DISCOMYCETACEAE 



143 



no other form of reproduction is known ; hence, if diseased 

 leaves are collected and burned before the ascospores are 

 liberated in the spring, the disease will be arrested. 



Fig. 28. Rhytisma acerinum. i, sycamore leaf with 

 Rhytisma blotches ; 2, section through a portion of a 

 black patch bearing spermatia, x 200 ; 3, ascus contain- 

 ing spores, also two curved paraphyses, x 300 ; 4, a 

 free spore, x 300. 



The following species of Rhytisma also occur as parasites, 

 and if injury is being done, the diseased leaves should be 

 removed, to prevent inoculation the following spring : 



Rhytisma punctatum^ Fr., forms large black blotches, 

 surrounded by a yellowish line, on living leaves of Acer 

 campestre^ L., A. pseudoplatanus, L., and A. spicatum> Lam. 

 In this species the black scab or crust is not continuous 

 as in R. acerinum^ but consists of closely crowded small 

 pieces. 



