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. z%. 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 It. acerinum, but consists of closely crowded small 

 pieces. 



