176 



BOTANY. 



habits of the one and the aerial and parasitic or saprophytic 

 habits of the other. 



317. In the Powdery Mildews, which are all para- 

 sitic, the jointed filaments closely cover the leaves and 

 other tender parts of their hosts, and draw nourishment 

 from them by means of suckers, which project as irregular 

 outgrowths from the side next to the epidermis (Fig. 96). 

 These suckers apply themselves closely to the epidermal 

 cells, and, in some cases, appear to penetrate them. 



318. The crossing and branching filaments soon send up 

 many vertical branches, in which partitions form at regu- 

 lar intervals. The cells thus formed are at first oblong 

 and cylindrical, with flattened ends ; but the topmost one 



Fig. 98. Fig. 99. 



Fig. 98.— The sexual process In a Powdery Mildew (Erysiphe). o, jointed 

 threads ; b, antherid ; c, carpogone ; d, young spore-fruit ; 6, older spore- 

 fruit. Magnified. 



Fig. 99.— Ripe spore-fruit of WUlow-mlldew (Unclnula salicis). The 

 appendages are curved or hooked. Magnified. 



soon becomes rounded at its extremities, and the others 

 follow in quick succession, thus giving rise to a row of 

 cells, the spores, or conidia (Fig. 97). These fall off and 

 germinate at once by pushing out a tube, which gives rise 

 to a new plant. 



319. The sexual process in most species takes place late 

 in the season. Two fi.laments crossing each other or com- 

 ing into close contact swell slightly and send out from each 



