of the bushes which appear to
be hermaphrodites never produce fruit, and these are in fact males. The species,
therefore, is polygamous in the sense in which I use the term, and trioecious.
The flowers are frequented by many Diptera and some small Hymenoptera for the
sake of the nectar secreted by the disc, but I did not see a single bee at work;
nevertheless the other insects sufficed to fertilise effectually female bushes
growing at a distance of even 30 yards from any polleniferous bush.
The small anthers borne by the short stamens of the female flowers are well
formed and dehisce properly, but I could never find in them a single grain of
pollen. It is somewhat difficult to compare the length of the pistils in the two
forms, as they vary somewhat in this respect and continue to grow after the
anthers are mature. The pistils, therefore, in old flowers on a polleniferous
plant are often of considerably greater length than in young flowers on a female
plant. On this account the pistils from five flowers from so many hermaphrodite
or male bushes were compared with those from five female bushes, before the
anthers had dehisced and whilst the rudimentary ones were of a pink colour and
not at all shrivelled. These two sets of pistils did not differ in length, or if
there was any difference those of the polleniferous flowers were rather the
longest. In one hermaphrodite plant, which produced during three years very few
and poor fruit, the pistil much exceeded in length the stamens bearing perfect
and as yet closed anthers; and I never saw such a case on any female plant. It
is a surprising fact that the pistil in the male and in the semi-sterile
hermaphrodite flowers has not been reduced in length, seeing that it performs
very poorly or not at all its proper function. The stigmas in the two forms are
exactly alike; and in some of the polleniferous plants which never produced any
fruit I found that the surface of the stigma was viscid, so that pollen-grains
adhered to it and had exserted their tubes. The ovules are of equal size in the
two forms. Therefore the most acute botanist, judging only by structure, would
never have suspected that some of the bushes were in function exclusively males.
Thirteen bushes growing near one another in a hedge consisted of eight females
quite destitute of pollen and of five hermaphrodites with well-developed
anthers. In the autumn the eight females were well covered with fruit, excepting
one, which bore only a moderate number. Of the five hermaphrodites, one bore a
dozen or two fruits, and the remaining four bushes several dozen; but their
number was as nothing compared with those on the female bushes, for a single
branch, between two and three feet in length, from one of the latter, yielded
more than any one of the hermaphrodite bushes. The difference in the amount of
fruit produced by the two sets of bushes is all the more striking, as from the
sketches above given it is obvious that the stigmas of the polleniferous flowers
can hardly fail to receive their own pollen; whilst the fertilisation of the
female flowers depends on pollen being brought to them by flies and the smaller
Hymenoptera, which are far from being such efficient carriers as bees.
I now determined to observe more carefully during successive seasons some bushes
growing in another place about a mile distant. As the female bushes were so
highly productive, I marked only two of them with the letters A and B, and five
polleniferous bushes with the letters C to G. I may premise that the year 1865
was highly favourable for the fruiting of all the bushes, especially for the
polleniferous ones, some of which were quite barren except under such favourable
conditions. The season of 1864 was unfavourable. In 1863 the female A produced
"some fruit;" in 1864 only 9; and in 1865, 97 fruit. The female B in 1863 was
"covered with fruit;" in 1864 it bore 28; and in 1865 "innumerable very fine
fruits." I may add, that three other female trees growing close by were
observed, but only during 1863, and they then bore abundantly. With respect to
the polleniferous bushes, the one marked C did not bear a single fruit during
the years 1863 and 1864, but during 1865 it produced no less than 92 fruit,
which, however, were very poor. I selected one of the finest branches with 15
fruit, and these contained 20 seeds, or on an average 1.33 per fruit. I then
took by hazard 15 fruit from an adjoining female bush, and these contained 43
seeds; that is, more than twice as many, or on an average 2.86 per fruit. Many
of the fruits from the female bushes included four seeds, and only one had a
single seed; whereas not one fruit from the polleniferous bushes contained four
seeds. Moreover when the two lots of seeds were compared, it was manifest that
those from the female bushes were the larger. The second polleniferous bush, D,
bore in 1863 about two dozen fruit,--in 1864 only 3 very poor fruit, each
containing a single seed,--and in 1865, 20 equally poor fruit. Lastly, the three
polleniferous bushes, E, F, and G, did not produce a single fruit during the
three years 1863, 1864, and 1865.
We thus see that the female bushes differ somewhat in their degree of fertility,
and the polleniferous ones in the most marked manner. We have a perfect
gradation from the female bush, B, which in 1865 was covered with "innumerable
fruits,"--through the female A, which produced during the same year 97,--through
the polleniferous bush C, which produced this year 92 fruits, these, however,
containing a very low average number of seeds of small size,--through the bush
D, which produced only 20 poor fruit,--to the three bushes, E, F, and G, which
did not this year, or during the two previous years, produce a single fruit. If
these latter bushes and the more fertile female ones were to supplant the
others, the spindle-tree would be as strictly dioecious in function as any plant
in the world. This case appears to me very interesting, as showing how gradually
an hermaphrodite plant may be converted into a dioecious one. (7/7. According to
Fritz Muller 'Botanische Zeitung' 1870 page 151, a Chamissoa (Amaranthaceae) in
Southern Brazil is in nearly the same state as our Euonymus. The ovules are
equally developed in the two forms. In the female the pistil is perfect, whilst
the anthers are entirely destitute of pollen. In the polleniferous form, the
pistil is short and the stigmas never separate from one another, so that,
although their surfaces are covered with fairly well-developed papillae, they
cannot be fertilised, these latter plants do not commonly yield any fruit, and
are therefore in function males. Nevertheless, on one occasion Fritz Muller
found flowers of this kind in which the stigmas had separated, and they produced
some fruit.)
Seeing how general it is for organs which are almost or quite functionless to be
reduced in size, it is remarkable that the pistils of the polleniferous plants
should equal or even exceed in length those of the highly fertile female plants.
This fact formerly led me to suppose that the spindle-tree had once been
heterostyled; the hermaphrodite and male plants having been originally long-
styled, with the pistils since reduced in length, but with the stamens retaining
their former dimensions; whilst the female plant had been originally short-
styled, with the pistil in its present state, but with the stamens since greatly
reduced and rendered rudimentary. A conversion of this kind is at least
possible, although it is the reverse of that which appears actually to have
occurred with some Rubiaceous genera and Aegiphila; for with these plants the
short-styled form has become the male, and the long-styled the female. It is,
however, a more simple view that sufficient time has not elapsed for the
reduction of the pistil in the male and hermaphrodite flowers of our Euonymus;
though this view does not account for the pistils in the polleniferous flowers
being sometimes longer than those in the female flowers.
Fragaria vesca, Virginiana, chiloensis, etc. (ROSACEAE).
A tendency to the separation of the sexes in the cultivated strawberry seems to
be much more strongly marked in the United States than in Europe; and this
appears to be the result of the direct action of climate on the reproductive
organs. In the best account which I have seen, it is stated that many of the
varieties in the United States consist of three forms, namely, females, which
produce a heavy crop of fruit,--of hermaphrodites, which "seldom produce other
than a very scanty crop of inferior and imperfect berries,"--and of males, which
produce none. (7/8. Mr. Leonard Wray 'Gardener's Chronicle' 1861 page 716.) The
most skilful cultivators plant "seven rows of female plants, then one row of
hermaphrodites, and so on throughout the field." The males bear large, the
hermaphrodites mid-sized, and the females small flowers. The latter plants
produce few runners, whilst the two other forms produce many; consequently, as
has been observed both in England and in the United States, the polleniferous
forms increase rapidly and tend to supplant the females. We may therefore infer
that much more vital force is expended in the production of ovules and fruit
than in the production of pollen. Another species, the Hautbois strawberry (F.
elatior), is more strictly dioecious; but Lindley made by selection an
hermaphrodite stock. (7/9. For references and further information on this
subject, see 'Variation under Domestication' chapter 10 2nd edition volume 1
page 375.)
Rhamnus catharticus (RHAMNEAE).
(FIGURE 7.13. Rhamnus catharticus (from Caspary.)
Left: Long-styled male.
Right: Short-styled male.)
(FIGURE 7.14. Rhamnus catharticus.
Left: Long-styled female.
Right: Short-styled female.)
This plant is well known to be dioecious. My son William found the two sexes
growing in about equal numbers in the Isle of Wight, and sent me specimens,
together with observations on them. Each sex consists of two sub-forms. The two
forms of the male differ in their pistils: in some plants it is quite small,
without any distinct stigma; in others the pistil is much more developed, with
the papillae on the stigmatic surfaces moderately large. The ovules in both
kinds of males are in an aborted condition. On my mentioning this case to
Professor Caspary, he examined several male plants in the botanic gardens at
Konigsberg, where there were no females, and sent me the drawings in Figure
7.13.
