86 



PLECTOMYCETES 



[CH. 



chromosomes is that of Winge who describes eight in the first and second 

 mitoses, and four in the third, and suggests that a brachymeiotic reduction 

 takes place. 



According to the interpretations of Dangeard and Winge fertiHzation 

 does not take place in the oogonium of Sphaerotheca Hitmuli. For them 

 the degenerating mass in the antheridium includes the male nucleus which 

 thus degenerates in situ and the two nuclei seen lying side by side in the 

 female organ represent the product of a premature division (fig. 42). 



Again Bezssonoff working on Sphaerotheca mors-uvae, the economically 

 important gooseberry mildew, records the entrance of the male nucleus into 

 the oogonium, but does not observe its fusion with the female nucleus. He 

 finds four chromosomes throughout the divisions in the ascus. 



ErysiphePolygoni^ occurs on the leaves and stems of a considerablevariety 

 of hosts belonging to a number of different families. The development of the 

 sexual organs takes place much as in Sphaerotheca Hiimuli, and, here also. 

 Harper has observed the entrance of the male nucleus into the oogonium, 

 and its fusion with the female nucleus (fig. 43«). After fertilization the 



Fig. 43. Erysiphe Polygoni; rt. fertilization; b. young perithecium 

 with ascogenous liyphae ; after Harper. 



protective hyphae begin to grow up, the oogonium elongates, the fusion 

 nucleus divides till a row of from five to eight nuclei is produced, transverse 

 walls appear, and a row of cells is formed of which the penultimate contains 

 two or more nuclei. 



From the surface of the penultimate cell, and perhaps sometimes from 

 that of its neighbours, filaments bud out (fig. 43(5), branch rapidly to form 

 a dense mass, and undergo septation. These are the ascogenous hyphae. 

 In them some six or eight intercalary cells, which will give rise to asci, 

 become distinguished by the fact that each contains two nuclei. The rest 



^ Erysiphe Polygoni DC. =Erysiphe communis (Wallr.) Link and Rabh., and E. Martii L^v. 



