PARASITIC FUNGI 363 



most important biological phenomena, and merits extended 

 study. 



748. The common mildews, lilac or grape mildew, which 

 are seen in late summer as a whitish "fur" on the surface of 

 leaves, is due to a mildew spore which, blown by the wind, 

 falls upon the leaf and germinates. It puts out a slender tube 

 which grows through a stoma into the mesophyll. Here it 



FIG. 228. Peridermium, a rust fungus parasitic on pine trees. The white 

 ridges are composed of masses of spores. X2/3. 



develops by absorbing its nourishment from the mesophyll 

 cells, until finally it puts numerous branches out through the 

 stomata, and on each of these are borne numerous spores. 

 The orange-colored or black specks which appear later on the 

 surface of the leaf are spore cases in which a second kind of 

 spore is produced from the same mycelium. 



749. The rusts which occur on our cereal grasses, wheat, 