In the English plants the petals are not so greatly reduced as represented in
this drawing. My son observed that those males which had their pistils
moderately well-developed bore slightly larger flowers, and, what is very
remarkable, their pollen-grains exceeded by a little in diameter those of the
males with greatly reduced pistils. This fact is opposed to the belief that the
present species was once heterostyled; for in this case it might have been
expected that the shorter-styled plants would have had larger pollen-grains.
In the female plants the stamens are in an extremely rudimentary condition, much
more so than the pistils in the males. The pistil varies considerably in length
in the female plants, so that they may be divided into two sub-forms according
to the length of this organ. Both the petals and sepals are decidedly smaller in
the females than in the males; and the sepals do not turn downwards, as do those
of the male flowers when mature. All the flowers on the same male or same female
bush, though subject to some variability, belong to the same sub-form; and as my
son never experienced any difficulty in deciding under which class a plant ought
to be included, he believes that the two sub-forms of the same sex do not
graduate into one another. I can form no satisfactory theory how the four forms
of this plant originated.
Rhamnus lanceolatus.
This plant exists in the United States, as I am informed by Professor Asa Gray,
under two hermaphrodite forms. In the one, which may be called the short-styled,
the flowers are sub-solitary, and include a pistil about two-thirds or only half
as long as that in the other form; it has also shorter stigmas. The stamens are
of equal length in the two forms; but the anthers of the short-styled contain
rather less pollen, as far as I could judge from a few dried flowers. My son
compared the pollen-grains from the two forms, and those from the long-styled
flowers were to those from the short-styled, on an average from ten
measurements, as 10 to 9 in diameter; so that the two hermaphrodite forms of
this species resemble in this respect the two male forms of R. catharticus. The
long-styled form is not so common as the short-styled. The latter is said by Asa
Gray to be the more fruitful of the two, as might have been expected from its
appearing to produce less pollen, and from the grains being of smaller size; it
is therefore the more highly feminine of the two. The long-styled form produces
a greater number of flowers, which are clustered together instead of being sub-
solitary; they yield some fruit, but as just stated are less fruitful than the
other form, so that this form appears to be the more masculine of the two. On
the supposition that we have here an hermaphrodite plant becoming dioecious,
there are two points deserving notice; firstly, the greater length of the pistil
in the incipient male form; and we have met with a nearly similar case in the
male and hermaphrodite forms of Euonymus compared with the females. Secondly,
the larger size of the pollen-grains in the more masculine flowers, which
perhaps may be attributed to their having retained their normal size; whilst
those in the incipient female flowers have been reduced. The long-styled form of
R. lanceolatus seems to correspond with the males of R. catharticus which have a
longer pistil and larger pollen-grains. Light will perhaps be thrown on the
nature of the forms in this genus, as soon as the power of both kinds of pollen
on both stigmas is ascertained. Several other species of Rhamnus are said to be
dioecious or sub-dioecious. (7/10. Lecoq 'Geogr. Bot.' tome 5 1856 pages 420-
26.) On the other hand, R. frangula is an ordinary hermaphrodite, for my son
found a large number of bushes all bearing an equal profusion of fruit.
Epigaea repens (ERICACEAE).
This plant appears to be in nearly the same state as Rhamnus catharticus. It is
described by Asa Gray as existing under four forms. (7/11. 'American Journal of
Science' July 1876. Also 'The American Naturalist' 1876 page 490.) (1.) With
long style, perfect stigma, and short abortive stamens. (2.) Shorter style, but
with stigma equally perfect, short abortive stamens. These two female forms
amounted to 20 per cent of the specimens received from one locality in Maine;
but all the fruiting specimens belonged to the first form. (3.) Style long, as
in Number 1, but with stigma imperfect, stamens perfect. (4.) Style shorter than
in the last, stigma imperfect, stamens perfect. These two latter forms are
evidently males. Therefore, as Asa Gray remarks, "the flowers may be classified
into two kinds, each with two modifications; the two main kinds characterised by
the nature and perfection of the stigma, along with more or less abortion of the
stamens; their modifications, by the length of the style." Mr. Meehan has
described the extreme variability of the corolla and calyx in this plant, and
shows that it is dioecious. (7/12. "Variations in Epigaea repens" 'Proc. Acad.
Nat. Soc. of Philadelphia' May 1868 page 153.) It is much to be wished that the
pollen-grains in the two male forms should be compared, and their fertilising
power tried on the two female forms.
Ilex aquifolium (AQUIFOLIACEAE).
In the several works which I have consulted, one author alone says that the
holly is dioecious. (7/13. Vaucher 'Hist. Phys. des Plantes d'Europe' 1841 tome
2 page 11.) During several years I have examined many plants, but have never
found one that was really hermaphrodite. I mention this genus because the
stamens in the female flowers, although quite destitute of pollen, are but
slightly and sometimes not at all shorter than the perfect stamens in the male
flowers. In the latter the ovary is small and the pistil is almost aborted. The
filaments of the perfect stamens adhere for a greater length to the petals than
in the female flowers. The corolla of the latter is rather smaller than that of
the male. The male trees produce a greater number of flowers than the females.
Asa Gray informs me that I. opaca, which represents in the United States our
common holly, appears (judging from dried flowers) to be in a similar state; and
so it is, according to Vaucher, with several other but not with all the species
of the genus.
GYNO-DIOECIOUS PLANTS.
The plants hitherto described either show a tendency to become dioecious, or
apparently have become so within a recent period. But the species now to be
considered consist of hermaphrodites and females without males, and rarely show
any tendency to be dioecious, as far as can be judged from their present
condition and from the absence of species having separated sexes within the same
groups. Species belonging to the present class, which I have called gyno-
dioecious, are found in various widely distinct families; but are much more
common in the Labiatae (as has long been noticed by botanists) than in any other
group. Such cases have been noticed by myself in Thymus serpyllum and vulgaris,
Satureia hortensis, Origanum vulgare, and Mentha hirsuta; and by others in
Nepeta glechoma, Mentha vulgaris and aquatica, and Prunella vulgaris. In these
two latter species the female form, according to H. Muller, is infrequent. To
these must be added Dracocephalum Moldavicum, Melissa officinalis and
clinopodium, and Hyssopus officinalis. (7/14. H. Muller 'Die Befruchtung der
Blumen' 1873 and 'Nature' 1873 page 161. Vaucher 'Plantes d'Europe' tome 3 page
611. For Dracocephalum Schimper as quoted by Braun 'Annals and Magazine of
Natural History' 2nd series volume 18 1856 page 380. Lecoq 'Geographie Bot. de
l'Europe' tome 8 pages 33, 38, 44, etc. Both Vaucher and Lecoq were mistaken in
thinking that several of the plants named in the text are dioecious. They appear
to have assumed that the hermaphrodite form was a male; perhaps they were
deceived by the pistil not becoming fully developed and of proper length until
some time after the anthers have dehisced.) In the two last-named plants the
female form likewise appears to be rare, for I raised many seedlings of both,
and all were hermaphrodites. It has already been remarked in the Introduction
that andro-dioecious species, as they may be called, or those which consist of
hermaphrodites and males, are extremely rare, or hardly exist.
Thymus serpyllum.
The hermaphrodite plants present nothing particular in the state of their
reproductive organs; and so it is in all the following cases. The females of the
present species produce rather fewer flowers and have somewhat smaller corollas
than the hermaphrodites; so that near Torquay, where this plant abounds, I
could, after a little practice, distinguish the two forms whilst walking quickly
past them. According to Vaucher, the smaller size of the corolla is common to
the females of most or all of the above-mentioned Labiatae. The pistil of the
female, though somewhat variable in length, is generally shorter, with the
margins of the stigma broader and formed of more lax tissue, than that of the
hermaphrodite. The stamens in the female vary excessively in length; they are
generally enclosed within the tube of the corolla, and their anthers do not
contain any sound pollen; but after long search I found a single plant with the
stamens moderately exserted, and their anthers contained a very few full-sized
grains, together with a multitude of minute empty ones. In some females the
stamens are extremely short, and their minute anthers, though divided into the
two normal cells or loculi, contained not a trace of pollen: in others again the
anthers did not exceed in diameter the filaments which supported them, and were
not divided into two loculi. Judging from what I have myself seen and from the
descriptions of others, all the plants in Britain, Germany, and near Mentone,
are in the state just described; and I have never found a single flower with an
aborted pistil. It is, therefore, remarkable that, according to Delpino, this
plant near Florence is generally trimorphic, consisting of males with aborted
pistils, females with aborted stamens, and hermaphrodites. (7/15. 'Sull' Opera,
la Distribuzione dei Sessi nelle Piante, etc' 1867 page 7. With respect to
Germany H. Muller 'Die Befruchtung etc.' page 327.)
