THALLOPHYTES: FUNGI 



271 



Algas ; 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. 238. Sexual reproduction of Mucor, showing tips of sex branches meeting (A), 

 the two gametangia cut off by partition walls (B), and the heavy-walled zygote 

 ( C). — Caldwell. 



182. Peronospora These are the " downy mildews," very 



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

 common kind attacking grape leaves. The mycelium is 

 ccenocytic and entirely internal, ramifying among the tis- 

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

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

 presence of the parasite is made known by discolored and 



