. A textbook of botany for colleges and universities ... Botany. 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 a
. A textbook of botany for colleges and universities ... Botany. 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. i8o). 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 blacli dots on the and antheridium are uninucleate cells at '"y«lium which spreads 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) the terminal cell of the ascogonium becomes the solitary ascus; in others (as ificrosphaera and Uncinula) the terminal cell gives rise to ascogenous 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, dichotomously branching haifs, hairs with hook
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