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, 

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



The white 



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, 



