FUNGI, SAC FUNGI 



289 



species cause rots of fruits, many others are harmless, while a 

 few are of great importance in the ripening and flavoring of 

 certain varieties of cheese* (Roquefort, Camembert, etc). The 

 ascus (or perfect) stage of the blue mold is difficult to obtain and 

 study. (This stage of the powdery mildews, however, is very 

 easy to obtain and study.) 



437. The powdery mildews (Perisporiales). The powdery 

 mildews are very common and conspicuous fungi parasitic upon 

 a great variety of plants, on leaves, 

 stems, flowers, and fruit. Many 

 of the common mildews belong 

 here. The mycelium grows on 

 the surface of the host, forming a 

 thin and irregular web-like whitish 

 layer, just visible to the eye. 

 Branches called haustoria pene- 

 trate to the epidermal cells, in 

 some species even to the deeper 

 cells, and draw nutriment from 

 the protoplasm. When very 

 young leaves and stems are 

 affected they are often checked in 

 growth. The large number of 

 white conidia (or conidia-spores) 

 in chains or in loose masses give a 

 powdery appearance to the surface of the plants affected, hence 

 the name powdery mildews. Some of the important diseases 

 caused by the powdery mildews are the gooseberry mildew; 



To THE TEACHER. The practical study should include at least one of the 

 powdery mildews. There are other members of the sac fungi which make 

 striking examples for illustration. These can be shown by the teacher, and at 

 his discretion may be included in the practical study where the time devoted 

 to the course will permit. One lichen can be studied and others compared. 

 The yeast should also be studied. 



* See Bulletin 118, Bureau of Animal Industry, U. S. Department of 

 Agriculture, "Cultural Studies of Species of Penicillium," 1910. 



Fig. 232. 



Leaves of willow, showing willow mil- 

 dew. The black dots are the fruit bodies 

 (perithecia) seated on the white mycelium. 



