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.