VARIATION AND DISEASE. 175 



and other factors of the environment. This 

 is true of parasites as well as of saprophytes. 

 Botrytis forms conidia only in darkness and in 

 moist air. Klebahn found that a Puccinia grow- 

 ing on Digraphis infected Polygonattivi readily 

 and completely, Convallaria imperfectly, whereas 

 if sown on Majanthevium it only just infected 

 the plant and then remained sterile, while it 

 refused to infect Paris at all. Magnus has shown 

 that Peronospora parasitica can only infect meriste- 

 matic tissues, and that when it co-exists with 

 Cystopus on Capsella, as is usually the case, it 

 enters the latter plant by infecting the gall- 

 like pustules of hypertrophied tissue induced by 

 that parasite. Numerous parasitic fungi can only 

 penetrate particular parts of plants. For in- 

 stance, the Ustilago of wheat can only infect 

 the young seedling, and grows for weeks as a 

 barren mycelium, only becoming a dominant 

 fungus in the endosperm. Numerous other 

 examples could be given, but these suffice to 

 show some of the ways in which the nature 

 of the food substratum supplied by the host 

 affects the fungus. It is obvious that if the 

 nature of this food changes, the fungus is also 

 affected, and no doubt this is the principal reason 

 why Rust-fungi, for instance, vary so much in 

 their vigour and reproductive power on different 

 wheats and grasses, though the other factors of 

 the environment must also be of influence on 

 them as well as on the hosts. 



But and this is the second point modern 



