FERTILIZATION OF FLOWERS. 353 



(1) Separation of the Sexes. In many flowers we find that only 

 stamens or pistil are developed, never both, so that we get 

 distinct male and female flowers, which renders self-fertilization 

 absolutely impossible, though it does not necessarily prevent 

 fertilization by pollen from other flowers on the same plant. 



(2) Physiological Self -sterility. In some flowers, to quote the 

 words of Darwin, " the ovules absolutely refuse to be fertilized 

 by pollen from the same plant, but can be fertilized by pollen 

 from any other individual of the same species." Again, in a 

 large number of plants, although self-fertilization may take place, 

 yet, if the pollen from another individual be brought to the 

 stigma, it takes precedence, so to speak, of the flower's own 

 pollen and renders the latter ineffectual ; it is said to be 

 prepotent. 



(3) Dichogamy. In a large number of flowers, in which both 

 male and female organs are present, the stamens and pistil 

 become mature at different times, so that self-fertilization 

 cannot possibly take place and, physiologically speaking, the 

 flowers are unisexual. Usually in these cases the pollen ripens 

 and is all shed before the stigma is ready to receive it. The 

 flower is then termed protandrous, as for example in the 

 common pink (Dianthus). More rarely the stigma ripens and 

 withers before the pollen is ripe, and such species are termed 

 protogynous, as in the fig-wort (Scroplmlaria nodosa). 



Of course it would be worse than useless to prevent self- 

 fertilization unless some means were provided at the same time 

 for securing cross-fertilization. We saw in Chapter VIII that 

 the principal agents in conveying pollen from one flower to 

 another are wind and insects, and that flowers are accordingly dis- 

 tinguished as anemophilous or wind-fertilized and entomophilous 

 or insect-fertilized. The fact that the former are usually very 

 small and inconspicuous, as for example in the grasses, while the 

 latter are large and brightly coloured, affords strong presumptive 

 evidence that entomophilous flowers have become modified in 

 relation to the insects which visit them in search of food or 

 shelter. A great many different kinds of insects have this habit, 

 bees, flies, butterflies, moths and even beetles ; while in South 

 America some of the humming birds in like manner play the 

 part of pollen carriers. 



We are in the habit of regarding bees as the most important 

 insects concerned in the cross-fertilization of flowers, and in 



B. A A 



