EFFECT OF SUBSTRATUM ON DEVELOPMENT OF PARASITE. 49 



while on Larix only little cushions appeared on the needles 

 (Caeoma laricis). 



These variations in the effect of the substratum on the 

 development and reproduction of the parasites assist us to 

 understand the well-known resistance of certain varieties and 

 species against epidemic diseases, which are sweeping off their 

 near allies. Thus, we know that some varieties of cereals suffer 

 from attacks of rust-fungi more than others grown under like 

 conditions. Similarly amongst the varieties of vine some are 

 known to be more sensitive to disease than others. These 

 points will be more fully discussed in a subsequent chapter. 



