[Chap. XLIV 



THE FUNGI 



551 



Conidmm 



Oogonium 



Arxthfindium 



Fig. 242. One of the tube fungi, Cystopiis, a parasite in leaves of many green 

 plants: A, section of a leaf of a host plant in which a pustule has been made by 

 the formation and growth of conidia on hyphae beneath the epidermis, zygotes 

 have formed from intercellular hyphae within the mesophyll of the leaf; B, part 

 of the fungus illustrated in A dissected out and enlarged; C, mesophyll cell sur- 

 rounded and penetrated by hyphae. Motile spores are formed when the conidia 

 and zygotes germinate. Courtesy of World Book Co. 



In late summer small black dots may appear among the hyphae on 

 the infected leaves. The dots are reproductive bodies and under the 

 microscope are seen to be spherical and thick-walled, with curiously 

 branched transparent appendages. These fruiting bodies, or ascocarps, 

 are the result of the growth of coalescent hyphae, following the fusion 

 of the protoplasts of an oogonium and an antheridium. The ascocarps 

 contain several sac-like structures termed asci (sing, asciis) (Fig. 241). 

 Within each ascus are usually 8 ascospores, although the number may 

 vary from 2 to many. The germination of an ascospore the following 

 spring results in a hypha which may grow upon other plants and start a 

 new life cycle of the fungus. 



Downy mildews. The downy mildews are internal obligate parasites 

 that are verv destructive to many plants, such as potato, onion, tobacco, 

 and grape. They are not classified with the powdery mildews because of 

 differences in structure and reproduction (Fig. 242). The downy mil- 

 dews have tubular, non-septate hyphae similar to those of bread mold 

 and water molds. More than 1000 species of molds are characterized by 



