THE FUNGI 



171 



FIG. 137. Microsphsera Alni. A, peri- 

 thecium, with dichotomously braiiched 

 appendages (X 150). B, asci. 



produced, which in some cases have led to confusion in their classi- 

 fication. The formation of the spore-fruit is in some cases preceded 

 by the development of sexual organs and a genuine fertilization, but 

 there is little question that in 

 many of them no trace of sexu- 

 ality remains. 



Perisporiales. The simplest of 



the Fyrenomycetes are the Mildews 



and their allies (Perisporiales). The 



best known of these are those belong- 



ing to the family Erysiphese, including 



many common plant-parasites. The 



Rose-mildew ( Sphcerotheca pannosa) 



is, perhaps, the best known. These 



Mildews are superficial parasites, the 



mycelium forming a delicate webby 



growth upon the surface of the leaves 



of the host, into which are sent short 



haustoria (Fig. 138). The mycelium 



sends up numerous upright conidio- 



phores, which divide into a series of 



short cells. These cells then swell 



somewhat, and break off as barrel- 



shaped conidia. It is these masses of conidia which give the powdery appear- 



ance to the actively growing Fungus. 



Sex-organs. The sexual organs have been especially studied in Sphcerotheca 



castagnei (Fig. 136), which is common upon the Dandelion, and upon a variety 



of other plants as well. The sex-organs are first formed after the production 



of conidia begins to decline. The oogonium is an oval cell with a single nucleus. 



From a branch close by, the antheridial branch grows up, in close contact with 



the oogonium. The antheridial 

 cell is cut off from its apex, 

 and 1 fuses with the oogonium, 

 into which its nucleus passes, 

 the nucleus fusing with that of 

 the oogonium (Fig. 136, F). The 

 fertilized oogonium divides 

 transversely, and forms a short 

 filament, the end-cell of which 

 becomes the single ascus found 

 in the ripe perithecium (G). 

 The wall of the perithecium is 

 formed from filaments growing 

 up about the oogonium and 

 ^^i^i.. enclosing it It is 

 ' 



not unn ke tnat of Aspergillus, 

 and also has its outer cells 



dark-colored, so that the ripe perithecia appear as black specks scattered over 



the whitish mycelium. 



In Erysiphe and other genera the ascogenous hypha derived from 

 the oogonium develops several asci (Fig. 137). From the outer 



Fm. m.-Erystphe sp. (on Chrysanthemum), 

 showing the haustoria, h. A, from above; B, 

 in section (x450). 



