PARASITIC FUNGI 7 



If one looks closely at the under surface of the leaf 

 beneath the brown spots, he will see, in case this fungus 

 is present, a w^hitish velvety mold. By making a thin 

 cross-section of the leaf at this point, and putting it 

 under the microscope, an appearance similar to Fig. 4 

 will be seen. At -the right is one of the long pointed 

 hairs which grow naturally upon the under surface of 

 the healthy leaf ; next to this toward the left is another 

 peculiar leaf-hair v/ith a club-like tip ; and beyond this 



FIG. 4. SECTION" OF POTATO LEAF SHOWING DEVELOPMENT OP 



MILDEW. 



project the hyph^e, or fruiting threads of the fungus, 

 which give off branches on which are borne small oval 

 bodies. The latter are the spores, or conidia, of the 

 fungus. They are the bodies by means of which the 

 fungus reproduces itself, and are developed on the 

 infested leaves in enormous numbers. 



These conidia are easily dispersed by slight air cur- 

 rents, and are carried far and wide en the wings of the 



