246 



REPRODUCTION OF PERISPORIALES 



twelve generations, at which time he states that the spores would 

 grow upon the untreated leaves. Perhaps this is the explanation 

 of some epidemics or the occasional sudden appearance of a plant 

 disease. A variety of circumstances might cause plants to form 

 substances attractive to the parasite or to fail to develop repellent 



Fig. 157. Sexual reproduction of a powdery mildew: A, meeting of the 

 male, an, and female, 0, gametangia. B, fertilization, the male gamete, m, 

 is seen approaching the female. C, section of young ascocarp showing the 

 early germination of the gametospore, which has become surrounded by 

 hyphae derived from the mycelium. D, later stage, the gametospore has 

 developed several cells and the second cell from the end, as, will produce 

 the ascus. — After Harper. 



materials. In either case they would become susceptible to the 

 disease. 



The spores are formed in chains (Fig. 156, c) from the end 

 of the erect hyphae that project from the surface of the leaf in 

 thick masses, causing the powdery appearance and the popular 

 name of these parasites. These spores germinate quickly and 

 rapidly spread the fungus. 



The reproductive organs have in this group become still 

 further reduced and appear as short branches as shown in Fig. 

 157, A. The solution of the walls at the point of contact of 

 these organs permits the male gamete to pass over and fuse 

 with the female (Fig. 157, B). The growth of the gametospore 

 forms a limited number of cells and one of them, usually the 



