A characteristic feature of the Mildew-Fungi is the thin, pseudo-parenchymatous covering of the ascocarp, enclosing _one_ (_Podosphæra_ and _Sphærotheca_; compare _Thelebolus_ among the Hemiasci) or _a few_ asci (Fig. 108 _c_), which do not form any hymenium, but are irregularly placed. The cells of the ascocarp-envelope are often prolonged into hair-like appendages. The ascocarps are developed from the mycelium at places where two hyphæ cross each other (Fig. 107). At these places two short and erect hyphæ are produced side by side. The one from the lower hypha (Fig. 107 _c_) assumes an ellipsoidal shape, and is known as the _archicarp_ or _ascogone_, while the other (“_pollinodium_”) arches over the ascogone. From the latter one ascus may be at once developed (_Sphærotheca_, etc.), or after its division several asci may be produced, each developed from one division. The sterile hypha (termed “pollinodium,” since it was formerly, but erroneously, supposed to fertilise the ascogone) produces a number of branches, and forms the pseudo-parenchymatous envelope of one cell in thickness, enclosing the asci. [Illustration: FIG. 108.--_Erysiphe communis._ A small portion of a leaf with this Fungus growing upon it (considerably magnified). The hyphæ b and d do not belong to this Fungus, but are reproductive organs of a pyrenomycetous Fungus parasitic upon it (_Cicinnobolus_).] Many plants, both cultivated and wild, are attacked by various species of Mildew. A common means of prevention against their attacks is to dust the diseased parts with sulphur. [Illustration: FIG. 109.--_Eurotium glaucum_: α portion of mycelium lying horizontally; β vertically-placed conidiophore; the mycelium gives rise to another branch near α; the conidia are abstricted from short flask-shaped cells; _b_ a ripe conidium; _c_, _d_ germinating conidia; _e_ spirally-twisted hypha, commencement of an ascocarp; _f_ a stage later; _g_ still later, the hypha at the base of the coil has given off branches which are applied to it; _h_, _i_ sections of young ascocarps.]