I found it very difficult to judge of the proportional number of the two forms
at Torquay. They often grow mingled together, but with large patches consisting
of one form alone. At first I thought that the two were nearly equal in number;
but on examining every plant which grew close to the edge of a little
overhanging dry cliff, about 200 yards in length, I found only 12 females; all
the rest, some hundreds in number, being hermaphrodites. Again, on an extensive
gently sloping bank, which was so thickly covered with this plant that, viewed
from the distance of half a mile it appeared of a pink colour, I could not
discover a single female. Therefore the hermaphrodites must greatly exceed in
number the females, at least in the localities examined by me. A very dry
station apparently favours the presence of the female form. With some of the
other above-named Labiatae the nature of the soil or climate likewise seems to
determine the presence of one or both forms; thus with Nepeta glechoma, Mr. Hart
found in 1873 that all the plants which he examined near Kilkenny in Ireland
were females; whilst all near Bath were hermaphrodites, and near Hertford both
forms were present, but with a preponderance of hermaphrodites. (7/16. 'Nature'
June 1873 page 162.) It would, however, be a mistake to suppose that the nature
of the conditions determines the form independently of inheritance; for I sowed
in the same small bed seeds of T. serpyllum, gathered at Torquay from the female
alone, and these produced an abundance of both forms. There is every reason to
believe, from large patches consisting of the same form, that the same
individual plant, however much it may spread, always retains the same form. In
two distant gardens I found masses of the lemon-thyme (T. citriodorus, a var. of
T. serpyllum, which I was informed had grown there during many years, and every
flower was female.
With respect to the fertility of the two forms, I marked at Torquay a large
hermaphrodite and a large female plant of nearly equal sizes, and when the seeds
were ripe I gathered all the heads. The two heaps were of very nearly equal
bulk; but the heads from the female plant numbered 160, and their seeds weighed
8.7 grains; whilst those from the hermaphrodite plant numbered 200, and their
seeds weighed only 4.9 grains; so that the seeds from the female plant were to
those from the hermaphrodite as 100 to 56 in weight. If the relative weight of
the seeds from an equal number of flower-heads from the two forms be compared,
the ratio is as 100 for the female to 45 for the hermaphrodite form.
Thymus vulgaris.
(FIGURE 7.15. Thymus vulgaris (magnified).
Left: Hermaphrodite.
Right: Two females.)
The common garden thyme resembles in almost every respect T. serpyllum. The same
slight differences between the stigmas of the two forms could be perceived. In
the females the stamens are not generally quite so much reduced as in the same
form of T. serpyllum. In some specimens sent me from Mentone by Mr. Moggridge,
together with the sketches in Figure 7.15, the anthers of the female, though
small, were well formed, but they contained very little pollen, and not a single
sound grain could be detected. Eighteen seedlings were raised from purchased
seed, sown in the same small bed; and these consisted of seven hermaphrodites
and eleven females. They were left freely exposed to the visits of bees, and no
doubt every female flower was fertilised; for on placing under the microscope a
large number of stigmas from female plants, not one could be found to which
pollen-grains of thyme did not adhere. The seeds were carefully collected from
the eleven female plants, and they weighed 98.7 grains; and those from the seven
hermaphrodites 36.5 grains. This gives for an equal number of plants the ratio
of 100 to 58; and we here see, as in the last case, how much more fertile the
females are than the hermaphrodites. These two lots of seeds were sown
separately in two adjoining beds, and the seedlings from both the hermaphrodite
and female parent-plants consisted of both forms.
Satureia hortensis.
Eleven seedlings were raised in separate pots in a hotbed and afterwards kept in
the greenhouse. They consisted of ten females and of a single hermaphrodite.
Whether or not the conditions to which they had been subjected caused the great
excess of females I do not know. In the females the pistil is rather longer than
that of the hermaphrodite, and the stamens are mere rudiments, with minute
colourless anthers destitute of pollen. The windows of the greenhouse were left
open, and the flowers were incessantly visited by humble and hive bees. Although
the ten females did not produce a single grain of pollen, yet they were all
thoroughly well fertilised by the one hermaphrodite plant, and this is an
interesting fact. It should be added that no other plant of this species grew in
my garden. The seeds were collected from the finest female plant, and they
weighed 78 grains; whilst those from the hermaphrodite, which was a rather
larger plant than the female, weighed only 33.2 grains; that is, in the ratio of
100 to 43. The female form, therefore, is very much more fertile than the
hermaphrodite, as in the two last cases; but the hermaphrodite was necessarily
self-fertilised, and this probably diminished its fertility.
We may now consider the probable means by which so many of the Labiatae have
been separated into two forms, and the advantages thus gained. H. Muller
supposes that originally some individuals varied so as to produce more
conspicuous flowers; and that insects habitually visited these first, and then
dusted with their pollen visited and fertilised the less conspicuous flowers.
(7/17. 'Die Befruchtung der Blumen' pages 319, 326.) The production of pollen by
the latter plants would thus be rendered superfluous, and it would be
advantageous to the species that their stamens should abort, so as to save
useless expenditure. They would thus be converted into females. But another view
may be suggested: as the production of a large supply of seeds evidently is of
high importance to many plants, and as we have seen in the three foregoing cases
that the females produce many more seeds than the hermaphrodites, increased
fertility seems to me the more probable cause of the formation and separation of
the two forms. From the data above given it follows that ten plants of Thymus
serpyllum, if half consisted of hermaphrodites and half of females, would yield
seeds compared with ten hermaphrodite plants in the ratio of 100 to 72. Under
similar circumstances the ratio with Satureia hortensis (subject to the doubt
from the self-fertilisation of the hermaphrodite) would be as 100 to 60. Whether
the two forms originated in certain individuals varying and producing more seed
than usual, and consequently producing less pollen; or in the stamens of certain
individuals tending from some unknown cause to abort, and consequently producing
more seed, it is impossible to decide; but in either case, if the tendency to
the increased production of seed were steadily favoured, the result would be the
complete abortion of the male organs. I shall presently discuss the cause of the
smaller size of the female corolla.
[Scabiosa arvensis (DIPSACEAE).
It has been shown by H. Muller that this species exists in Germany under an
hermaphrodite and female form. (7/18. 'Die Befruchtung der Blumen' page 368. The
two forms occur not only in Germany, but in England and France. Lecoq
'Geographie Bot.' 1857 tome 6 pages 473, 477, says that male plants as well as
hermaphrodites and females coexist; it is, however, possible that he may have
been deceived by the flowers being so strongly proterandrous. From what Lecoq
says, S. succisa likewise appears to occur under two forms in France.) In my
neighbourhood (Kent) the female plants do not nearly equal in number the
hermaphrodites. The stamens of the females vary much in their degree of
abortion; in some plants they are quite short and produce no pollen; in others
they reach to the mouth of the corolla, but their anthers are not half the
proper size, never dehisce, and contain but few pollen-grains, these being
colourless and of small diameter. The hermaphrodite flowers are strongly
proterandrous, and H. Muller shows that, whilst all the stigmas on the same
flower-head are mature at nearly the same time, the stamens dehisce one after
the other; so that there is a great excess of pollen, which serves to fertilise
the female plants. As the production of pollen by one set of plants is thus
rendered superfluous, their male organs have become more or less completely
aborted. Should it be hereafter proved that the female plants yield, as is
probable, more seeds than the hermaphrodites, I should be inclined to extend the
same view to this plant as to the Labiatae. I have also observed the existence
of two forms in our endemic S. succisa, and in the exotic S. atro-purpurea. In
the latter plant, differently to what occurs in S. arvensis, the female flowers,
especially the larger circumferential ones, are smaller than those of the
hermaphrodite form. According to Lecoq, the female flower-heads of S. succisa
are likewise smaller than those of what he calls the male plants, but which are
probably hermaphrodites.
Echium vulgare (BORAGINEAE).
The ordinary hermaphrodite form appears to be proterandrous, and nothing more
need be said about it. The female differs in having a much smaller corolla and
shorter pistil, but a well-developed stigma. The stamens are short; the anthers
do not contain any sound pollen-grains, but in their place yellow incoherent
cells which do not swell in water. Some plants were in an intermediate
condition; that is, had one or two or three stamens of proper length with
perfect anthers, the other stamens being rudimentary. In one such plant half of
one anther contained green perfect pollen-grains, and the other half yellowish-
green imperfect grains. Both forms produced seed, but I neglected to observe
whether in equal numbers. As I thought that the state of the anthers might be
due to some fungoid growth, I examined them both in the bud and mature state,
but could find no trace of mycelium. In 1862 many female plants were found; and
in 1864, 32 plants were collected in two localities, exactly half of which were
hermaphrodites, fourteen were females, and two in an intermediate condition. In
1866, 15 plants were collected in another locality, and these consisted of four
hermaphrodites and eleven females. I may add that this season was a wet one,
which shows that the abortion of the stamens can hardly be due to the dryness of
the sites where the plants grew, as I at one time thought probable. Seeds from
an hermaphrodite were sown in my garden, and of the 23 seedlings raised, one
belonged to the intermediate form, all the others being hermaphrodites, though
two or three of them had unusually short stamens. I have consulted several
botanical works, but have found no record of this plant varying in the manner
here described.
Plantago lanceolata (PLANTAGINEAE).
