| Figure 239. | Figure 240. |
| Portion of vesiculose trama in the pileus of a Russula. | Portion of a floccose trama. |
As an example of the most usual form of the pileus, we may take that of the common mushroom (Agaricus campestris) when it is nearly expanded. The pileus is then quite regular in outline and evenly convex (Fig. 243). Many mushrooms during the early stages of their development have this form, which is variously changed by later growth. The convex pileus usually becomes plane or expanded as it grows. If the convexity is greater it is said to be campanulate (Fig. [245]), conical hemispherical, etc., terms which need no explanation. The pileus is umbilicate when it has an abrupt, sharp depression at the center (Fig. 241), infundibuliform when the margin is much higher than the center, so that the cap resembles a funnel (Fig. [244]), and depressed when the center is less, or irregularly, sunken. When the center of the pileus is raised in the form of a boss or knob it is umbonate (Fig. [242]). The umbo may have the form of a sharp elevation at the center, or it may be rounded or obtuse, occupying a larger part of the disc. When it is irregular or indistinct the pileus is said to be gibbous (Fig. [246]).
- Figure 241.—Omphalia campanella, pileus umbilicate, gills decurrent.
- Figure 242.—Lepiota procera, pileus convex, umbonate; annulus free, movable; gills free.
- Figure 243.—Agaricus campestris, pileus convex, gills free.
The gills.—The gills or lamellæ are thin blades on the under side of the pileus, radiating from the stem to the margin. When the pileus is cut in halves the general outline of the gills may be observed. In outline they may be broad, narrow, lanceolate, triangular, etc. In respect to their ends they are attenuate when gradually narrowed to a sharp point, acute when they end in a sharp angle, and obtuse when the ends are rounded. Again, the gills are arcuate when they arch from the stem to the edge of the pileus, and ventricose when they are bellied out vertically toward the earth.
| Figure 244. | Figure 245. |
| Clitocybe infundibuliformis, pileus infundibuliform, gills decurrent. | Mycena galericulata, pileus conic to campanulate, gills decurrent by a tooth, stem fistulose. |
The terms given above are often used in descriptive works, but the most important feature to be noted in the section of the plant is the relation of the gills to the stem. This relation is represented by several distinct types which are sometimes used to limit genera or sub-genera, since the mode of attachment is usually constant in all species of a group. The principal relations of the gills to the stem are described as follows: Adnate when they reach the stem and are set squarely against it (Fig. [247]); decurrent when they run down the stem (Fig. [244]); sinuate or emarginate when they have a notch or vertical curve at the posterior end (Fig. [246]); and free when they are rounded off without reaching the stem (Fig. [243]). In all cases when the lamellæ reach the stem and are only attached by the upper angle they are said to be adnexed. This term is often used in combination with others, as sinuate-adnexed (Fig. [248], small figure), or ascending adnexed (Fig. 248, larger plant). Sometimes the lamellæ are adnate, adnexed, etc., and have a slight decurrent process or tooth as in Mycena galericulata (Fig. [245]). In many plants the gills separate very readily from the stem when the plants are handled. Sometimes merely the expansion of the pileus tears them away, so that it is necessary to use great caution, and often to examine plants in different stages of development to determine the real condition of the lamellæ.