ASCOMYCETES 



363 



the carpogone and form the single-layered outer perithecial wall. From 

 the inner surface of the cells composing this wall there subsequently 

 arise a number of other cells forming an inner wall several cells thick. 

 The growth of these separates the antherid from the carpogone, and it 

 takes part in the formation of the outer wall. From the outer wall, the 

 cells of which have become larger and brown in colour, fine rhizoids are 

 produced near the base, and in some species a few fine hairs at the apex 

 termed appendicula. Meanwhile the carpogone has divided into two^ 

 cells, one the ultimate ascus, and the other its pedicel-cell. Within the 

 ascus finally eight ascospores are formed. 



In Erysiphe (Hedw.) the chief points of difference from Podosphsra 



!FlG. 303. — /, //, Podosplupra pannosa de By. and Wor, /, chain of spores on sporophore and 

 mycele. //, ripe sporocarp with ascus, a, emerging through wall of sporocarp, h^ III — V^ P. 

 Castagnei de By. and Wor., fertilisation. //i, c, carpogone : /, antherid. /K, older state ; 

 A, hyphal branches of envelope. K, still older state in optical longitudinal section ; a, ascus 

 ■.< X 600). (/, //, after Tulasne, ///— K, after de Bary.) 



to be noted are these. The antherid winds spirally round the club- 

 ■shaped carpogone, which divides into a series of cells — produces a 

 number of asci — and the inner wall of the perithece is more developed. 

 The germinating ascospore gives rise to a mycele provided with 

 haustoria, on a suitable host, and from this thallus there spring short 

 sporophores which produce successively a series of acrospores. The 

 acrospores in turn produce a mycele exactly like the primary one from 

 the ascospore — which like it, if completely developed, ends by bearing 

 the sporocarp again. But owing to external conditions such as varying 

 weather, nutrition, and the like, this consummation is frequently not 

 reached, and acrospores only are then formed generation after generation. 