Delpino states that this plant presents in Italy three forms, which graduate
from an anemophilous into an entomophilous condition. According to H. Muller,
there are only two forms in Germany, neither of which show any special
adaptation for insect fertilisation, and both appear to be hermaphrodites.
(7/19. 'Die Befruchtung' etc. page 342.) But I have found in two localities in
England female and hermaphrodite forms existing together; and the same fact has
been noticed by others. (7/20. Mr. C.W. Crocker 'The Gardener's Chronicle' 1864
page 294. Mr. W. Marshall writes to me to the same effect from Ely.) The females
are less frequent than the hermaphrodites; their stamens are short, and their
anthers, which are of a brighter green whilst young than those of the other
form, dehisce properly, yet contain either no pollen, or a small amount of
imperfect grains of variable size. All the flower-heads on a plant belong to the
same form. It is well known that this species is strongly proterogynous, and I
found that the protruding stigmas of both the hermaphrodite and female flowers
were penetrated by pollen-tubes, whilst their own anthers were immature and had
not escaped out of the bud. Plantago media does not present two forms; but it
appears from Asa Gray's description, that such is the case with four of the
North American species. (7/21. 'Manual of the Botany of the Northern United
States' 2nd edition 1856 page 269. See also 'American Journal of Science'
November 1862 page 419 and 'Proceedings of the American Academy of Science'
October 14, 1862 page 53.) The corolla does not properly expand in the short-
stamened form of these plants.
Cnicus, Serratula, Eriophorum.
In the Compositae, Cnicus palustris and acaulis are said by Sir J.E. Smith to
exist as hermaphrodites and females, the former being the more frequent. With
Serratula tinctoria a regular gradation may be followed from the hermaphrodite
to the female form; in one of the latter plants the stamens were so tall that
the anthers embraced the style as in the hermaphrodites, but they contained only
a few grains of pollen, and these in an aborted condition; in another female, on
the other hand, the anthers were much more reduced in size than is usual.
Lastly, Dr. Dickie has shown that with Eriophorum angustifolium (Cyperaceae)
hermaphrodite and female forms exist in Scotland and the Arctic regions, both of
which yield seed. (7/22. Sir J.E. Smith 'Transactions of the Linnean Society'
volume 13 page 599. Dr. Dickie 'Journal of the Linnean Society Botany' volume 9
1865 page 161.)]
It is a curious fact that in all the foregoing polygamous, dioecious, and gyno-
dioecious plants in which any difference has been observed in the size of the
corolla in the two or three forms, it is rather larger in the females, which
have their stamens more or less or quite rudimentary, than in the hermaphrodites
or males. This holds good with Euonymus, Rhamnus catharticus, Ilex, Fragaria,
all or at least most of the before-named Labiatae, Scabiosa atro-purpurea, and
Echium vulgare. So it is, according to Von Mohl, with Cardamine amara, Geranium
sylvaticum, Myosotis, and Salvia. On the other hand, as Von Mohl remarks, when a
plant produces hermaphrodite flowers and others which are males owing to the
more or less complete abortion of the female organs, the corollas of the males
are not at all increased in size, or only exceptionally and in a slight degree,
as in Acer. (7/23. 'Botanische Zeitung' 1863 page 326.) It seems therefore
probable that the decreased size of the female corollas in the foregoing cases
is due to a tendency to abortion spreading from the stamens to the petals. We
see how intimately these organs are related in double flowers, in which the
stamens are readily converted into petals. Indeed some botanists believe that
petals do not consist of leaves directly metamorphosed, but of metamorphosed
stamens. That the lessened size of the corolla in the above case is in some
manner an indirect result of the modification of the reproductive organs is
supported by the fact that in Rhamnus catharticus not only the petals but the
green and inconspicuous sepals of the female have been reduced in size; and in
the strawberry the flowers are largest in the males, mid-sized in the
hermaphrodites, and smallest in the females. These latter cases,--the
variability in the size of the corolla in some of the above species, for
instance in the common thyme,--together with the fact that it never differs
greatly in size in the two forms--make me doubt much whether natural selection
has come into play;--that is whether, in accordance with H. Muller's belief, the
advantage derived from the polleniferous flowers being visited first by insects
has been sufficient to lead to a gradual reduction of the corolla of the female.
We should bear in mind that as the hermaphrodite is the normal form, its corolla
has probably retained its original size. (7/24. It does not appear to me that
Kerner's view 'Die Schutzmittel des Pollens' 1873 page 56, can be accepted in
the present cases, namely that the larger corolla in the hermaphrodites and
males serves to protect their pollen from rain. In the genus Thymus, for
instance, the aborted anthers of the female are much better protected than the
perfect ones of the hermaphrodite.) An objection to the above view should not be
passed over; namely, that the abortion of the stamens in the females ought to
have added through the law of compensation to the size of the corolla; and this
perhaps would have occurred, had not the expenditure saved by the abortion of
the stamens been directed to the female reproductive organs, so as to give to
this form increased fertility.
CHAPTER VIII.
CLEISTOGAMIC FLOWERS.
General character of cleistogamic flowers.
List of the genera producing such flowers, and their distribution in the
vegetable series.
Viola, description of the cleistogamic flowers in the several species; their
fertility compared with that of the perfect flowers.
Oxalis acetosella.
O. sensitiva, three forms of cleistogamic flowers.
Vandellia.
Ononis.
Impatiens.
Drosera.
Miscellaneous observations on various other cleistogamic plants.
Anemophilous species producing cleistogamic flowers.
Leersia, perfect flowers rarely developed.
Summary and concluding remarks on the origin of cleistogamic flowers.
The chief conclusions which may be drawn from the observations in this volume.
It was known even before the time of Linnaeus that certain plants produced two
kinds of flowers, ordinary open, and minute closed ones; and this fact formerly
gave rise to warm controversies about the sexuality of plants. These closed
flowers have been appropriately named cleistogamic by Dr. Kuhn. (8/1.
'Botanische Zeitung' 1867 page 65.) They are remarkable from their small size
and from never opening, so that they resemble buds; their petals are rudimentary
or quite aborted; their stamens are often reduced in number, with the anthers of
very small size, containing few pollen-grains, which have remarkably thin
transparent coats, and generally emit their tubes whilst still enclosed within
the anther-cells; and, lastly, the pistil is much reduced in size, with the
stigma in some cases hardly at all developed. These flowers do not secrete
nectar or emit any odour; from their small size, as well as from the corolla
being rudimentary, they are singularly inconspicuous. Consequently insects do
not visit them; nor if they did, could they find an entrance. Such flowers are
therefore invariably self-fertilised; yet they produce an abundance of seed. In
several cases the young capsules bury themselves beneath the ground, and the
seeds are there matured. These flowers are developed before, or after, or
simultaneously with the perfect ones. Their development seems to be largely
governed by the conditions to which the plants are exposed, for during certain
seasons or in certain localities only cleistogamic or only perfect flowers are
produced.
Dr. Kuhn, in the article above referred to, gives a list of 44 genera including
species which bear flowers of this kind. To this list I have added some genera,
and the authorities are appended in a footnote. I have omitted three names, from
reasons likewise given in the footnote. But it is by no means easy to decide in
all cases whether certain flowers ought to be ranked as cleistogamic. For
instance, Mr. Bentham informs me that in the South of France some of the flowers
on the vine do not fully open and yet set fruit; and I hear from two experienced
gardeners that this is the case with the vine in our hothouses; but as the
flowers do not appear to be completely closed it would be imprudent to consider
them as cleistogamic. The flowers of some aquatic and marsh plants, for instance
of Ranunculus aquatalis, Alisma natans, Subularia, Illecebrum, Menyanthes, and
Euryale, remain closely shut as long as they are submerged, and in this
condition fertilise themselves. (8/2. Delpino 'Sull' Opera, la Distribuzione dei
Sessi nelle Piante' etc. 1867 page 30. Subularia, however, sometimes has its
flowers fully expanded beneath the water, see Sir J.E. Smith 'English Flora'
volume 3 1825 page 157. For the behaviour of Menyanthes in Russia see Gillibert
in 'Act. Acad. St. Petersb.' 1777 part 2 page 45.--On Euryale 'Gardener's
Chronicle' 1877 page 280.) They behave in this manner, apparently as a
protection to their pollen, and produce open flowers when exposed to the air; so
that these cases seem rather different from those of true cleistogamic flowers,
and have not been included in the list. Again, the flowers of some plants which
are produced very early or very late in the season do not properly expand; and
these might perhaps be considered as incipiently cleistogamic; but as they do
not present any of the remarkable peculiarities proper to the class, and as I
have not found any full record of such cases, they are not entered in the list.
When, however, it is believed on fairly good evidence that the flowers on a
plant in its native country do not open at any hour of the day or night, and yet
set seeds capable of germination, these may fairly be considered as
cleistogamic, notwithstanding that they present no peculiarities of structure. I
will now give as complete a list of the genera containing cleistogamic species
as I have been able to collect.
TABLE 8.38. List of genera including cleistogamic species (chiefly after Kuhn).
