THALLOPHYTE8 : FUNGI 



55 



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 gametarigia. Mucor, therefore, is isogamous. 



FIG. 38. 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 

 (f). CALDWELL. 



41. 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 cceno- 

 cytic and entirely internal, ramifying among the tissues 

 within the leaf, and piercing the living cells with haustoria 

 which rapidly absorb their contents (Fig. 39). The pres- 

 ence of the parasite is made known by discolored and 



