44 



A TEXT-BOOK OF BOTANY. 



oogonium (Fig. 22, o) ; 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- 



Fii;. jj. A, Cy-stopus candidus; B, Peronospora calntheca. Mycelia (m) with haustoria 

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

 ridium. At the Ict't the antheridium is in contact with ooKonium; the next stage shows the 

 antherHhr .niuni and discharging its contents; at the right the resulting 



oospore is shown. After De Uary. 



tive to many cultivated plants, constituting mildews or blights, 

 as tlii'M- iK\-urriiig on the lcavc> of hyo.-cyamus, tobacco, anthe- 

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

 group ha- revived the name 'downy mildews' because of the 



