128 



MORPHOLOGY. 



Downy Mildews. 



285. The downy mildews make up a group of plants which are closely 

 related to the water moulds, but they are parasitic on land plants, and some 

 species produce very serious diseases. The mycelium grows between the 

 cells of the leaves, stems, etc., of their hosts, and sends haustoria into the 

 cells to take up nutriment. Gonidia are formed on threads which grow 

 through the stomates to the outside and branch as shown in figs. 137-140. 



The gonidia are borne on the tips 

 of the branches. The kind of 

 branching bears some relation to 

 thedifferent genera. Fig. 137 is 

 from Peronospora alsinearum on 

 leaves of cerastium; figs. 138 

 and 139 are Plasmopara viticola, 

 the grape mildew, while figs. 140 

 and 141 are from Phytophthora 

 infestans, which causes a disease 

 known as potato blight. The 

 gonidia of peronospora germinate 

 by a germ tube, those of plasmop- 

 ara first form zoogonidia, while 

 in phytophthora the gonidium 

 may either germinate forming a. 

 thread, or each gonidium may 

 first form several zoogonidia as shown in fig. 142. 



386. In sexual reproduction oogonia and antheridia are developed on the 

 mycelium within the tissues. Fig. 143 represents the antheridium entering 

 the oogonium, and the male nucleus fusing with the female nucleus in fertili- 

 zation. The sexual organs of Phytophthora infestans are not known. 



287. Mucor, saprolegnia, peronospora, and their relati\^es have few or 

 no septa in the mycelium. In this respect they resemble certain of the algse 

 like vaucheria, but they lack chlorophyll. They are sometimes called the 

 alga-like fungi and belong to a large group called Phycomycetcs. 



Fig I44- 

 Ripe oospore of Peronospora alsinearum. 



