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 
(Perisporiales), 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. 
When COnidium production declines, mildew): ascocarps (cleistothecia) 
the sex organs appear. The oogonium appearing as black dots on the 
j ,i i- -i 11 * mycelium which spreads over the 
and anthendmm are unmucleate cells at , , , - 
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). 
1 
FIG. 180. Microsphaera (lilac 
