VII 



NATURAL CROSSING 



121 



liability to natural crossing ; we have just stated that such 

 difterences cannot be due to any obvious cause such as 

 position or floral structure, so we are driven back upon a 

 more abstruse explanation. 



The hypothesis which has Iteen framed to account for 

 the facts observed is based upon an analogy drawn by 

 Prof. Marshall Ward many years ag(i, between the p)ollen 



Fi(!. .52b. — The Breedinci Plot. 

 Irrigating Sudanese Tree-Cottons. 



tube and the hyplia of a parasitic fungus. Wc find in 

 mycology that within the same strain of host-plant, difterent 

 species and varieties of the same fungus possess different 

 infection-capabilities. Conversely, the same fungus may 

 l:)e able to attack one strain of its host-plant with ease, 

 while another strain may be practically immune. 



Using these facts to help us informing conceptions as to 

 the possiljle behaviour of pollen, we see at once that some 



