262 GENERAL MORPHOLOGY OF PLANTS 



and propagate the fungus rapidly. These stages are called im- 

 perfect. Another stage, which is the final stage in the life history, 

 or life cycle, bears the sacs or asci (containing the spores or asco- 

 spores) which make the characteristic fruit form of the members 

 of the class. This stage is called the perfect stage of the fungus. 

 It is often developed as the result of a sexual act, and thus repre- 

 sents the sexual reproduction in the class. Many of the species, 

 however, are believed to have lost the function of sexuality and are 

 supposed to develop the asci independently of a true fertilization. 

 In a few of the simple forms the asci are scattered without order in 

 loose wefts or knots of mycelium. In the majority of the species 

 the asci are closely crowded into extended surfaces (forming a 

 fruiting surface, the hymenium) or grouped in cup-shaped or 

 globose fruit bodies partly or entirely surrounded by a special 

 fungus tissue (a poth'ecium) , or entirely surrounded by fungus 

 tissue (perithecium} .* In some species, the lichen fungi, the 

 plant body is made up of an intimate association of fungus 



mycelium and algal cells. 



423. The powdery mildews 

 (Perisporiales). The powdery 

 mildews are very common and 

 conspicuous fungi parasitic upon 



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 de- 

 voted to the course will permit. One 

 lichen can be studied and others com- 



Fig. 221. ^ oared. The yeast should be studied 



Leaves of willow, showing willow mil- ***** J 



dew. The black dots are the iruit bodies unless this has already been done 

 (.pei ithecla) seated on the white mycelium. . T 



in Part I. 



* These fruit bodies of the Ascomycetes are sometimes called ascocarps, 

 i.e., sac fruits. 



