In Knuth's "Handbuch der Blutenbiologie" ("Handbook of Flower Pollination", Oxford, 1906) as many as 3792 papers on this subject are enumerated as having been published before January 1, 1904. These describe not only the different mechanisms of flowers, but deal also with a series of remarkable adaptations in the pollinating insects. As a fertilising rain quickly calls into existence the most varied assortment of plants on a barren steppe, so activity now reigns in a field which men formerly left deserted.
This development of the biology of flowers is of importance not only on theoretical grounds but also from a practical point of view. The rational breeding of plants is possible only if the flower-biology of the plants in question (i.e. the question of the possibility of self-pollination, self-sterility, etc.) is accurately known. And it is also essential for plant-breeders that they should have "the power of fixing each fleeting variety of colour, if they will fertilise the flowers of the desired kind with their own pollen for half-a-dozen generations, and grow the seedlings under the same conditions." ("Cross and Self fertilisation" (1st edition), page 460.)But the influence of Darwin on floral biology was not confined to the development of this branch of Botany. Darwin's activity in this domain has brought about (as Asa Gray correctly pointed out) the revival of teleology in Botany and Zoology. Attempts were now made to determine, not only in the case of flowers but also in vegetative organs, in what relation the form and function of organs stand to one another and to what extent their morphological characters exhibit adaptation to environment. A branch of Botany, which has since been called Ecology (not a very happy term) has been stimulated to vigorous growth by floral biology.
While the influence of the work on the biology of flowers was extraordinarily great, it could not fail to elicit opinions at variance with Darwin's conclusions. The opposition was based partly on reasons valueless as counterarguments, partly on problems which have still to be solved; to some extent also on that tendency against teleological conceptions which has recently become current. This opposing trend of thought is due to the fact that many biologists are content with teleological explanations, unsupported by proof; it is also closely connected with the fact that many authors estimate the importance of natural selection less highly than Darwin did. We may describe the objections which are based on the widespread occurrence of self-fertilisation and geitonogamy as of little importance. Darwin did not deny the occurrence of self-fertilisation, even for a long series of generations; his law states only that "Nature abhors PERPETUAL self-fertilisation." (It is impossible (as has been attempted) to express Darwin's point of view in a single sentence, such as H. Muller's statement of the "Knight-Darwin law." The conditions of life in organisms are so various and complex that laws, such as are formulated in physics and chemistry, can hardly be conceived.) An exception to this rule would therefore occur only in the case of plants in which the possibility of cross-pollination is excluded. Some of the plants with cleistogamous flowers might afford examples of such cases. We have already seen, however, that such a case has not as yet been shown to occur. Burck believed that he had found an instance in certain tropical plants (Anonaceae, Myrmecodia) of the complete exclusion of cross-fertilisation.
The flowers of these plants, in which, however,--in contrast to the cleistogamous flowers--the corolla is well developed, remain closed and fruit is produced.
Loew (E. Loew, "Bemerkungen zu Burck...", "Biolog. Centralbl." XXVI.
(1906).) has shown that cases occur in which cross-fertilisation may be effected even in these "cleistopetalous" flowers: humming birds visit the permanently closed flowers of certain species of Nidularium and transport the pollen. The fact that the formation of hybrids may occur as the result of this shows that pollination may be accomplished.
The existence of plants for which self-pollination is of greater importance than it is for others is by no means contradictory to Darwin's view. Self-fertilisation is, for example, of greater importance for annuals than for perennials as without it seeds might fail to be produced. Even in the case of annual plants with small inconspicuous flowers in which self-fertilisation usually occurs, such as Senecio vulgaris, Capsella bursa-pastoris and Stellaria media, A. Bateson (Anna Bateson, "The effects of cross-fertilisation on inconspicuous flowers", "Annals of Botany", Vol. I.