PERI SPORI ALES 197 



whose upper cells form conidia. When this falls off, the subterminal cell 

 (or conidial mother cell) divides again into two daughter cells, the upper 

 of which develops to a new conidium. The development of Phyllactinia 

 proceeds similarly, except the conidiophores are formed only on the 

 superficial mycelium. 



In the second type, e.g., Sphaerotheca Humuli (S. Castagnei) a protru- 

 sion forms on a hypha above a nucleus (Fig. 123, 1); this elongates and 

 divides into two cells of which one remains connected with the sporiferous 

 hypha while the other projects over it (Fig. 123, 2). The latter is the 

 mother cell of all future conidia; it cuts off successively a series of daughter 

 cells which round off to conidia, or first divided into two and the resulting 

 daughter cells change to conidia (Fig. 123, 3 to 6). Thus the mother cells 

 in this group, which includes Erysiphe graminis, E. dehor acearum, 

 Sphaerothecca pannosa and S. mors-uvae, lie directly on the sporiferous 

 hypha. In another group, as Erysiphe Polygoni and Uncinula Salicis, 

 it is separated from the sporiferous hyphae by a longer or shorter stipe 

 cell. 



The conidia are hyaline, uninucleate and generally only capable of 

 germination for a short time. As they are formed in enormous numbers, 

 they facilitate a very rapid dissemination. In damp air or water, they 

 develop one or more germ tubes on their narrow side. 



As they often appear months before the perithecia and as on many 

 hosts the perithecia are unknown, many Erysiphaceae have been 

 described as imperfects; thus the conidial forms of Leveillula taurica type 

 were placed in the imperfect genus Oidiopsis, the conidial forms of the 

 Phyllactinia type were in Ovulariopsis and the conidial forms of the ecto- 

 phytic type in Oidium. Occasionally (although the expression has 

 nothing to do with oidial formation) this designation is still used in plant 

 pathology. 



The perithecia generally begin to appear in the course of the summer. 

 From two neighboring hyphae there arises a thick, slightly ovoid asco- 

 gonium and a slender, somewhat curved antheridium; often they coil 

 about each other. Like the hyphal cells, they are both originally uni- 

 nucleate, and separated from the sporiferous hypha by a septum. The 

 male nucleus divides, and a basal stipe cell is cut off from the apical 

 antheridium (Figs. 124, 2; 126, 1 and 2), exceptionally the ascogonium 

 may also undergo a division. 



The further development of both copulation branches has been the 

 object of considerable controversy. In a first group, as in Erysiphe 

 Polygoni (E. communis) on Trifolium and Mellilotus (Harper, 1896), in 

 Sphaerotheca Humuli on Humulus (Harper, 1895; Blackman and Fraser, 

 1905, denied by Dangeard, 1907), in Phyllactinia corylea on Fraxinus 

 americana, Corylus americana, Celastrus scandens and Betula lutea 

 (Harper, 1905) and in Sphaerotheca mors-uvae on Ribes sp. (Bezsonov, 



