244 INTRODUCTION TO BOTANY 



temperature it develops in a very short time, growing by 

 means of food material that has been gathered by the under- 

 ground growth. In the deep, wrinkle-bordered pits of the 



mushroom are the spore- 

 bearing hyphse. The spores 

 form in great numbers ; 

 they are very light and are 

 easily earned long distances 

 by currents of air. Most 

 of the mushrooms do not 

 belong to the Aeomyeete9. 

 232. Lilac mildew. Mil- 

 dews are frequently found 

 upon the surface of leaves 

 of lilac (fig. 190) and 

 upon the willow, oak, some 

 of the smartweeds, and 

 many other plants. The 

 fungus is a superficial 

 parasite with haustoria 

 penetrating the host cells. 

 At times upright hyphse form conidia, and to their presence 

 the powdery appearance of these mildews is largely due. The 

 conidia, if favorably placed, are the means of producing new 



FIG. 190. Lilac mildew 



A, leaves of lilac upon which lilac mildew 

 appears iu whitish patches; the small, dark 

 reproductive bodies are also shown. B, the 

 central, heavy-walled body (ascocarp) which 

 contains the sacs (asci) in which spores are 

 formed. Upon the wall of the ascocarp are 

 stalks, sometimes called arms, which have 

 peculiar branches at their tips. About 60 

 times natural size 



