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. [Illustration: 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.]