se 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).

[Illustration: FIGURE 241. FIGURE 242. FIGURE 243.

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 _lamellae_ 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.

[Illustration: FIGURE 244.

Clitocybe infundibuliformis, pileus infundibuliform, gills decurrent.]

[Illustration: FIGURE 245.

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