THALLOPHYTES: FUNGI 
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. 238. Sexual reproduction of Mucor, showing tips of sex branches meeting (A), 
the two gametangia cut off by partition walls (), 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 
coenocytic 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 