(8/3. I have omitted Trifolium and Arachis from the list, because Von Mohl says
'Botanische Zeitung' 1863 page 312, that the flower-stems merely draw the
flowers beneath the ground, and that these do not appear to be properly
cleistogamic. Correa de Mello 'Journal of the Linnean Society Botany' volume 11
1870 page 254, observed plants of Arachis in Brazil, and could never find such
flowers. Plantago has been omitted because as far as I can discover it produces
hermaphrodite and female flower-heads, but not cleistogamic flowers.
Krascheninikowia (vel Stellaria) has been omitted because it seems very doubtful
from Maximowicz' description whether the lower flowers which have no petals or
very small ones, and barren stamens or none, are cleistogamic; the upper
hermaphrodite flowers are said never to produce fruit, and therefore probably
act as males. Moreover in Stellaria graminea, as Babington remarks 'British
Botany' 1851 page 51, "shorter and longer petals accompany an imperfection of
the stamens or germen."
I have added to the list the following cases:
Several Acanthaceae, for which see J. Scott in 'Journal of Botany' London new
series volume 1 1872 page 161.
With respect to salvia see Dr. Ascherson in 'Botanische Zeitung' 1871 page 555.
For Oxybaphus and Nyctaginia see Asa Gray in 'American Naturalist' November 1873
page 692.
From Dr. Torrey's account of Hottonia inflata 'Bulletin of the Torrey Botanical
Club' volume 2 June 1871, it is manifest that this plant produces true
cleistogamic flowers.
For Pavonia see Bouche in 'Sitzungsberichte d. Gesellsch. Natur. Freunde'
October 20, 1874 page 90.
I have added Thelymitra, as from the account given by Mr. Fitzgerald in his
magnificent work on 'Australian Orchids' it appears that the flowers of this
plant in its native home never open, but they do not appear to be reduced in
size. Nor is this the case with the flowers of certain species of Epidendron,
Cattleya, etc. see second edition of my 'Fertilisation of Orchids' page 147,
which without expanding produce capsules. It is therefore doubtful whether these
Orchideae ought to have been included in the list. From what Duval-Jouve says
about Cryptostachys in 'Bulletin of the Soc. Bot. de France' tome 10 1863 page
195, this plant appears to produce cleistogamic flowers.
the other additions to the list are noticed in my text.)
DICOTYLEDONS.
BORAGINEAE:
Eritrichium.
CONVOLVULACEAE:
Cuscuta.
SCROPHULARINEAE:
Scrophularia.
Linaria.
Vandellia.
ACANTHACEAE:
Cryphiacanthus.
Eranthemum.
Daedalacanthus.
Dipteracanthus.
Aechmanthera.
Ruellia.
LABIATAE:
Lamium.
Salvia.
NYCTAGINEAE:
Oxybaphus.
Nyctaginia.
ASCLEPIADAE:
Stapelia.
CAMPANULACEAE:
Specularia.
Campanula.
PRIMULACEAE:
Hottonia.
COMPOSITAE:
Anandria.
CRUCIFERAE:
Heterocarpaea.
VIOLACEAE:
Viola.
CISTINEAE:
Helianthemum.
Lechea.
MALVEACEAE:
Pavonia.
MALPIGHIACEAE:
Gaudichaudia.
Aspicarpa.
Camarea.
Janusia.
POLYGALEAE:
Polygala.
BALSAMINEAE:
Impatiens.
GERANIACAEA:
Oxalis.
LEGUMINOSAE:
Ononis.
Parochaetus.
Chapmannia.
Stylosanthus.
Lespedeza.
Vicia.
Lathyrus.
Martinsia vel Neurocarpum.
Amphicarpaea.
Glycine.
Galactia.
Voandzeia.
DROSERACEAE:
Drosera.
MONOCOTYLEDONS.
JUNCEAE:
Juncus.
GRAMINEAE:
Leersia.
Hordeum.
Cryptostachys.
COMMELINEAE:
Commelina.
PONTEDERACEAE:
Monochoria.
ORCHIDEAE:
Schomburgkia.
Cattleya.
Epidendron.
Thelymitra.
The first point that strikes us in considering this list of 55 genera, is that
they are very widely distributed in the vegetable series. They are more common
in the family of the Leguminosae than in any other, and next in order in that of
the Acanthaceae and Malpighiaceae. A large number, but not all the species, of
certain genera, as of Oxalis and Viola, bear cleistogamic as well as ordinary
flowers. A second point which deserves notice is that a considerable proportion
of the genera produce more or less irregular flowers; this is the case with
about 32 out of the 55 genera, but to this subject I shall recur.
I formerly made many observations on cleistogamic flowers, but only a few of
them are worth giving, since the appearance of an admirable paper by Hugo Von
Mohl, whose examination was in some respects much more complete than mine. (8/4.
'Botanische Zeitung' 1863 page 309-28.) His paper includes also an interesting
history of our knowledge on the subject.
Viola canina.
The calyx of the cleistogamic flowers differs in no respect from that of the
perfect ones. The petals are reduced to five minute scales; the lower one, which
represents the lower lip, is considerably larger than the others, but with no
trace of the spur-like nectary; its margins are smooth, whilst those of the
other four scale-like petals are papillose. D. Muller of Upsala says that in the
specimens which he observed the petals were completely aborted. (8/5. Ibid. 1857
page 730. This paper contains the first full and satisfactory account of any
cleistogamic flower.) The stamens are very small, and only the two lower ones
are provided with anthers, which do not cohere together as in the perfect
flowers. The anthers are minute, with the two cells or loculi remarkably
distinct; they contain very little pollen in comparison with those of the
perfect flowers. The connective expands into a membranous hood-like shield which
projects above the anther-cells. These two lower stamens have no vestige of the
curious appendages which secrete nectar in the perfect flowers. The three other
stamens are destitute of anthers and have broader filaments, with their terminal
membranous expansions flatter or not so hood-like as those of the two
antheriferous stamens. The pollen-grains have remarkably thin transparent coats;
when exposed to the air they shrivel up quickly; when placed in water they
swell, and are then 8-10/7000 of an inch in diameter, and therefore of smaller
size than the ordinary pollen-grains similarly treated, which have a diameter of
13-14/7000 of an inch. In the cleistogamic flowers, the pollen-grains, as far as
I could see, never naturally fall out of the anther-cells, but emit their tubes
through a pore at the upper end. I was able to trace the tubes from the grains
some way down the stigma. The pistil is very short, with the style hooked, so
that its extremity, which is a little enlarged or funnel-shaped and represents
the stigma, is directed downwards, being covered by the two membranous
expansions of the antheriferous stamens. It is remarkable that there is an open
passage from the enlarged funnel-shaped extremity to within the ovarium; this
was evident, as slight pressure caused a bubble of air, which had been drawn in
by some accident, to travel freely from one end to the other: a similar passage
was observed by Michalet in V. alba. The pistil therefore differs considerably
from that of the perfect flower; for in the latter it is much longer, and
straight with the exception of the rectangularly bent stigma; nor is it
perforated by an open passage.
The ordinary or perfect flowers have been said by some authors never to produce
capsules; but this is an error, though only a small proportion of them do so.
This appears to depend in some cases on their anthers not containing even a
trace of pollen, but more generally on bees not visiting the flowers. I twice
covered with a net a group of flowers, and marked with threads twelve of them
which had not as yet expanded. This precaution is necessary, for though as a
general rule the perfect flowers appear considerably before the cleistogamic
ones, yet occasionally some of the latter are produced early in the season, and
their capsules might readily be mistaken for those produced by the perfect
flowers. Not one of the twelve marked perfect flowers yielded a capsule, whilst
others under the net which had been artificially fertilised produced five
capsules; and these contained exactly the same average number of seeds as some
capsules from flowers outside the net which had been fertilised by bees. I have
repeatedly seen Bombus hortorum, lapidarius, and a third species, as well as
hive-bees, sucking the flowers of this violet: I marked six which were thus
visited, and four of them produced fine capsules; the two others were gnawed off
by some animal. I watched Bombus hortorum for some time, and whenever it came to
a flower which did not stand in a convenient position to be sucked, it bit a
hole through the spur-like nectary. Such ill-placed flowers would not yield any
seed or leave descendants; and the plants bearing them would thus tend to be
eliminated through natural selection.
The seeds produced by the cleistogamic and perfect flowers do not differ in
appearance or number. On two occasions I fertilised several perfect flowers with
pollen from other individuals, and afterwards marked some cleistogamic flowers
on the same plants; and the result was that 14 capsules produced by the perfect
flowers contained on an average 9.85 seeds; and 17 capsules from the
cleistogamic ones contained 9.64 seeds,--an amount of difference of no
significance. It is remarkable how much more quickly the capsules from the
cleistogamic flowers are developed than those from the perfect ones; for
instance, several perfect flowers were cross-fertilised on April 14th, 1863, and
a month afterwards (May 15th) eight young cleistogamic flowers were marked with
threads; and when the two sets of capsules thus produced were compared on June
3rd, there was scarcely any difference between them in size.
