The Stem or Stipe.—The stem in this plant, as in many other kinds, is attached to the pileus in the center. The purpose of the stem seems quite surely to be that of lifting the cap and the gills up above the ground, so that the spores can float in the currents of air and be readily scattered. The stem varies in length from 2–10 cm. and is about 1–1-1/2 cm. in diameter. It is cylindrical in form, and even, quite firm and compact, though sometimes there is a central core where the threads are looser. The stem is also white and fleshy, and is usually smooth.
The Ring.—There is usually present in the mature plant of Agaricus campestris a thin collar (annulus) or ring around the upper end of the stem. It is not a movable ring, but is joined to the stem. It is very delicate, easily rubbed off, or may be even washed off during rains.
Parts Present in Other Mushrooms—The Volva.—Some other mushrooms, like the deadly Amanita (Amanita phalloides) and other species of the genus Amanita, have, in addition to the cap, gills, stem, and ring, a more or less well formed cup-like structure attached to the lower end of the stem, and from which the stem appears to spring. (Figs. [55], [72], etc.) This is the volva, sometimes popularly called the "death cup," or "poison cup." This structure is a very important one to observe, though its presence by no means indicates in all cases that the plant is poisonous. It will be described more in detail in treating of the genus Amanita, where the illustrations should also be consulted.
Figure 2.—Agaricus campestris. "Buttons" just appearing through the sod. Some spawn at the left lower corner. Soil removed from the front. (Natural size.)
Presence or Absence of Ring or Volva.—Of the mushrooms which have stems there are four types with respect to the presence or absence of the ring and volva. In the first type both the ring and volva are absent, as in the common fairy ring mushroom, Marasmius oreades; in the genus Lactarius, Russula, Tricholoma, Clitocybe, and others. In the second type the ring is present while the volva is absent, as in the common mushroom, Agaricus campestris, and its close allies; in the genus Lepiota, Armillaria, and others. In the third type the volva is present, but the ring is absent, as in the genus Volvaria, or Amanitopsis. In the fourth type both the ring and volva are present, as in the genus Amanita.
The Stem is Absent in Some Mushrooms.—There are also quite a large number of mushrooms which lack a stem. These usually grow on stumps, logs, or tree trunks, etc., and one side of the cap is attached directly to the wood on which the fungus is growing. The pileus in such cases is lateral and shelving, that is, it stands out more or less like a shelf from the trunk or log, or in other cases is spread out flat on the surface of the wood. The shelving form is well shown in the beautiful Claudopus nidulans, sometimes called Pleurotus nidulans, and in other species of the genus Pleurotus, Crepidotus, etc. These plants will be described later, and no further description of the peculiarities in form of the mushrooms will be now attempted, since these will be best dealt with when discussing species fully under their appropriate genus. But the brief general description of form given above will be found useful merely as an introduction to the more detailed treatment. Chapter XXI should also be studied. For those who wish the use of a glossary, one is appended at the close of the book, dealing only with the more technical terms employed here.
