SEXUALITY OF PLANTS 41 



react in determining the distribution of the Bee. The 

 methods of transfer of the pollen may be very varied. 

 But the essential feature of them all is the same, viz. 

 the conveyance of an immobile body essential to pro- 

 pagation from the pollen-sac where it is produced to the 

 surface of the stigma, where it can germinate. 



The study of the structure of Flowers as pollinating 

 mechanisms has caught public attention, and the facts 

 are often presented in sensational language. Floral 

 mechanisms and their evolution parallel to the forms of 

 their visitants are certainly wonderful instances of adapta- 

 tion. But in studying them it should always be re- 

 membered that it is the immobility of the Plant that 

 gives these adaptations their special value. Vegetation 

 was originally aquatic. The spread of Plant Life to the 

 Land raised a thousand difficult life-problems. One of 

 the most urgent was how it was possible for Plants, being 

 immobile, to maintain sexual reproduction under con- 

 ditions of life in air instead of in water. A first step in 

 the solution of that problem, the transfer of the immobile 

 pollen, is carried out in that wonderful structure, the 

 Flower, which is as beautiful to the understanding as it 

 is to any of the senses. 



Once landed on the surface of the stigma the pollen- 

 grain germinates and forms a pollen-tube, which, pene- 

 trating the tissue, traverses the carpel downwards to the 

 cavity in which the ovule or ovules lie (Fig. 21). There it 

 is led to the apex of the ovule ; it enters the micropyle, 

 and impinges directly upon the embryo-sac where the egg 

 is attached. Since it conveys two male gametes, these 

 can be discharged at the apex into the embryo-sac. It 

 is one of these which entering the egg fertilizes it. These 

 are, briefly told, the steps leading to fertilization or 



