Cross-fertilization

“ Then comes cross-fertilizing hermaphroditism; and Nature takes much pains and uses many ingenious devices to prevent self-fertilization and insure cross-fertilization. Now, for the first time, we have slight individual differences funded in a common offspring. Then, in order to absolutely forbid self-fertilization, and at the same time allow greater differences in the crossing individuals than could be attained in hermaphroditic individuals, the sex organs are separated in different individuals, and fertilization can only take place by voluntary union. ”

“ In these cases self-fertilization is often absolutely excluded; it may be physically impossible, and hermaphroditism therefore secures cross-fertilization in such cases just as effectively as if the sexes were separate. ”

“ Cross-fertilization among the plants does not, of course, depend upon man as an agent. Since cross-fertilization is so valuable, it is not surprising to find many devices in the plant world for securing it. Honey and color, which attract winged messengers, are among the most universal helps to cross-fertilization. ”

“ That normal development or growth by cell division is advantaged by cross-fertilization may mean that the cells divide more readily and normally when they contain protoplasmic 'elements' of a proper degree of diversity than when they have only one kind of protoplasm, as would happen in narrow inbreeding, and also when cross-breeding is too wide for the intimate cooperation required for true fertilization. ”

“ Darwin, by his careful studies on the effects of cross- and self-fertilization in the vegetable kingdom, contributed more largely than any one else to the discovery of this law. He watched, from germination to maturity, more than a thousand individual plants, produced by crossing and self-fertilization, belonging to fifty-seven species, fifty-two genera, and thirty large families, and including natives of the most various countries.1980 The result established by this research was, that cross-fertilization is generally beneficial, and self-fertilization injurious ”

“ Thus we see that in the animal kingdom hermaphroditism is always associated with cross-fertilization in some way or other, even though the latter may occur rarely, being usually periodically interpolated, and thus bringing new ids into the germ-plasm which is rapidly becoming monotonous or uniform. ”

“ Formation of Seed—Fertilization.—In fertilization—the influence in flowering plants of the male-cell in the pollen tube upon the egg-cell in the ovule (see Botany) —there are many circumstances of importance horticulturally, to which, therefore, brief reference must be made. Flowers, generally speaking, are either self-fertilized, cross-fertilized or hybridized. Self-fertilization occurs when the pollen of a given flower affects the egg-cell of the same individual flower. Cross-fertilization varies both in manner and degree. ”

“ There may be a few other cases of such drastic results of self-fertilization, but in any case, and disregarding these apparently suicidal fanatics, cross-fertilization is so very nearly universal that nature must find it of enormous advantage. Only in this way is it possible to explain the intricate adjustments of insects and flowers, which work together in such wonderful harmony that cross-fertilization hardly ever fails in those flowers where it appears to be necessary. ”

“ He recognized that, in general, self-fertilization is disadvantageous to plants; that they produce fewer seeds, and that these produce feebler plants, than when they are cross-fertilized; that, therefore, those flowers which are arranged to secure cross-fertilization have an advantage over those which are self-fertilized. In many species, as Sprengel had already pointed out, self-fertilization leads to actual infertility; only a few plants are as fertile with their own pollen as with that of another plant ”

“ But cases of exclusive and constant self-fertilization do also seem to occur, and even these may be brought into harmony with our conception, according to which cross-fertilization is an advantage, but only an advantage which must be weighed against others, and which may eventually be given up in favour of greater advantages. ”

“ After cross-fertilization has been effected, it is the pistil's turn to keep out of the way, and give the imprisoned stamens a chance: the styles curve until the stigmas are pressed against the sides of the ovary, that not a grain of pollen may touch them; the petals spread and release the stamens; but so great is the flower's zeal not to be fertilized with its own pollen that it sometimes holds the anthers tightly between the petals until all the vitalizing dust has been shed! Around the hermaphrodite florets are a large number of male florets in each hemispheric cluster. ”

“ In the flower where the father and mother part matures at the same time, self-fertilization is the rule. Cross-fertilization occurs in instances where either the father part or mother part ripen at different times, in these cases the pollen is carried from plant to plant by the wind or by the nectar-seeking bees. ”

“ Thus the first batch of eggs liberated by the females are fertilized by the minute free-swimming 'primordial males,' but after that the females themselves develop testes, and then fertilize themselves; the males die very soon after copulation, and only appear the following year in a new generation. They are therefore far from being mere historic reminiscences, vestiges of the early history of the modern species, for they are the instruments of a regular cross-fertilization of the species, and therefore of a constant mingling of new ids in the germ-plasm. ”

“ That the advantages of cross-fertilization do not follow from some mysterious virtue in the mere union of two distinct individuals, but from such individuals having been subjected during previous generations to different conditions, or to their having varied in a manner commonly called spontaneous, so that in either case their sexual elements have been in some degree differentiated ”

“ Monoecious plants can hardly fail to be to a large extent dioecious in function, owing to the lightness of their pollen and to the wind blowing laterally, with the great additional advantage of occasionally or often producing some self-fertilised seeds. When they are also dichogamous, they are necessarily dioecious in function. Lastly, hermaphrodite plants can generally produce at least some self-fertilised seeds, and they are at the same time capable, through the various means specified in this chapter, of cross-fertilisation. ”

“ We should always keep in mind the obvious fact that the production of seed is the chief end of the act of fertilisation; and that this end can be gained by hermaphrodite plants with incomparably greater certainty by self-fertilisation, than by the union of the sexual elements belonging to two distinct flowers or plants. Yet it is as unmistakably plain that innumerable flowers are adapted for cross-fertilisation, as that the teeth and talons of a carnivorous animal are adapted for catching prey ”