Viola odorata (WHITE-FLOWERED, SINGLE, CULTIVATED VARIETY).
The petals are represented by mere scales as in the last species; but
differently from in the last, all five stamens are provided with diminutive
anthers. Small bundles of pollen-tubes were traced from the five anthers into
the somewhat distant stigma. The capsules produced by these flowers bury
themselves in the soil, if it be loose enough, and there mature themselves.
(8/6. Vaucher says 'Hist. Phys. des Plantes d'Europe' tome 3 1844 page 309, that
V. hirta and collina likewise bury their capsules. See also Lecoq 'Geograph.
Bot.' tome 5 1856 page 180.) Lecoq says that it is only these latter capsules
which possess elastic valves; but I think this must be a misprint, as such
valves would obviously be of no use to the buried capsules, but would serve to
scatter the seeds of the sub-aerial ones, as in the other species of Viola. It
is remarkable that this plant, according to Delpino, does not produce
cleistogamic flowers in one part of Liguria, whilst the perfect flowers are
there abundantly fertile (8/7. 'Sull' Opera, la Distribuzione dei Sessi nelle
Piante' etc. 1867 page 30.); on the other hand, cleistogamic flowers are
produced by it near Turin. Another fact is worth giving as an instance of
correlated development: I found on a purple variety, after it had produced its
perfect double flowers, and whilst the white single variety was bearing its
cleistogamic flowers, many bud-like bodies which from their position on the
plant were certainly of a cleistogamic nature. They consisted, as could be seen
on bisecting them, of a dense mass of minute scales closely folded over one
another, exactly like a cabbage-head in miniature. I could not detect any
stamens, and in the place of the ovarium there was a little central column. The
doubleness of the perfect flowers had thus spread to the cleistogamic ones,
which therefore were rendered quite sterile.
Viola hirta.
The five stamens of the cleistogamic flowers are provided, as in the last case,
with small anthers, from all of which pollen-tubes proceed to the stigma. The
petals are not quite so much reduced as in V. canina, and the short pistil
instead of being hooked is merely bent into a rectangle. Of several perfect
flowers which I saw visited by hive-and humble-bees, six were marked, but they
produced only two capsules, some of the others having been accidentally injured.
M. Monnier was therefore mistaken in this case as in that of V. odorata, in
supposing that the perfect flowers always withered away and aborted. He states
that the peduncles of the cleistogamic flowers curve downwards and bury the
ovaries beneath the soil. (8/8. These statements are taken from Professor
Oliver's excellent article in the 'Natural History Review' July 1862 page 238.
With respect to the supposed sterility of the perfect flowers in this genus see
also Timbal-Lagrave 'Botanische Zeitung' 1854 page 772.) I may here add that
Fritz Muller, as I hear from his brother, has found in the highlands of Southern
Brazil a white-flowered species of violet which bears subterranean cleistogamic
flowers.
Viola nana.
Mr. Scott sent me seeds of this Indian species from the Sikkim Terai, from which
I raised many plants, and from these other seedlings during several successive
generations. They produced an abundance of cleistogamic flowers during the whole
of each summer, but never a perfect one. When Mr. Scott wrote to me his plants
in Calcutta were behaving similarly, though his collector saw the species in
flower in its native site. This case is valuable as showing that we ought not to
infer, as has sometimes been done, that a species does not bear perfect flowers
when growing naturally, because it produces only cleistogamic flowers under
culture. The calyx of these flowers is sometimes formed of only three sepals;
two being actually suppressed and not merely coherent with the others; this
occurred with five out of thirty flowers which were examined for this purpose.
The petals are represented by extremely minute scales. Of the stamens, two bear
anthers which are in the same state as in the previous species, but, as far as I
could judge, each of the two cells contained only from 20 to 25 delicate
transparent pollen-grains. These emitted their tubes in the usual manner. The
three other stamens bore very minute rudimentary anthers, one of which was
generally larger than the other two, but none of them contained any pollen. In
one instance, however, a single cell of the larger rudimentary anther included a
little pollen. The style consists of a short flattened tube, somewhat expanded
at its upper end, and this forms an open channel leading into the ovarium, as
described under V. canina. It is slightly bent towards the two fertile anthers.
Viola Roxburghiana.
This species bore in my hothouse during two years a multitude of cleistogamic
flowers, which resembled in all respects those of the last species; but no
perfect ones were produced. Mr. Scott informs me that in India it bears perfect
flowers only during the cold season, and that these are quite fertile. During
the hot, and more especially during the rainy season, it bears an abundance of
cleistogamic flowers.
Many other species, besides the five now described, produce cleistogamic
flowers; this is the case, according to D. Muller, Michalet, Von Mohl, and
Hermann Muller, with V. elatior, lancifolia, sylvatica, palustris, mirabilis,
bicolor, ionodium, and biflora. But V. tricolor does not produce them.
Michalet asserts that V. palustris produces near Paris only perfect flowers,
which are quite fertile; but that when the plant grows on mountains cleistogamic
flowers are produced; and so it is with V. biflora. The same author states that
he has seen in the case of V. alba flowers intermediate in structure between the
perfect and cleistogamic ones. According to M. Boisduval, an Italian species, V.
Ruppii, never bears in France "des fleurs bien apparentes, ce qui ne l'empeche
pas de fructifier."
It is interesting to observe the gradation in the abortion of the parts in the
cleistogamic flowers of the several foregoing species. It appears from the
statements by D. Muller and Von Mohl that in V. mirabilis the calyx does not
remain quite closed; all five stamens are provided with anthers, and some
pollen-grains probably fall out of the cells on the stigma, instead of
protruding their tubes whilst still enclosed, as in the other species. In V.
hirta all five stamens are likewise antheriferous; the petals are not so much
reduced and the pistil not so much modified as in the following species. In V.
nana and elatior only two of the stamens properly bear anthers, but sometimes
one or even two of the others are thus provided. Lastly, in V. canina never more
than two of the stamens, as far as I have seen, bear anthers; the petals are
much more reduced than in V. hirta, and according to D. Muller are sometimes
quite absent.
Oxalis acetosella.
The existence of cleistogamic flowers on this plant was discovered by Michalet.
(8/9. 'Bulletin Soc. Bot. de France' tome 7 1860 page 465.) They have been fully
described by Von Mohl, and I can add hardly anything to his description. In my
specimens the anthers of the five longer stamens were nearly on a level with the
stigmas; whilst the smaller and less plainly bilobed anthers of the five shorter
stamens stood considerably below the stigmas, so that their tubes had to travel
some way upwards. According to Michalet these latter anthers are sometimes quite
aborted. In one case the tubes, which ended in excessively fine points, were
seen by me stretching upwards from the lower anthers towards the stigmas, which
they had not as yet reached. My plants grew in pots, and long after the perfect
flowers had withered they produced not only cleistogamic but a few minute open
flowers, which were in an intermediate condition between the two kinds. In one
of these the pollen-tubes from the lower anthers had reached the stigmas, though
the flower was open. The footstalks of the cleistogamic flowers are much shorter
than those of the perfect flowers, and are so much bowed downwards that they
tend, according to Von Mohl, to bury themselves in the moss and dead leaves on
the ground. Michalet also says that they are often hypogean. In order to
ascertain the number of seeds produced by these flowers, I marked eight of them;
two failed, one cast its seed abroad, and the remaining five contained on an
average 10.0 seeds per capsule. This is rather above the average 9.2, which
eleven capsules from perfect flowers fertilised with their own pollen yielded,
and considerably above the average 7.9, from the capsules of perfect flowers
fertilised with pollen from another plant; but this latter result must, I think,
have been accidental.
Hildebrand, whilst searching various Herbaria, observed that many other species
of Oxalis besides O. acetosella produce cleistogamic flowers (8/10.
'Monatsbericht der Akad. der Wiss. zu Berlin' 1866 page 369.); and I hear from
him that this is the case with the heterostyled trimorphic O. incarnata from the
Cape of Good Hope.
Oxalis (Biophytum) sensitiva.
This plant is ranked by many botanists as a distinct genus, but as a sub-genus
by Bentham and Hooker. Many of the early flowers on a mid-styled plant in my
hothouse did not open properly, and were in an intermediate condition between
cleistogamic and perfect. Their petals varied from a rudiment to about half
their proper size; nevertheless they produced capsules. I attributed their state
to unfavourable conditions, for later in the season fully expanded flowers of
the proper size appeared. But Mr. Thwaites afterwards sent me from Ceylon a
number of long-styled, mid-styled, and short-styled flower-stalks preserved in
spirits; and on the same stalks with the perfect flowers, some of which were
fully expanded and others still in bud, there were small bud-like bodies
containing mature pollen, but with their calyces closed. These cleistogamic
flowers do not differ much in structure from the perfect ones of the
corresponding form, with the exception that their petals are reduced to
extremely minute, barely visible scales, which adhere firmly to the rounded
bases of the shorter stamens. Their stigmas are much less papillose, and smaller
in about the ratio of 13 to 20 divisions of the micrometer, as measured
transversely from apex to apex, than the stigmas of the perfect flowers. The
styles are furrowed longitudinally, and are clothed with simple as well as
glandular hairs, but only in the cleistogamic flowers produced by the long-
styled and mid-styled forms. The anthers of the longer stamens are a little
smaller than the corresponding ones of the perfect flowers, in about the ratio
of 11 to 14. They dehisce properly, but do not appear to contain much pollen.
