20 



BOTANY AND PHARMACOGNOSY. 



tive to many cultivated plants, constituting mildews or' blights, 

 as those occurring on the leaves of hyoscyamus, tobacco, anthe- 

 mis, matricaria, aconite, grape vine, lima bean, potato, etc. The 

 group has received the name " downy mildews " because of the 



Fig. 12. A, Cystopus candidus;. B, Peronospora calotheca. Mycelia (m) with haus- 

 storia penetrating cells (z) qf hosts. C', Oospore formation in Peronospora: o, oogonium; 

 n, antheridium. At the left the antheridium is in contact with oogonium; the next stage 

 shows the antheridium penetrating oogonium and discharging its contents; at the right the 

 resulting oospore is shown. — After De Bary. 



fact that the conidiophores rise to the surface of the leaves 

 where the spores are discharged, forming powdery patches. 



Black Mold. — A common example of the Zygomycetes 

 is furnished by the " black mold," Mucor mucedo. The myce- 

 lium of this plant is coenocytic, thread-like, very much branched. 



