EFFECT OF SUBSTRATUM ON DEVELOPMENT "F PAKAsITK. 4'.' 



while on L<tri>- only little cushions appeared on the needle^ 

 (Caeoma /"/-/v/.s). 



These variations in the effect of the substratum on tin- 

 development and reproduction of the parasites assi.-t us to 

 understand the well-known resistance of certain varieties ami 

 species against epidemic diseases, which are sweeping otT tin-li- 

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

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

 conditions. Similarly amongst the varieties of vine sonn- are 

 known to be more sensitive to disease than others. These 

 points will be more fully discussed in a subsequent chapter. 



