CH.XJ 



THE MILDEWS 



455 



Order 3. Erysiphem: the 

 These are familiar on leaves 



Mildews. 

 of Lilacs, 



sumes, and itself replaces, all of the organic matter within 

 the chitinous skin. Later it carries to the surface of the 

 ground a club-shaped fructification, containing perithecia 

 with ascospores. The stalk grows to a 

 length sufficient to attain the surface, and 

 often that is considerable. 



Another small group of Ascomycetes para- 

 sitic upon insects, especially water beetles, 

 is the Laboulbeniales. They are notable for 

 the clear definition of their sexual reproduc- 

 tion, which bears striking resemblance to 

 that of Red Algae. Thus they tend to show . 

 a direct connection between that group ^f\A^^^ 

 and the Ascomycetes. *^" 



Fig. 317. — Cor- 

 dyceps militaris ; 

 natural size. The 

 swellings on the 

 stalked sporopho res 



Alders, Roses, and others, where they appear are the perithecia. 



as delicate, white, Superficial growths, (After Errera and 



mold-like or cobweb-like. In autumn they 

 develop minute black perithecia amidst the white mycelium, 

 justifying the name of " powdery mildews" sometimes ap- 

 plied to distinguish them from the "downy mildews" which 

 are Phy corny cetes, earlier studied (page 436). Some 100 

 species are known, of which a number are mildly injurious 

 parasites. 



A typical form, Microsphcera Alni, occurs on the Alder. A 

 loose mycelium on the surface of the leaf sends haustorial 

 hyphae into the epidermal cells, and therefrom absorbs nour- 

 ishment. Up from the surface it sends conidiophores, from 

 which abundant spores are abstricted and carried by the 

 wind. In sexual reproduction ascogonia and antheridia 

 develop from neighboring hyphae and fuse ; and as result of 

 this fertilization there is formed a peculiar, globular, hard, 

 black perithecium, visible to the eye as a black spot amid 

 the white mycelium on the leaf (Fig. 318). From these 



