Chapter 26

DESCRIPTION OF TERMS APPLIED TO CERTAIN STRUCTURAL CHARACTERS OF MUSHROOMS.

By H. HASSELBRING.

In fungi, as in higher plants, each organ or part of the plant is subject to a great number of variations which appeal to the eye of the student, and by which he recognizes relationship among the various individuals, species, and genera of this group. For the purpose of systematic studies of mushrooms or even for the recognition of a few species, it is of primary importance to be acquainted with terms used in describing different types of variation. Only a few of the more important terms, such as are employed in this book, together with diagrams illustrating typical cases to which they are applied, will be given here.

=The pileus.=--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 _hyphæ_, 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 _hyphæ_ 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 _hyphæ_ 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 _vesiculose_ 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.

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).

=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.

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.

In certain genera the gills have special characteristics which may be noted here. Usually the edge of the lamellæ is _acute_ or sharp like the blade of a knife, but in _Cantharellus_ and _Trogia_ the edges are very blunt or obtuse. In extreme forms the lamellæ are reduced to mere veins or ridges. Again, the edge is generally _entire_, i. e., not noticeably toothed, but in _Lentinus_ it is often toothed or cut in various ways. In some other plants the edges are _serrulate_, _crenulate_, etc. In _Schizophyllum alneum_, a small whitish plant very common on dead sticks, the gills are split lengthwise along the edge with the halves revolute, i. e., rolled back. In _Coprinus_ the gills and often a large part of the pileus melt at maturity into a dark, inky fluid.

=The hymenium.=--The term _hymenium_ is applied to the spore-bearing tissue of many fungi. In the _Agaricaceæ_ the hymenium covers the entire surface of the gills and usually the portion of the pileus between the gills. It originates in the following manner: the threads forming the trama of the gills grow out from the lower side of the pileus and perpendicular to its under surface. As growth advances many branches of the threads turn outward toward either surface of the gill and finally terminate in club-shaped cells. These cells, therefore, lie side by side, perpendicular to the surface, forming a pavement, as it were, over the entire surface of the gills. Some of them put out four little prongs, on each of which a spore is borne, while others simply remain as sterile cells (Figs. 249, 250). The spore-bearing cells are _basidia_; the others are called _paraphyses_. They resemble each other very much, except that the basidia bear four _sterigmata_ and a spore on each. In a few species the number of sterigmata is reduced to two and in some low forms the number is variable. The layer just beneath the basidia is usually more or less modified, being often composed of small cells different from the rest of the trama. This is called the _sub-hymenial_ layer or _sub-hymenium_ (Fig. 250).

Other cells called _cystidia_ occur in the hymenia of various species distributed through nearly all the genera of the agarics. Cystidia are large, usually inflated, cells which project above the rest of the hymenium (Fig. 250). They originate either like the basidia, from the sub-hymenial cells (Fig. 250), or from special hyphæ deeper down in the trama of the gill (Fig. 249). They are scattered over the entire surface of the hymenium, but become more numerous on the edge of the lamellæ. Their number is much smaller than that of the basidia, but in some species where they are colored they may greatly change the appearance of the gills. Cystidia often secrete moisture which collects in drops at their tips, a phenomenon common to all free fungous cells.

=The stem.=--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 larvæ, 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 _Agaricaceæ_ 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 _appendiculate_ (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 _Coprinus_, or forms a hanging annular curtain in _Amanita_, or a thick, felty ring in _Agaricus_, 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.