Part 24 (1/2)
GENTIANACEAE: Menyanthes.
Limnanthemum.
Villarsia.
POLEMONIACEAE: Gilia.
CORDIEAE: Cordia.
BORAGINEAE: Pulmonaria.
VERBENACEAE: Aegiphila.
POLYGONEAE: Polygonum.
THYMELEAE: Thymelea.
MONOCOTYLEDONS.
PONTEDERIACEAE: Pontederia.
In some of these families the heterostyled condition must have been acquired at a very remote period. Thus the three closely allied genera, Menyanthes, Limnanthemum, and Villarsia, inhabit respectively Europe, India, and South America. Heterostyled species of Hedyotis are found in the temperate regions of North and the tropical regions of South America. Trimorphic species of Oxalis live on both sides of the Cordillera in South America and at the Cape of Good Hope. In these and some other cases it is not probable that each species acquired its heterostyled structure independently of its close allies. If they did not do so, the three closely connected genera of the Menyantheae and the several trimorphic species of Oxalis must have inherited their structure from a common progenitor. But an immense lapse of time will have been necessary in all such cases for the modified descendants of a common progenitor to have spread from a single centre to such widely remote and separated areas. The family of the Rubiaceae contains not far short of as many heterostyled genera as all the other thirteen families together; and hereafter no doubt other Rubiaceous genera will be found to be heterostyled, although a large majority are h.o.m.ostyled.
Several closely allied genera in this family probably owe their heterostyled structure to descent in common; but as the genera thus characterised are distributed in no less than eight of the tribes into which this family has been divided by Bentham and Hooker, it is almost certain that several of them must have become heterostyled independently of one another. What there is in the const.i.tution or structure of the members of this family which favours their becoming heterostyled, I cannot conjecture. Some families of considerable size, such as the Boragineae and Verbenaceae, include, as far as is at present known, only a single heterostyled genus. Polygonum also is the sole heterostyled genus in its family; and though it is a very large genus, no other species except P.
f.a.gopyrum is thus characterised. We may suspect that it has become heterostyled within a comparatively recent period, as it seems to be less strongly so in function than the species in any other genus, for both forms are capable of yielding a considerable number of spontaneously self-fertilised seeds. Polygonum in possessing only a single heterostyled species is an extreme case; but every other genus of considerable size which includes some such species likewise contains h.o.m.ostyled species. Lythrum includes trimorphic, dimorphic, and h.o.m.ostyled species.
Trees, bushes, and herbaceous plants, both large and small, bearing single flowers or flowers in dense spikes or heads, have been rendered heterostyled. So have plants which inhabit alpine and lowland sites, dry land, marshes and water.
(6/3. Out of the 38 genera known to include heterostyled species, about eight, or 21 per cent, are more or less aquatic in their habits. I was at first struck with this fact, for I was not then aware how large a proportion of ordinary plants inhabit such stations. Heterostyled plants may be said in one sense to have their s.e.xes separated, as the forms must mutually fertilise one another.
Therefore it seemed worth while to ascertain what proportion of the genera in the Linnean cla.s.ses, Monoecia, Dioecia and Polygamia, contained species which live ”in water, marshes, bogs or watery places.” In Sir W.J. Hooker's 'British Flora' 4th edition 1838, these three Linnean cla.s.ses include 40 genera, 17 of which (i.e. 43 per cent) contain species inhabiting the just-specified stations.
So that 43 per cent of those British plants which have their s.e.xes separated are more or less aquatic in their habits, whereas only 21 per cent of heterostyled plants have such habits. I may add that the hermaphrodite cla.s.ses, from Monandria to Gynandria inclusive, contain 447 genera, of which 113 are aquatic in the above sense, or only 25 per cent. It thus appears, as far as can be judged from such imperfect data, that there is some connection between the separation of the s.e.xes in plants and the watery nature of the sites which they inhabit; but that this does not hold good with heterostyled species.)
When I first began to experimentise on heterostyled plants it was under the impression that they were tending to become dioecious; but I was soon forced to relinquish this notion, as the long-styled plants of Primula which, from possessing a longer pistil, larger stigma, shorter stamens with smaller pollen- grains, seemed to be the more feminine of the two forms, yielded fewer seeds than the short-styled plants which appeared to be in the above respects the more masculine of the two. Moreover, trimorphic plants evidently come under the same category with dimorphic, and the former cannot be looked at as tending to become dioecious. With Lythrum salicaria, however, we have the curious and unique case of the mid-styled form being more feminine or less masculine in nature than the other two forms. This is shown by the large number of seeds which it yields in whatever manner it may be fertilised, and by its pollen (the grains of which are of smaller size than those from the corresponding stamens in the other two forms) when applied to the stigma of any form producing fewer seeds than the normal number. If we suppose the process of deterioration of the male organs in the mid-styled form to continue, the final result would be the production of a female plant; and Lythrum salicaria would then consist of two heterostyled hermaphrodites and a female. No such case is known to exist, but it is a possible one, as hermaphrodite and female forms of the same species are by no means rare. Although there is no reason to believe that heterostyled plants are regularly becoming dioecious, yet they offer singular facilities, as will hereafter be shown, for such conversion; and this appears occasionally to have been effected.