Many pollen-grains were attached by short tubes to the stigmas; but many others,
still adhering to the anthers, had emitted their tubes to a considerable length,
without having come in contact with the stigmas. Living plants ought to be
examined, as the stigmas, at least of the long-styled form, project beyond the
calyx, and if visited by insects (which, however, is very improbable) might be
fertilised with pollen from a perfect flower. The most singular fact about the
present species is that long-styled cleistogamic flowers are produced by the
long-styled plants, and mid-styled as well as short-styled cleistogamic flowers
by the other two forms; so that there are three kinds of cleistogamic and three
kinds of perfect flowers produced by this one species! Most of the heterostyled
species of Oxalis are more or less sterile, many absolutely so, if
illegitimately fertilised with their own-form pollen. It is therefore probable
that the pollen of the cleistogamic flowers has been modified in power, so as to
act on their own stigmas, for they yield an abundance of seeds. We may perhaps
account for the cleistogamic flowers consisting of the three forms, through the
principle of correlated growth, by which the cleistogamic flowers of the double
violet have been rendered double.
Vandellia nummularifolia.
Dr. Kuhn has collected all the notices with respect to cleistogamic flowers in
this genus, and has described from dried specimens those produced by an
Abyssinian species. (8/11. 'Botanische Zeitung' 1867 page 65.) Mr. Scott sent me
from Calcutta seeds of the above common Indian weed, from which many plants were
successively raised during several years. The cleistogamic flowers are very
small, being when fully mature under 1/20 of an inch (1.27 millimetres) in
length. The calyx does not open, and within it the delicate transparent corolla
remains closely folded over the ovarium. There are only two anthers instead of
the normal number of four, and their filaments adhere to the corolla. The cells
of the anthers diverge much at their lower ends and are only 5/700 of an inch
(.181 millimetres) in their longer diameter. They contain but few pollen-grains,
and these emit their tubes whilst still within the anther. The pistil is very
short, and is surmounted by a bilobed stigma. As the ovary grows the two anthers
together with the shrivelled corolla, all attached by the dried pollen-tubes to
the stigma, are torn off and carried upwards in the shape of a little cap. The
perfect flowers generally appear before the cleistogamic, but sometimes
simultaneously with them. During one season a large number of plants produced no
perfect flowers. It has been asserted that the latter never yield capsules; but
this is a mistake, as they do so even when insects are excluded. Fifteen
capsules from cleistogamic flowers on plants growing under favourable conditions
contained on an average 64.2 seeds, with a maximum of 87; whilst 20 capsules
from plants growing much crowded yielded an average of only 48. Sixteen capsules
from perfect flowers artificially crossed with pollen from another plant
contained on an average 93 seeds, with a maximum of 137. Thirteen capsules from
self-fertilised perfect flowers gave an average of 62 seeds, with a maximum of
135. Therefore the capsules from the cleistogamic flowers contained fewer seeds
than those from perfect flowers when cross-fertilised, and slightly more than
those from perfect flowers self-fertilised.
Dr. Kuhn believes that the Abyssinian V. sessiflora does not differ specifically
from the foregoing species. But its cleistogamic flowers apparently include four
anthers instead of two as above described. The plants, moreover, of V.
sessiflora produce subterranean runners which yield capsules; and I never saw a
trace of such runners in V. nummularifolia, although many plants were
cultivated.
Linaria spuria.
Michalet says that short, thin, twisted branches are developed from the buds in
the axils of the lower leaves, and that these bury themselves in the ground.
(8/12. 'Bulletin Soc. Bot. de France' tome 7 1860 page 468.) They there produce
flowers not offering any peculiarity in structure, excepting that their
corollas, though properly coloured, are deformed. These flowers may be ranked as
cleistogamic, as they are developed, and not merely drawn, beneath the ground.
Ononis columnae.
Plants were raised from seeds sent me from Northern Italy. The sepals of the
cleistogamic flowers are elongated and closely pressed together; the petals are
much reduced in size, colourless, and folded over the interior organs. The
filaments of the ten stamens are united into a tube, and this is not the case,
according to Von Mohl, with the cleistogamic flowers of other Leguminosae. Five
of the stamens are destitute of anthers, and alternate with the five thus
provided. The two cells of the anthers are minute, rounded and separated from
one another by connective tissue; they contain but few pollen-grains, and these
have extremely delicate coats. The pistil is hook-shaped, with a plainly
enlarged stigma, which is curled down, towards the anthers; it therefore differs
much from that of the perfect flower. During the year 1867 no perfect flowers
were produced, but in the following year there were both perfect and
cleistogamic ones.
Ononis minutissima.
My plants produced both perfect and cleistogamic flowers; but I did not examine
the latter. Some of the former were crossed with pollen from a distinct plant,
and six capsules thus obtained yielded on an average 3.66 seeds, with a maximum
of 5 in one. Twelve perfect flowers were marked and allowed to fertilise
themselves spontaneously under a net, and they yielded eight capsules,
containing on an average 2.38 seeds, with a maximum of 3 in one. Fifty-three
capsules produced by the cleistogamic flowers contained on an average 4.1 seeds,
so that these were the most productive of all; and the seeds themselves looked
finer even than those from the crossed perfect flowers. According to Mr. Bentham
O. parviflora likewise bears cleistogamic flowers; and he informs me that these
flowers are produced by all three species early in the spring; whilst the
perfect ones appear afterwards, and therefore in a reversed order compared with
those of Viola and Oxalis. Some of the species, for instance Ononis columnae,
bear a fresh crop of cleistogamic flowers in the autumn.
Lathyrus nissolia.
This plant apparently offers a case of the first stage in the production of
cleistogamic flowers, for on plants growing in a state of nature, many of the
flowers never expand and yet produce fine pods. Some of the buds are so large
that they seem on the point of expansion; others are much smaller, but none so
small as the true cleistogamic flowers of the foregoing species. As I marked
these buds with thread and examined them daily, there could be no mistake about
their producing fruit without having expanded.
Several other Leguminous genera produce cleistogamic flowers, as may be seen in
Table 8.38; but much does not appear to be known about them. Von Mohl says that
their petals are commonly rudimentary, that only a few of their anthers are
developed, their filaments are not united into a tube and their pistils are
hook-shaped. In three of the genera, namely Vicia, Amphicarpaea, and Voandzeia,
the cleistogamic flowers are produced on subterranean stems. The perfect flowers
of Voandzeia, which is a cultivated plant, are said never to produce fruit
(8/13. Correa de Mello 'Journal of the Linnean Society Botany' volume 11 1870
page 254, particularly attended to the flowering and fruiting of this African
plant, which is sometimes cultivated in Brazil.); but we should remember how
often fertility is affected by cultivation.
Impatiens fulva.
Mr. A.W. Bennett has published an excellent description, with figures, of this
plant. (8/14. 'Journal of the Linnean Society Botany' volume 13 1872 page 147.)
He shows that the cleistogamic and perfect flowers differ in structure at a very
early period of growth, so that the existence of the former cannot be due merely
to the arrested development of the latter,--a conclusion which indeed follows
from most of the previous descriptions. Mr. Bennett found on the banks of the
Wey that the plants which bore cleistogamic flowers alone were to those bearing
perfect flowers as 20 to 1; but we should remember that this is a naturalised
species. The perfect flowers are usually barren in England; but Professor Asa
Gray writes to me that after midsummer in the United States some or many of them
produce capsules.
Impatiens noli-me-tangere.
I can add nothing of importance to Von Mohl's description, excepting that one of
the rudimentary petals shows a vestige of a nectary, as Mr. Bennett likewise
found to be the case with I. fulva. As in this latter species all five stamens
produce some pollen, though small in amount; a single anther contains, according
to Von Mohl, not more than 50 grains, and these emit their tubes while still
enclosed within it. The pollen-grains of the perfect flowers are tied together
by threads, but not, so far as I could see, those of the cleistogamic flowers;
and a provision of this kind would here have been useless, as the grains can
never be transported by insects. The flowers of I. balsamina are visited by
humble-bees (8/15. H. Muller 'Die Befruchtung' etc. page 170.), and I am almost
sure that this is the case with the perfect flowers of I. noli-me-tangere. From
the perfect flowers of this latter species covered with a net eleven
spontaneously self-fertilised capsules were produced, and these yielded on an
average 3.45 seeds. Some perfect flowers with their anthers still containing an
abundance of pollen were fertilised with pollen from a distinct plant; and the
three capsules thus produced contained, to my surprise, only 2, 2, and 1 seed.
