438 VEGETABLE PHYSIOLOGY 



advantageous to a plant than the fusion of gametes which 

 are both produced by the same individual. In the same 

 way certain advantages are secured by the process of cross- 

 pollination or the application of the pollen of one flower to 

 the stigma of a different one of the same species. In the 

 case of flowering plants or any others which are hetero- 

 sporous, self-fertilisation is of course in the strictest sense 

 impossible, as the male and female cells which fuse 

 together are necessarily borne upon gametophytes which 

 originate from different spores and cannot thus be derived 

 immediately from the same individual. Self-pollination, or 

 the transference- of pollen from the stamens to the stigma 

 of the same flower, is however possible, and in many cases 

 occurs in the ordinary course of events. Cross-pollination, 

 or the bringing together of spores from different flowers 

 of the, same species, has been found to yield more and 

 better seeds than self-pollination. 



Very many mechanisms have been developed in different 

 plants to secure this end. Pollen may be carried from 

 flower to flower by wind or water, or by the agency of 

 insects or other animals. From this point of view flowers 

 have been classed as anemophilous, or wind-pollinated, 

 hydrophilous, or water-pollinated, entomophilous, or insect- 

 pollinated, and zoophilous, or pollinated by other animals. 



Of these methods of cross-pollination, the anemophilous 

 and the entomophilous are most widespread. The former 

 is the more primitive ; indeed, the latter has been gradually 

 supplanting it. We find cases now of nearly allied genera 

 which illustrate the transition from the one to the other. 

 Among the Eanunculaceae the flowers of the genus Thalic- 

 trum are pollinated by the wind, while those of the more 

 specialised genera Aconitum and Delphinium depend upon 

 insects. The Plantains also afford instances of the replace- 

 ment of the one method by the other. 



Anemophilous flowers exhibit certain structural features 

 which are associated with their mode of transference of 

 the pollen. It is produced in such flowers in great abun- 



