THALLOPHYTES 



75 



tissue, through which the asci are scattered. There is thus no definite 

 layer of asci (hymenium), as in other groups, a feature that char- 

 acterizes the Plectascales. 



(g) Pyrenomycetales 



This is an enormous group of fungi, comprising thousands of species. 

 There are two well-defined subgroups: the mildews and their allies 

 (Perispcriales) , and the black fungi (Pyre- 

 nomycetes proper). A representative or 

 two from each subgroup will serve as 

 illustrations. 



Mildews. These fungi form a family 

 of Perisporiales known as the Erysiphaceae 

 (often written Erysipheae). They are 

 superficial parasites on the higher plants, 

 the cobweb-like mycelium especially run- 

 ning over leaves, and sending out small 

 haustoria into the epidermal cells (fig. 

 1 80). From the mycelium there arises 

 a profusion of simple sporophores, each 

 producing a terminal row of conidia, 

 which multiply the parasite rapidly. FK lgo .- Micros fhaera (lilac 



When COnidium production declines, mildew): ascocarps (cleistothecia) 

 the sex organs appear. The OOgonium appearing as black dots on the 



and antheridium are uninucleate cells at ^eiium which spreacis over the 



surface of the leaf. 



the tips of branches, develop in contact, 



and through the usual perforation developed in such cases the male 

 nucleus enters the oogonium and fuses with the female nucleus. As 

 a result of fertilization, the oogonium becomes a short filament, 

 the ascogenous filament or ascogonium. In some of the mildews 

 (as Sphaerotheca) a cell of the ascogonium becomes the solitary ascus; 

 in others (as Microsphaera and Uncinula) one of the cells gives rise to as- 

 cogenous hyphae that produce several asci. From the cell beneath the 

 oogonium (the stalk cell), the sterile hyphae arise that form the sheath 

 of the closed ascocarp (cleistothecium), and from the sheath cells there 

 arise the characteristic appendages in the form of 'simple hairs, dichoto- 

 mously branching hairs, hairs with hooked tips, etc. (figs. 181, 182). 

 The ascocarps appear on the mycelium as small black or brownish dots 

 irregularly scattered (fig. 180), 



