Pileus
(from pileus, a cap). The name given to the broad, expanded part in Mushrooms and allied groups of the larger Fungi. It is very frequently, e.g., in the common Mushroom, shaped like an inverted saucer, or a shallow skull-cap but shows a considerable diversity of shape in different species; e.g., in Marasmius oreades and Hydnum repandum It is usually fixed by the middle of its lower surface, as in all the examples quoted, to the stop of a stalk; but in a good many Fungi that possess a Pileus, the stalk is fixed to one side (e.g., in many species of Polyporus), and others want it altogether. The upper surface of the Pileus is usually covered with a skin or coat, which in many species is slimy, but in some is scaly, in others hairy, &c. The Pileus bears generally, on its lower surface, the spore-producing surface, or hymenium--in some genera, in the form of gills; in others, like tubes or fleshy teeth (see Mushrooms). The Pileus and stalk together form the hymenophore.