XI] LEAVES, AND LEAF-DISEASES. 255 



that we find the most extraordinary cases of para- 

 sitism, and since some of these exhibit the most 

 highly developed and complex adaptations known to 

 us, I propose to select one of them as the type of these 

 so-called ''leaf diseases." This form is Coleosjporium 

 Senecionis {Peridermmm Pint), rendered classical by 

 the reseaches of several excellent botanists. 



It is true, Coleospormm Senecionis is not in some 

 respects the most dangerous of these fungi — or, rather, 

 it has not hitherto been found to be so — but in view 

 of the acknowledged fact that foresters have not as 

 yet been able to devise practical measures against the 

 ravages of these numerous rust-fungi, and since we 

 are still very ignorant of the details of the biology of 

 most of them, it seems advisable to choose for illustra- 

 tion a form which shows in a distinct manner the 

 complexities of the subject, so that those interested 

 may see in what directions botanists may look for 

 new results. That the story of this fungus is both 

 complicated and of great biological interest will be 

 sufficiently evident from the mere recital of what we 

 know concerning it. 



