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 calothcca. Mycelia (m) with haus- 

 storia penetrating cells (z) of 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." Mitcor mucedo. The myce- 

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



