212 REPRODUCTION OF THE ASPERGILLALES 



dogs and pigs are trained to locate the truffles by smell. This 

 industry amounts to more than fi, 000,000 annually in France 

 and Italy. 



FIG. 144. 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. 



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86. Order d. 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 sur- 

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

 Virginia creeper, etc. (Fig. 145), 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. 146, h. The spores are formed in chains (Fig. 146, c} 

 from the end of the erect hyphae that project from the surface 

 of the leaf in thick masses, causing the powdery appearance and 

 the popular name of these parasites. These spores germinate 

 quickly and rapidly spread the fungus. 