We may feel sure that plants have been rendered heterostyled to ensure cross- fertilisation, for we now know that a cross between the distinct individuals of the same species is highly important for the vigour and fertility of the offspring. The same end is gained by dichogamy or the maturation of the reproductive elements of the same flower at different periods,--by dioeciousness--self-sterility--the prepotency of pollen from another individual over a plant's own pollen,--and lastly, by the structure of the flower in relation to the visits of insects. The wonderful diversity of the means for gaining the same end in this case, and in many others, depends on the nature of all the previous changes through which the species has pa.s.sed, and on the more or less complete inheritance of the successive adaptations of each part to the surrounding conditions. Plants which are already well adapted by the structure of their flowers for cross-fertilisation by the aid of insects often possess an irregular corolla, which has been modelled in relation to their visits; and it would have been of little or no use to such plants to have become heterostyled.
We can thus understand why it is that not a single species is heterostyled in such great families as the Leguminosae, l.a.b.i.atae, Scrophulariaceae, Orchideae, etc., all of which have irregular flowers. Every known heterostyled plant, however, depends on insects for its fertilisation, and not on the wind; so that it is a rather surprising fact that only one genus, Pontederia, has a plainly irregular corolla.
Why some species are adapted for cross-fertilisation, whilst others within the same genus are not so, or if they once were, have since lost such adaptation and in consequence are now usually self-fertilised, I have endeavoured elsewhere to explain to a certain limited extent. (6/4. 'The Effects of Cross and Self- fertilisation' 1876 page 441.) If it be further asked why some species have been adapted for this end by being made heterostyled, rather than by any of the above specified means, the answer probably lies in the manner in which heterostylism originated,--a subject immediately to be discussed. Heterostyled species, however, have an advantage over dichogamous species, as all the flowers on the same heterostyled plant belong to the same form, so that when fertilised legitimately by insects two distinct individuals are sure to intercross. On the other hand, with dichogamous plants, early or late flowers on the same individual may intercross; and a cross of this kind does hardly any or no good.
Whenever it is profitable to a species to produce a large number of seeds and this obviously is a very common case, heterostyled will have an advantage over dioecious plants, as all the individuals of the former, whilst only half of the latter, that is the females, yield seeds. On the other hand, heterostyled plants seem to have no advantage, as far as cross-fertilisation is concerned, over those which are sterile with their own pollen. They lie indeed under a slight disadvantage, for if two self-sterile plants grow near together and far removed from all other plants of the same species, they will mutually and perfectly fertilise one another, whilst this will not be the case with heterostyled dimorphic plants, unless they chance to belong to opposite forms.
It may be added that species which are trimorphic have one slight advantage over the dimorphic; for if only two individuals of a dimorphic species happen to grow near together in an isolated spot, the chances are even that both will belong to the same form, and in this case they will not produce the full number of vigorous and fertile seedlings; all these, moreover, will tend strongly to belong to the same form as their parents. On the other hand, if two plants of the same trimorphic species happen to grow in an isolated spot, the chances are two to one in favour of their not belonging to the same form; and in this case they will legitimately fertilise one another, and yield the full complement of vigorous offspring.
THE MEANS BY WHICH PLANTS MAY HAVE BEEN RENDERED HETEROSTYLED.
This is a very obscure subject, on which I can throw little light, but which is worthy of discussion. It has been shown that heterostyled plants occur in fourteen natural families, dispersed throughout the whole vegetable kingdom, and that even within the family of the Rubiaceae they are dispersed in eight of the tribes. We may therefore conclude that this structure has been acquired by various plants independently of inheritance from a common progenitor, and that it can be acquired without any great difficulty--that is, without any very unusual combination of circ.u.mstances.
It is probable that the first step towards a species becoming heterostyled is great variability in the length of the pistil and stamens, or of the pistil alone. Such variations are not very rare: with Amsinckia spectabilis and Nolana prostrata these organs differ so much in length in different individuals that, until experimenting on them, I thought both species heterostyled. The stigma of Gesneria pendulina sometimes protrudes far beyond, and is sometimes seated beneath the anthers; so it is with Oxalis acetosella and various other plants. I have also noticed an extraordinary amount of difference in the length of the pistil in cultivated varieties of Primula veris and vulgaris.
As most plants are at least occasionally cross-fertilised by the aid of insects, we may a.s.sume 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 const.i.tution) 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 s.e.xes 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.