“ It is, however, incredible that so peculiar a form of mutual infertility should have been specially acquired unless it were highly beneficial to the species; and although it may be beneficial to an individual plant to be sterile with its own pollen, cross-fertilisation being thus ensured, how can it be any advantage to a plant to be sterile with half its brethren, that is, with all the individuals belonging to the same form? ”

“ In noting the advantage accruing to crossed offspring, we have particularly referred to gain in constitutional vigor. We have occasion now to speak of gain in fertility. Seeing that hybrids—the product of a cross between species—are invariably sterile, it is clear that, if the conception that varieties are incipient species is a valid one, we are bound to expect that the more marked, distinct, and widely divergent varieties are, the greater will be their sterility. The mere circumstance that such an effect is not observable, goes far to invalidate the conception. ”

“ There is the clearest evidence, as we shall presently see, that the advantage of a cross depends wholly on the plants differing somewhat in constitution; and that the disadvantages of self-fertilisation depend on the two parents, which are combined in the same hermaphrodite flower, having a closely similar constitution. A certain amount of differentiation in the sexual elements seems indispensable for the full fertility of the parents, and for the full vigour of the offspring. ”

“ The results can be considered as only approximately accurate, for fertility is so variable a character, that each species ought to have been tried many times. The above number of species, namely, 125, is as nothing to the host of living plants; but the mere fact of more than half of them being sterile within the specified degree, when insects are excluded, is a striking one; for whenever pollen has to be carried from the anthers to the stigma in order to ensure full fertility, there is at least a good chance of cross-fertilisation. ”

“ The simple fact of the necessity in many cases of extraneous aid for the transport of the pollen, and the many contrivances for this purpose, render it highly probable that some great benefit is thus gained; and this conclusion has now been firmly established by the proved superiority in growth, vigour, and fertility of plants of crossed parentage over those of self-fertilised parentage. But we should always keep in mind that two somewhat opposed ends have to be gained; the first and more important one being the production of seeds by any means, and the second, cross-fertilisation. ”

“ Yet more extraordinary is it to find numerous cases in which the special arrangements for cross-fertilisation appear to have been a failure, since they have either been supplemented by special means for self-fertilisation, or have reverted back in various degrees to simpler forms in which self-fertilisation becomes the rule. ”

“ The offspring from self-fertilised flowers are themselves more or less sterile, sometimes highly sterile, and their pollen is sometimes in an imperfect condition; but I have not met with any case of complete sterility in self-fertilised seedlings, as is so common with hybrids. The degree of their sterility does not correspond with that of the parent-plants when first self-fertilised. The offspring of self-fertilised plants suffer in stature, weight, and constitutional vigour more frequently and in a greater degree than do the hybrid offspring of the greater number of crossed species. ”

“ In the chapter on Fertility, when we consider the effects of cross-fertilisation and self-fertilisation on the productiveness of the parent-plants we shall arrive at nearly the same result, namely, that a cross between the flowers on the same plant does not at all increase the number of the seeds, or only occasionally and to a slight degree. ”

“ I do not doubt that this is usually the case, and that the fertility often suddenly decreases in the first few generations. Nevertheless I believe that in all these experiments the fertility has been diminished by an independent cause, namely, from close interbreeding. I have collected so large a body of facts, showing that close interbreeding lessens fertility, and, on the other hand, that an occasional cross with a distinct individual or variety increases fertility, that I cannot doubt the correctness of this almost universal belief amongst breeders. ”

“ The fertility once re-established, the sterility does not return in the later progeny, a fact strongly suggestive of segregation. Now if the sterility of the cross-bred be really the consequence of the meeting of two complementary factors, we see that the phenomenon could only be produced among the divergent offspring of one species by the acquisition of at least TWO new factors ”

“ Ova were, therefore, unisexual, always female, while spermatozoa were either male or female, and when a female egg was fertilized by a female spermatozoon the result naturally was a female, but when it was fertilized by a male spermatozoon the result was a sex-hybrid appearing as a male because of the dominance of male characters. Correns's theory avoids the unlikely supposition of selective fertilization, but breaks down in those cases of parthenogenesis where the unfertilized egg produced by a female gives rise to a male. ”

“ We cannot escape this dilemma by making the further assumption, which is also incapable of proof, that a smaller amount of reproductive force is required for the development of a male individual than for the development of a female. It is true that the unfertilized eggs of the bee produce male individuals, while the fertilized ones develope into females, but in certain other species the converse association holds good, while in others, again, fertilization bears no relation to the sex of the offspring. ”

“ That certain plants, for instance, Viola tricolor, Digitalis purpurea, Sarothamnus scoparius, Cyclamen persicum, etc., which have been naturally cross-fertilised for many or all previous generations, should suffer to an extreme degree from a single act of self-fertilisation is a most surprising fact. Nothing of the kkind has been observed in our domestic animals ”

“ There are some few species which have been self-fertilised for a vast number of generations, and yet are vigorous enough to compete successfully with a host of surrounding plants. We can form no conception why the advantage from a cross is sometimes directed exclusively to the vegetative system, and sometimes to the reproductive system, but commonly to both. It is equally inconceivable why some individuals of the same species should be sterile, whilst others are fully fertile with their own pollen; why a change of climate should either lessen or increase the sterility of self-sterile species ”