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 stem is usually fixed to the center of the pileus, but it may be eccentric, i. e., fixed to one side of the center, or entirely lateral. When the stem is wanting the pileus is sessile. With regard to its interior the stem is solid, when it is evenly fleshy throughout (Fig. 246), or hollow when the interior is occupied by a cavity (Fig. 248). If the cavity is narrow and tubular the stem is fistulose (Fig. 245); and if the center is filled with a pithy substance it is stuffed (Fig. 243). These terms apply only to the natural condition of the stem, and not the condition brought about by larvae, which eat out the interior of the stem, causing it to be hollow or fistulose.
The terms applicable to the consistency of the stem are difficult to define. In general, stems may be either fleshy or cartilaginous. The meaning of these terms can best be learned by careful study of specimens of each, but a few general characters can be given here. Fleshy, fibrous stems occur in the genera Clitocybe and Tricholoma, among the white-spored forms. Their consistency is like that of the pileus, namely, made up of fleshy, fibrous tissue. They are usually stout, compared with the size of the plant, and when bent or broken they seem to be more or less spongy or tough, fibrous, so that they do not snap readily. Cartilaginous stems have a consistency resembling that of cartilage. Their texture is always different from that of the pileus, which is fleshy or membranous. In general such stems are rather slender, in many genera rather thin, but firm. When bent sufficiently they either snap suddenly, or break like a green straw, without separating. In regard to their external appearance some resemble fibrous stems, while others are smooth and polished as in Mycena and Omphalia.
The veil - In the young stages of development the margin of the pileus lies in close contact with the stipe, the line of separation being indicated by a kind of furrow which runs around the young button mushroom. In many genera, as Collybia, Mycena, Omphalia, etc., the pileus simply expands without having its margin ever united to the stipe by any special structure, but in other forms, which include by far the greater number of genera of the Agaricaceae. and some Boleti, the interval between the stem and pileus is bridged over by threads growing from the margin of the pileus and from the outer layers of the stem. These threads interlace to form a delicate membrane, known as the veil, which closes the gap between the stem and pileus and covers over the young hymenium.
The veil remains firm for a time, but it is finally torn by the expanding pileus, and its remnants persist on the cap and stem in the form of various appendages, whose character depends on the character of the veil. In Cortinarius the veil is made up of delicate threads extending radially from the stem to the margin of the cap without forming a true membrane. From its resemblance to a spider's web such a veil is said to be arachnoid. At maturity mere traces of it can be found on the stem. In many genera the veil consists of a delicate membrane which tears away from the stem and hangs in flakes to the margin of the pileus. In these cases the veil is appen-diculate (Fig. 248). Frequently it is so delicate that no trace of it remains on the mature plant. Where the veil is well developed it usually remains on the stem as a ring or annulus which becomes free and movable in species of Lepiota (Fig. 242) and Coprimis, or forms a hanging annular curtain in Amanita, or a thick, felty ring in Agar-icus, etc. In some plants (species of Lepiota) the annulus is continuous with the outer cortex of the stem, which then appears as if it were partially enclosed in a sheath, with the annulus forming a fringe on the upper end of the sheath, from which the apex of the stem projects.
No reference is here made to the volva, which encloses the entire plant, and which is described in connection with the genera in which it occurs.
The few typical characters described here will help the student to become familiar with terms applied to them. In nature, however, typical cases rarely exist, and it is often necessary to draw distinction between differences so slight that it is almost impossible to describe them. Only by patient study and a thorough acquaintance with the characters of each genus can one hope to become familiar with the many mushrooms growing in our woods and fields.