POWDERY MILDEWS 221 



cells below by cross partitions. The cells thus formed are 

 at first oblong and cylindrical, with flattened ends; but 

 the topmost ones soon become rounded at their extremi- 

 ties, thus giving rise to a row of cells, the spores, or 

 conidia. These fall off successively and germinate at once 

 by pushing out a tube, which gives rise to a new plant. 



364. The sexual process (generation) in most species 

 takes place late in the season. Two 



filaments crossing each other or coming 

 into close contact swell slightly and send 

 out from each a short branch; one of 

 these becomes the oogone, and the other 

 the antherid, both organs being very FIG. 

 much reduced. 



365. Fertilization is effected by the direct union of 

 protoplasm. Eight or ten branches then bud out below 

 the oogone, and growing upward soon completely enclose 

 it in a cellular coat which eventually becomes hardened 

 and turns brownish in color, constituting the spore-fruit 

 (perithecium) . 



366. The oogone inside of the perithecium gives rise, 

 by' branching, to one or more large cells (young asci) 

 filled at first with granular protoplasm, which soon forms 

 two to eight spores (ascospores) . Upon its outer surface 

 the spore-fruit develops long filaments (known as 

 "appendages"), probably for holdfasts. In some genera 

 these terminate in hooks; in others they are dichotom- 

 ously branched; in still others, needle-shaped; while in 

 many species they end irregularly. The spore-fruits re- 

 main during the winter upon the fallen and decaying 

 leaves, and finally, by rupturing, permit the asci, with 

 the contained spores, to escape. 



367. The Herbarium-mold (Aspergillus) is related to 

 the Mildews and belongs to the order of Little Tubers 



