This section is from the book "Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc.", by George Francis Atkinson. Also available from Amazon: Studies of American Fungi: Mushrooms, Edible, Poisonous, Etc..
The pileus or cap is the first part of a mushroom which attracts the attention of the collector. It is the fleshy fruit body of the plant. This, like all other parts of the mushroom, is made up, not of cellular tissue as we find it in flowering plants, but of numerous interwoven threads, called hyphae, which constitute the flesh or trama of the pileus. Ordinarily, the filamentous structure of the flesh is very obvious when a thin section of the cap is examined under the microscope, but in certain genera, as Russula and Lactarius, many branches of the hyphae become greatly enlarged, forming little vesicles or bladders. These vesicles lie in groups all through the flesh of the pileus, sometimes forming the greater part of its substance. The filamentous hyphae pass around and through these groups, filling up the interstices. In cross section this tissue resembles parenchyma, and appears as if it were made up of rounded cells. Such a trama is said to be vesiculosa to distinguish it from the ordinary or floccose trama. The threads on the outer surface of the pileus constitute the cortex or cuticle. They are thick walled and often contain coloring matter which gives the plants their characteristic color. In many species their walls become gelatinized, covering the outside of the pileus with a viscid, slimy, or glutinous layer, often called pellicle. In other instances the corticle layer ceases to grow with the pileus. It is then torn and split by the continued expanding of the rest of the plant, and remains on the surface in the form of hairs, fibers, scales, etc.
Portion of vesiculose trama in the pileus of a Russula.
Figure 240. 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).
Lepiota procera, pileus convex, umbonate; annulus free, movable; gills free. Figure 243. - Agaricus campestris, pileus convex, gills free.