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 
SS 
Fie. 190. Lilac mildew 
A, leaves of lilac upon which lilac mildew 
appears iu whitish patches; the small, dark 
reproductive bodies are also shown. 3B, 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 
mushroom are the spore- 
bearing hyphee. The spores 
form in great numbers; 
they are very light and are 
easily carried long distances 
by currents of air. Most 
of the mushrooms do not 
belong to the Ascomycetes. 
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 hyphe 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 
