244 THE POWDERY MILDEWS 



91. Order f. Perisporiales or Powdery Mildews. This name 

 is given to a common and widely distributed group of largely 

 parasitic fungi that form cobwebby mycelia on the under surface 

 of the leaves of the elm, maple, willow, lilac, rhododendron, 



A 



FIG. 154. Further development of the ascocarp: A, sectional view, show- 

 ing the branches, s, derived from the germinating gametospore, that are 

 forming numerous lateral branchlets. B, one of the branchlets enlarged, 

 showing how it divides into cells which round off, forming the asci, as. C, 

 ascospore. D, germinating ascospore. After Brefeld. 



Virginia creeper, etc. (Fig. 155), and a great variety of herba- 

 ceous plants, as the dandelion and cocklebur. The majority of 

 them do not seriously interfere with the health of the plant, but 

 others produce serious diseases in young cherry and plum trees, 

 hop vines, gooseberry, etc. These parasites are external and 

 obtain their food by means of short branches, haustoria, which 

 dissolve the cell wall and absorb the cell contents as shown in 

 Fig. 156, h. The members of this order are of special interest 

 in that they throw considerable light upon the nature of para- 

 sitism. Frequently one and the same form will grow upon a 

 wide variety of plants, just as we will see to be the case in another 

 group, i. e., the rusts. Erysiphe graminis, for example, is found 

 upon barley, wheat, rye, oats and several genera of grasses. 

 But the form growing upon any one of these kinds of plants wil> 



