PERISPORIALES 201 



ceae (and of the Perisporiales altogether) ; for only by this limitation does 

 there result that tufted arrangement of asci which forms the character- 

 istic feature of the Perisporiales. 



This degeneration goes still further in the Sphaerotheca type, for here 

 the development of the subterminal cell to a secondary ascogenous hypha 

 is also suppressed and the subterminal cell itself develops directly to an 

 ascus. 



If the interpretation is correct, the Erysiphaceae may be regarded as 

 a branch of the Plectascales in which the first and then the second phase 

 of development of the ascogenous hypha gradually disappears reaching 

 an end stage in Sphaerotheca. It is, however, not permissible (as repeat- 

 edly occurs in the literature) to contrast the subterminal formation of 

 asci in Sphaerotheca and their terminal formation in Erysiphe and Phyllac- 

 tinia and to find fault with one or the other observation; for the sporo- 

 genous ascogenous hypha belongs in the two groups to different stages of 

 development. 



The second problem which arises in the consideration of the life cycle 

 of the Erysiphaceae, is double fertilization. If the interpretation which 

 Harper and others have given to their observations were correct, a first 

 caryogamy occurs in the ascogonium and a second in the ascus. As was 

 briefly discussed in the introduction to the Ascomycetes, it is at present 

 impossible to clear up controversy over this point. As occasionally 

 observations on material from different hosts have been combined, a 

 source of error might lie here, as similar misinterpretations, resulting 

 from mixing material of different biological strains, have occurred also 

 in the Ustilaginales. In this case, one must assume that in the Erysipha- 

 ceae (as in the Agaricales of the Hygrophorus conicus type in the Basidio- 

 mycetes) the place of caryogamy varies, in one form, in the ascus, in the 

 other in the ascogonium, and that, by a combination of these types, there 

 is a deceptive picture of double fertilization. 



At maturity, the perithecia agree in the essential characters of their 

 structure with those of the Plectascales. They consist of a homogeneous, 

 brown, hard, brittle rind consisting of plates, regular polygonal cells and 

 of a hyaline nurse tissue in which the asci are imbedded (Hein 1927). 

 This collective tissue within the brown rind, together with the asci, is 

 occasionally referred to in systematic literature as "nucleus," centrum, 

 kernel or core. In contrast to the Plectascales, the perithecia in the 

 Erysiphaceae have assumed a protective function by wintering over, as 

 well as the task of propagation. Perhaps the limitation of the number of 

 asci to one and consequent reduction of the dimensions of the perithecia 

 is connected with this new function and hence in the Erysiphaceae there 

 appear problems similar to those which we have met in the Oomycetes. 



This propagative function of the perithecia is facilitated by two impor- 

 tant facts, by the development of special appendages and, in some forms, 



