[Illustration: FIG. 112.--_Penicillium crustaceum_: _a_ two spirally-coiled hyphæ arise from the mycelium, from one of which (archicarp) the asci are produced; _b_ a further step in the development of the ascocarp; the branching archicarp is surrounded by sterile hyphæ; _c_ section of young ascocarp; the larger hyphæ in the centre are the ascogenous hyphæ; these are enclosed by a pseudo-parenchyma of sterile hyphæ (× 300); _d_ series of ripe asci with spores; _e_ four ascopores seen laterally; _f_ germinating ascospores (× 800).] _Penicillium crustaceum_ (_P. glaucum_, Figs. 111, 112) is an exceedingly common “Mould.” Its mycelium appears very frequently on any organic matter which is permitted to remain untouched, and soon covers it with a dense mass of blue-green conidiophores. These branch at their summits and bear flask-shaped cells from which the conidia are abstricted. The ascocarps which, both in size and colour, resemble grains of sand, have only been obtained in luxuriant cultivation with a limited supply of oxygen. _Capnodium salicinum_ (_Fumago salicina_, _Cladosporium fumago_), a common Mildew, forms dark overgrowths on the leaves and branches of various shrubs (Poplars, Elms, Willows) and on Hops. The conidia appear in various forms, as on conidiophores, in conidiocarps with large multicellular conidia, and in conidiocarps with small unicellular conidia; in nutritive solutions yeast-like conidia are also developed.--_Apiosporium pinophilum_ produces mildew on the leaves of _Abies alba_ and _Picea excelsa_. (The conidial-forms were formerly described as “_Antennaria pinophila_”). Order 3. =Tuberaceæ, Truffles.= The Fungi belonging to this order are entirely subterranean. The mycelium is filamentous, and partly parasitic upon the roots of plants, especially trees, in its neighbourhood; it is then known as _Mycorhiza_. The fruit-body is relatively large, in some cases about the size of a hen’s egg. Internally it is traversed by a number of winding passages (Fig. 113 _a_), the walls of which are coated with the asci. The asci (_b_) contain only a small number of spores, and these are set free by the putrefaction of the fruit-body. Conidia are unknown. [Illustration: FIG. 113.--_Tuber melanosporum_: _a_ fruit-body (nat. size), a portion having been removed to show the internal structure; _b_ an ascus with ascospores.] _Tuber melanosporum_, _T. brumale_, _T. æstivum_, and other species are edible. _Terfezia leonis_ and _Choiromyces mæandriformis_ are also edible. The Truffles are always found in woods and under trees, and disappear when these are destroyed. France and Italy produce the best and the largest number of Truffles, which are hunted by specially trained dogs and pigs.