different plants, forming the class Dioecia of Linnaeus. In these cases the pollen may be carried to the stigmas either by the wind or by the agency of insects.
Now these four methods are all apparently very simple, and easily produced by variation and selection. They are applicable to flowers of any shape, requiring only such size and colour as to attract insects, and some secretion of nectar to ensure their repeated visits, characters common to the great majority of flowers. All these methods are common, except perhaps the second; but there are many flowers in which the pollen from another plant is prepotent over the pollen from fertilisation, the same flower, and this has nearly the same effect as self-sterility if the flowers are frequently crossed by insects. We cannot help asking, therefore, why have other and much more elaborate methods been needed? And how have the more complex arrangements of so many flowers been brought about? Before attempting to answer these questions, and in order that the reader may appreciate the difficulty of the problem and the nature of the facts to be explained, it will be necessary to give a summary of the more elaborate modes of securing cross-fertilisation.
(1) We first have dimorphism and heteromorphism, the phenomena of which have been already sketched in our seventh chapter.
Here we have both a mechanical and a physiological modification, the stamens and pistil being variously modified in length and position, while the different stamens in the same flower have widely different degrees of fertility when applied to the same stigma,—a phenomenon which, if it were not so well established, would have appeared in the highest degree improbable. The most remarkable case is that of the three different forms of the loosestrife (Lythrum salicaria) here figured (Fig. 29 on next page).
