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. [Illustration: FIGURE 246. FIGURE 247. FIGURE 248. FIGURE 246.--Tricholoma, gills sinuate, stipe solid. FIGURE 247.--Panæolus papilionaceus, gills adnate. FIGURE 248.--Left-hand small plant, Hygrophorus, gills sinuate, adnexed. Right-hand plant Panæolus retirugis, gills ascending adnexed, veil appendiculate.] 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. [Illustration: FIGURE 249.--Section of portion of gill of Marasmius cohærens. _t_, trama of gill; _sh_, sub-hymenium; _h_, hymenium layer. The long, dark cells are brown cystidia, termed spicules by some to distinguish them from the colorless cystidia. The long cells bearing the oval spores are the basidia.]