VII 



NATURAL CROSSING 



121 



liability to natural crossing; we have just stated that such 

 differences 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 been framed to account for 

 the facts observed is based upon an analogy drawn by 

 Prof. Marshall Ward many years ago, between the pollen 



Fia. 52u. THE BREEDING PLOT. 

 Irrigating Sudanese Tree-Cottons. 



tube and the hypha of a parasitic fungus. We find in 

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

 species and varieties of the same fungus possess different 

 infection-capabilities. Conversely, the same fungus may 

 be able to attack one strain of its host-plant with ease, 

 while another strain may be practically immune. 



Using these facts to help us in forming conceptions as to 

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



