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. .h.i.therto 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 a.s.sumed 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 h.o.m.ostyled 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 const.i.tution 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 s.e.xes separated; for the two s.e.xes are combined in a monstrous hermaphrodite in a somewhat similar manner as the two s.e.xual 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 s.e.xes 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 cl.u.s.ter 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 s.e.xual 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 s.e.xual powers. We shall, perhaps, best perceive the complex and extraordinary nature of the marriage arrangements of a trimorphic plant by the following ill.u.s.tration. 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 a.s.sociated with the two s.e.xes, that they feel no surprise at almost any amount of difference; but differences in s.e.xual nature have been thought to be the very touchstone of specific distinction. We now see that such s.e.xual 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 s.e.xes 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 s.e.x 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 s.e.xes 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 s.e.xes combined in the same flower. It is a curious and obscure problem how and why such hermaphrodites have been rendered bis.e.xual.
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 s.e.xes, 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 a.s.sume that cross-fertilisation was a.s.sured 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 compet.i.tion 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 s.e.xes 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 a.s.sumption 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 circ.u.mstances, 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 quant.i.ty 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 a.n.a.logous 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 s.e.xes 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 s.e.xes 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 s.e.x 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 a.n.a.logy, 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 h.o.m.ologous 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 Mitch.e.l.la, 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, Mitch.e.l.la 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, Mitch.e.l.la 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 dest.i.tute 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 h.o.m.ostyled 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 s.e.xual 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 dest.i.tute 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 antic.i.p.ated 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 pa.s.s 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 dest.i.tute 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.