TIIALLOPIIYTES: FDNGl 



271 



Algae ; that the sexual spore is a zygote ; and that the two 

 pairing tip cells cut off from the main body by partition 

 walls are gametangia. Mucor, therefore, is isogamous. 



Fig. 338. Sexual reproduction of Jhiror, showiiiij fips of sex branches meeting (A), 

 the two gametantria cut off by partition walls (/;), and the heavy-walled zygote 

 ( C). — Caliiwei.l. 



182. Peronospora. — Tliese are the " downy mildews," very 

 common parasites on seed plants as hosts, one of the most 

 common kind attacking grape leaves. The mycelium is 

 ca3noc3'tic and entirely internal, ramifying among the tis- 

 sues within tlie leaf, and piercing the living cells with haus- 

 toria which rapidly ahsorb their contents (Fig. 239). The 

 presence of the parasite is made known by discolored and 



