44 



A TEXT-BOOK OF BOTANY. 



oogonium (Fig. 22, 0); the contents unite with the egg-cell, 

 after which a heavy membrane develops, forming an oospore, 

 which germinates when it finds a suitable host. The plants 

 belonging to Peronospora as well as related genera are destruc- 



FiG. 22. A. Cystopus candidus; B, Peronospora calotheca. Mycelia (w) with haustoria 

 penetrating cells (z) of hosts. C, Oospore formation in Peronospora: o, oogonium; n, anthe- 

 ridium. 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. 



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 



