WHITE MILDEWS OE BLIGHTS. 175 



finds that the mildew is more prolific than his peas. 

 The leaves become sickly and yeUow as the myce- 

 lium of the fungus spreads over them^ when they 

 present a peculiar appearance, as if growing beside 

 a chalky road in dry dusty weather, and had become 

 covered with comminuted chalk. Soon the con- 

 ceptacles appear, profusely scattered over the white 

 threads, like grains of gunpowder (fig. 237), and after 

 a brief struggle for existence the pea and its parasite 

 die together. In this species {Erysiphe Ma/rtii, 

 Lev.), the appendages are nearly transparent, short, 

 and much interwoven with the mycelium (fig. 238), 

 the globose sporangia containing from four to 

 eight spores (fig. 239). It is not confined to peas, 

 although that habitat has been here given for it, 

 because it is so common upon them. Beans, melilot, 

 St. John^s wort, some umbelhferous plants, and the 

 meadow-sweet, have all been found afiected. 



The species found on grasses, especially the cocks- 

 foot, has been already alluded to. The conceptacles 

 contain from twenty to twenty-four ovate sporangia, 

 each enclosing eight spores. The appendages and 

 mycelium are much interwoven. 



Another of these " white mildews," not only on 

 account of its frequency of occurrence on certain 

 plants, but also from the numerous species of 

 phanerogamous plants on which it is found 

 (fig. 240), may be truly designated "common" 

 {Erysiphe comm/unifs, Lk.) ; many kinds of crow- 

 foot, especially Ranunaulus aeris, are suDJect to its 



