The Evolution Theory, Vol. 2 of 2

by August Weismann

By what adaptations the pollen of a flower can be made ineffective for that flower is a question which we must leave the botanists to answer; in any case it must have been possible, and we see clearly that it depends upon adaptation when we consider the numerous stages which occur—from the rare case of the actually poisonous influence of self-pollination already noticed, to complete sterility, and from lessened fertility to greater or even perfect fertility. It is possible that chemical products, secretions of the stigma or the pollen-grain, or the so-called synergid-cells, have to do with this, or that the size and therewith the penetrating power of the pollen-cell in self-fertilization stand in inverse ratio to the length of the pistil, as has been proved in regard to heterostylism by Strasburger; but in any case it was possible for Nature, by means of slight variations in the characters of the male and female parts of the flower, to diminish the certainty of the meeting of the two germ-cells, even to the total exclusion of the possibility of any union of these.

If, then, self-fertilization had to be guarded against or at least rendered difficult because its consequences were injurious, all variations pointing in the direction of safeguarding would necessarily be preserved and increased. In many cases variations in the structure of the flower were sufficient; but when, as in Corydalis cava, the pollen could not readily be prevented from falling upon the stigma, the pollen might be made sterile as far as its own flower was concerned by a process of selection, in which on an average those plants would remain successful which produced the largest number of cross-fertilized seeds, and in this case those which did so were those whose pollen reacted most feebly to the stimulus of their own stigma.

Fig. 128. Heterostylism (Primula sinensis), after Noll. Two heterostylic flowers from different plants. L, the long-styled form. K, the short-styled form. G, style. S, anthers. P, p, pollen-grains. N, n, stigmatic papillæ of the long-styled and short-styled forms respectively. P, p, N, n, magnified 110 times.

That self-sterility in all these different degrees is not a primary character of the species, but an adaptation to the advantages of cross-fertilization, is apparent—if indeed it seems doubtful to any one—especially from cases of heterostylism. I refer to the dimorphism and trimorphism which Darwin discovered in many flowers, and which shows itself in the fact that flowers otherwise almost exactly alike, as, for instance, primroses, may exhibit a long style in some individuals, and in others a short one (Fig. 128). At the same time, there is a difference in the position of the stamens, which are placed higher up in flowers with short styles, and much lower down in those with long styles. Experiments have proved that the dusting of the stigma has the best results if pollen from the short-styled reaches the stigma of the long-styled form, or if pollen from the long-styled form reaches the stigma of the short-styled. Thus we have again to deal with an arrangement for crossing, an adaptation to the advantages of cross-fertilization, and we can in this case see the reason why the pollen has a different effect upon the two stigmas; the pollen-grains of the flowers with short style are larger than those of the flowers with long style, and as the length of the pollen-tube that can be sent out must depend upon the mass of protoplasm within the pollen-grain, it follows that the smaller pollen-grains will send out too short a tube to reach through the long style to the embryo-sac. In addition to this there is a difference in the papillæ of the stigmas, and it is possible that these may form an obstacle to the penetrating of pollen from a similar type. The process of selection which gives rise to such arrangements as we find in Primulas may easily be imagined, as soon as we are able to assume that cross-fertilization is more advantageous than self-fertilization as regards progeny, that is, as regards the continuance of the species.

We have already seen that uninterrupted self-fertilization is unknown among animals, but that it is not even very rare among plants, and this emphatically corroborates our previous conclusion, that the reason for which amphimixis was introduced as a normal event in nature is not to be sought for in the necessity for a renewing of life, or 'rejuvenation.' It cannot be a necessity, but only an advantage, which can in certain circumstances be dispensed with.

Although it is obvious enough that continued inbreeding in its most extreme form, self-fertilization, does not imply an absolute abandonment of amphimixis, the adherents of the rejuvenescence theory have regarded the unfavourable consequences of pure inbreeding as a confirmation of their assumption, according to which amphimixis is indispensable to the continuance of the life of the species, and it is therefore an important fact, if it can be proved, that continued self-fertilization can occur persistently, among plants at least, and yet not cause any injurious results to the species.

But how can this fact be understood from our point of view? How does it happen that crossing is striven after in so many different ways and yet so often given up again, and continued self-fertilization resorted to?

To this it may be answered, in the first place, that it is not, as far as we can see, for internal reasons that persistent self-fertilization becomes the rule; there is no peculiar condition of the germ-plasm which makes it disadvantageous or superfluous that the diversity of the id-combinations should be maintained; self-fertilization is due to external influences which bring it about that the plant has only the alternative of producing no seeds at all or of producing them by self-fertilization. In this connexion Darwin's experiments with orchids are particularly noteworthy.

In this very diversified order of plants there are numerous species whose flowers are infertile with their own pollen, although it does not reach the stigma in natural conditions, and therefore there was no necessity—as far as we can see—for guarding against self-fertilization by 'self-sterility.' These flowers are thus doubly adapted, so to speak, for crossing by means of insects. But as regards many of these, as well as many other modern orchids, insect-visits are very rare, and in some cases do not occur at all, and therefore these species cannot produce seed or can do so only exceptionally.