The genus _Pleurotus_ is usually recognized without difficulty among the fleshy, white-spored agarics, because of the eccentric (not quite in the center of the pileus) or lateral stem, or by the pileus being attached at one side in a more or less shelving position, or in some species where the upper side of the pileus lies directly against the wood on which the plant is growing, and is then said to be _resupinate_. The gills are either decurrent (extending downward) on the stem, or in some species they are rounded or notched at the junction with the stem. There is no annulus, though sometimes a veil, and the genus resembles both _Tricholoma_ and _Clitocybe_, except for the position of the stem on the pileus. In _Tricholoma_ and _Clitocybe_ the stem is usually attached at the center, and the majority of the species grow on the ground, while the species of _Pleurotus_ are especially characterized by growing on wood. Some species, at least, appear to grow from the ground, as in Pleurotus petaloides, which is sometimes found growing on buried roots or portions of decayed stumps which no longer show above ground. On the other hand species of _Clitocybe_, as in C. candida (Fig. 91), often have an eccentric stem. This presents to us one of the many difficulties which students, especially beginners, of this group of fungi meet, and also suggests how unsatisfactory any arrangement of genera as yet proposed is. =Pleurotus ulmarius= Bull. =Edible.=--The elm pleurotus is so called because it is often found growing on dead elm branches or trunks, or from wounds in living trees, but it is not confined to the elm. It is a large species, easily distinguished from the oyster agaric and the other related species by its long stem attached usually near the center of the cap, and by the gills being rounded or notched at their inner extremity. The cap is 5--12 cm. broad, the stem 5--10 cm. long, and 1--2 cm. in thickness. [Illustration: FIGURE 106.--Pleurotus ulmarius. Under view and section (natural size). Copyright.] The =pileus= is convex, the margin incurved, then nearly expanded, smooth, firm, white or whitish, or with shades of yellow or brown on the center, and the flesh is white. The =gills= are broad, rather distant, sinuate, white or nearly so. The =spores= are globose, 5--8 µ in diameter. The =stem= is firm, eccentric, usually curved because of its lateral attachment on the side of the tree, and the horizontal position of the pileus.