8 4 



PLECTOMYCETES 



[CH. 



de Bary, who was able to recognize an antheridium and oogonium and the 

 formation of an ascus or asci from the latter. These and several subsequent 

 investigations have rendered the reproductive processes in the Erysiphaceae 

 better known than perhaps in any other group of fungi. 



Spliaerotlieca Humuli* occurs on a variety of common plants, on the 

 cultivated strawberry, where it is responsible for strawberry mildew, 

 and especially on the hop. On the latter it is widely distributed in 

 autumn, and, if the female inflorescences are infected, may do considerable 

 damage. 



The male and female organs arise as lateral branches from the mycelium, 

 and project at right angles to the infected surface; they are borne on dif- 

 ferent hyphae, but there is no evidence that these are derived from distinct 

 mycelia. The oogonium, when fully grown, is an oval, uninucleate structure 

 two or three times the size of an ordinary vegetative cell; it is cut off from 

 the parent hyphae and a stalk cell may be differentiated below it (fig. 41 a). 



Fig. 4 1 . Sphaerotheca Humuli (DC.) Burr.; a. young oogonium and antheridium ; b. entrance of male 

 nucleus; c. male and female nuclei in oogonium; d. fertilization; e. fusion nucleus; f. nuclei 

 produced by first division of fusion nucleus; g. young perithecium with binucleate ascogenous 

 cell; x 1360; after Blackman and Fraser. 



1 S. Humuli (DC.) Burr. = 5. Castagnei Lev. 