As I. balsamina is proterandrous, so probably is the present species; and if so,
cross-fertilisation was effected by me at too early a period, and this may
account for the capsules yielding so few seeds.
Drosera rotundifolia.
The first flower-stems which were thrown up by some plants in my greenhouse bore
only cleistogamic flowers. The petals of small size remained permanently closed
over the reproductive organs, but their white tips could just be seen between
the almost completely closed sepals. The pollen, which was scanty in amount, but
not so scanty as in Viola or Oxalis, remained enclosed within the anthers,
whence the tubes proceeded and penetrated the stigma. As the ovarium swelled the
little withered corolla was carried upwards in the form of a cap. These
cleistogamic flowers produced an abundance of seed. Later in the season perfect
flowers appeared. With plants in a state of nature the flowers open only in the
early morning, as I have been informed by Mr. Wallis, who particularly attended
to the time of their flowering. In the case of D. Anglica, the still folded
petals on some plants in my greenhouse opened just sufficiently to leave a
minute aperture; the anthers dehisced properly, but the pollen-grains adhered in
a mass to them, and thence emitted their tubes, which penetrated the stigmas.
These flowers, therefore, were in an intermediate condition, and could not be
called either perfect or cleistogamic.
A few miscellaneous observations may be added with respect to some other
species, as throwing light on our subject. Mr. Scott states that Eranthemum
ambiguum bears three kinds of flowers,--large, conspicuous, open ones, which are
quite sterile,--others of intermediate size, which are open and moderately
fertile--and lastly small closed or cleistogamic ones, which are perfectly
fertile. (8/16. 'Journal of Botany' London new series volume 1 1872 pages 161-
4.) Ruellia tuberosa, likewise one of the Acanthaceae, produces both open and
cleistogamic flowers; the latter yield from 18 to 24, whilst the former only
from 8 to 10 seeds; these two kinds of flowers are produced simultaneously,
whereas in several other members of the family the cleistogamic ones appear only
during the hot season. According to Torrey and Gray, the North American species
of Helianthemum, when growing in poor soil, produce only cleistogamic flowers.
The cleistogamic flowers of Specularia perfoliata are highly remarkable, as they
are closed by a tympanum formed by the rudimentary corolla, and without any
trace of an opening. The stamens vary from 3 to 5 in number, as do the sepals.
(8/17. Von Mohl 'Botanische Zeitung' 1863 pages 314 and 323. Dr. Bromfield
'Phytologist' volume 3 page 530, also remarks that the calyx of the cleistogamic
flowers is usually only 3-cleft, while that of the perfect flower is mostly 5-
cleft.) The collecting hairs on the pistil, which play so important a part in
the fertilisation of the perfect flowers, are here quite absent. Drs. Hooker and
Thomson state that some of the Indian species of Campanula produce two kinds of
flowers; the smaller ones being borne on longer peduncles with differently
formed sepals, and producing a more globose ovary. (8/18. 'Journal of the
Linnean Society' volume 2 1857 page 7. See also Professor Oliver in 'Natural
History Review' 1862 page 240.) The flowers are closed by a tympanum like that
in Specularia. Some of the plants produce both kinds of flowers, others only one
kind; both yield an abundance of seeds. Professor Oliver adds that he has seen
flowers on Campanula colorata in an intermediate condition between cleistogamic
and perfect ones.
The solitary almost sessile cleistogamic flowers produced by Monochoria
vaginalis are differently protected from those in any of the previous cases,
namely, within "a short sack formed of the membranous spathe, without any
opening or fissure." There is only a single fertile stamen; the style is almost
obsolete, with the three stigmatic surfaces directed to one side. Both the
perfect and cleistogamic flowers produce seeds. (8/19. Dr. Kirk 'Journal of the
Linnean Society' volume 8 1864 page 147.)
The cleistogamic flowers on some of the Malpighiaceae seem to be more profoundly
modified than those in any of the foregoing genera. According to A. de Jussieu
they are differently situated from the perfect flowers; they contain only a
single stamen, instead of 5 or 6; and it is a strange fact that this particular
stamen is not developed in the perfect flowers of the same species. (8/20.
'Archives du Museum' tome 3 1843 pages 35-38, 82-86, 589, 598.) The style is
absent or rudimentary; and there are only two ovaries instead of three. Thus
these degraded flowers, as Jussieu remarks, "laugh at our classifications, for
the greater number of the characters proper to the species, to the genus, to the
family, to the class disappear." I may add that their calyces are not glandular,
and as, according to Kerner, the fluid secreted by such glands generally serves
to protect the flowers from crawling insects, which steal the nectar without
aiding in their cross-fertilisation (8/21. 'Die Schutzmittel der Bluthen gegen
unberufene Gaste' 1876 page 25.), the deficiency of the glands in the
cleistogamic flowers of these plants may perhaps be accounted for by their not
requiring any such protection.
As the Asclepiadous genus Stapelia is said to produce cleistogamic flowers, the
following case may be worth giving. I have never heard of the perfect flowers of
Hoya carnosa setting seeds in this country, but some capsules were produced in
Mr. Farrer's hothouse; and the gardener detected that they were the product of
minute bud-like bodies, three or four of which could sometimes be found on the
same umbel with the perfect flowers. They were quite closed and hardly thicker
than their peduncles. The sepals presented nothing particular, but internally
and alternating with them, there were five small flattened heart-shaped
papillae, like rudiments of petals; but the homological nature of which appeared
doubtful to Mr. Bentham and Dr. Hooker. No trace of anthers or of stamens could
be detected; and I knew from having examined many cleistogamic flowers what to
look for. There were two ovaries, full of ovules, quite open at their upper
ends, with their edges festooned, but with no trace of a proper stigma. In all
these flowers one of the two ovaries withered and blackened long before the
other. The one perfect capsule, 3 1/2 inches in length, which was sent me, had
likewise been developed from a single carpel. This capsule contained an
abundance of plumose seeds, many of which appeared quite sound, but they did not
germinate when sown at Kew. Therefore the little bud-like flower which produced
this capsule probably was as destitute of pollen as were those which I examined.
Juncus bufonius and Hordeum.
All the species hitherto mentioned which produce cleistogamic flowers are
entomophilous; but four genera, Juncus, Hordeum, Cryptostachys, and Leersia are
anemophilous. Juncus bufonius is remarkable by bearing in parts of Russia only
cleistogamic flowers, which contain three instead of the six anthers found in
the perfect flowers. (8/22. See Dr. Ascherson's interesting paper in 'Botanische
Zeitung' 1871 page 551.) In the genus Hordeum it has been shown by Delpino that
the majority of the flowers are cleistogamic, some of the others expanding and
apparently allowing of cross-fertilisation. (8/23. 'Bollettini del Comizio
agrario Parmense.' Marzo e Aprile 1871. An abstract of this valuable paper is
given in 'Botanische Zeitung' 1871 page 537. See also Hildebrand on Hordeum in
'Monatsbericht d. K. Akad Berlin' October 1872 page 760.) I hear from Fritz
Muller that there is a grass in Southern Brazil, in which the sheath of the
uppermost leaf, half a metre in length, envelopes the whole panicle; and this
sheath never opens until the self-fertilised seeds are ripe. On the roadside
some plants had been cut down, whilst the cleistogamic panicles were developing,
and these plants afterwards produced free or unenclosed panicles of small size,
bearing perfect flowers.
Leersia oryzoides.
It has long been known that this plant produces cleistogamic flowers, but these
were first described with care by M. Duval-Jouve. (8/24. 'Bulletin Bot. Soc. de
France' tome 10 1863 page 194.) I procured plants from a stream near Reigate,
and cultivated them for several years in my greenhouse. The cleistogamic flowers
are very small, and usually mature their seeds within the sheaths of the leaves.
These flowers are said by Duval-Jouve to be filled by slightly viscid fluid; but
this was not the case with several that I opened; but there was a thin film of
fluid between the coats of the glumes, and when these were pressed the fluid
moved about, giving a similarly deceptive appearance of the whole inside of the
flower being thus filled. The stigma is very small and the filaments extremely
short; the anthers are less than 1/50 of an inch in length or about one-third of
the length of those in the perfect flowers. One of the three anthers dehisces
before the two others. Can this have any relation with the fact that in some
other species of Leersia only two stamens are fully developed? (8/25. Asa Gray
'Manual of Botany of the United States' 1856 page 540.) The anthers shed their
pollen on the stigma; at least in one instance this was clearly the case, and by
tearing open the anthers under water the grains were easily detached. Towards
the apex of the anther the grains are arranged in a single row and lower down in
two or three rows, so that they could be counted; and there were about 35 in
each cell, or 70 in the whole anther; and this is an astonishingly small number
for an anemophilous plant. The grains have very delicate coats, are spherical
and about 5/7000 of an inch (.0181 millimetres), whilst those of the perfect
flowers are about 7/7000 of an inch (.0254 millimetres) in diameter.
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