356 THALLOPHYTES 
common and important one of the many Downy Mildews and 
often causes much loss in grape-growing districts. Its downy 
white growth occurs most commonly on the leaves, but the Fun- 
gus often attacks the green shoots and fruit (Fig. 306). Some- 
times it destroys the fruit crop and weakens the vines. 
The mycelium consists of coenocytic hyphae, which extend 
through the tissues of the part at- 
tacked. The hyphae grow between 
cells and send into the cells short 
branches (haustoria) which absorb 
the cell contents of the host (Fv. 
307). The death of the leaf cells re- 
sulting from the attack is indicated 
by the occurrence of yellow or brown 
areas which may involve much of 
the leaf. This destruction of leaf 
tissue diminishes the carbohydrates 
Fie. 306. — A bunch 
of Grapes partially de- Fic. 307.— The haustoria of the 
stroyed by the Downy Downy Mildew reaching into the 
Mildew. From Farmer’s cells of the grape. h, hypha; a, 
Bulletin 284, U.S. Dept. haustoria. From Bulletin 214, Ohio 
of Agriculture. Agr. Exp. Sta. 
furnished by the leaves and as a result both fruit and vine 
may suffer. Often the fruit is directly attacked and de- 
stroyed. After the Mildew is well established within the tissues 
of the hosts, it sends through the stomata numerous branches 
which constitute the superficial downy patches characteristic 
of the parasite (Fig. 808). On the tips of these protruding 
hyphae are produced small globular bodies known as conidio- 
spores or conidia, and the hyphae- bearing them are called 
conidiophores which means ‘‘conidia bearing.”’ 
The conidia are really small sporangia which break off and are 
