i 4 o TEXT-BOOK OF FUNGI 



These experiments are considered to demonstrate, as 

 they certainly do, the fact that the infective powers of a 

 * biologic form ' are not altered by producing one genera- 

 tion on a mutilated, strange host-plant. Considering that 

 the fungus was grown on mutilated or paralysed portions 

 of the strange host, it was hardly to be expected that its 

 innate affinity for a given host, probably the outcome of an 

 adaptation extending over centuries, should be neutralised 

 or in any way affected. It would be interesting to know 

 what would have happened if the fungus had been culti- 

 vated on a mutilated alien host-plant for, say, twenty 

 generations. The experiment, however, came perilously 

 near to demonstrating that what has hitherto been con- 

 sidered as an obligate parasite could live as a saprophyte ; 

 if, as suggested above, the experiment had been repeated 

 for twenty times, always using the spores of the previous 

 generation for inoculation, in all probability a saprophyte 

 would have resulted. In other words, the fungus would 

 have been weaned of its predilection for its former host- 

 plant. As the fungus in these experiments was growing 

 on a stratum of cells so treated as to be deprived of the 

 power of exercising the function of either producing or 

 receiving from other cells, those special substances re- 

 pellent to the fungus when uninjured, it would appear that 

 the fungus responded to some general chemotactic sub- 

 stance, as sugar. 



The details of the modes of entry, or infection of a 

 host-plant by a germinating spore, is a somewhat com- 

 plicated process, and is described by Ward in the 

 case of uredo-spores infecting leaves of Bromus, as 

 follows : 



'The germination of the uredo-spore on the epidermis 



