=419. The large group of fungi= to which the mushroom belongs is called the basidiomycetes because in all of them a structure resembling a club, or basidium, is present, and bears a limited number of spores, usually four, though in some genera the number is variable. Some place the rusts (Uredineæ) in the same series (basidium series), because of the short promycelium and four sporidia developed from each cell of the teleutospore. =420. The gill-bearing fungi (Agaricaceæ).=—A good example for this study is the common mushroom (Agaricus campestris). This occurs from July to November in lawns and grassy fields. The plant is somewhat umbrella-shaped, as shown in fig. 232, and possesses a cylindrical stem attached to the under side of the convex cap or pileus. On the under side of the pileus are thin radiating plates, shaped somewhat like a knife blade. These are the gills, or lamellæ, and toward the stem they are rounded on the lower angle and are not attached to the stem. The longer ones extend from near the stem to the margin of the pileus, and the V-shaped spaces between them are occupied by successively shorter ones. Around the stem a little below the gills is a collar, termed the ring or annulus. [Illustration: Fig. 232. Agaricus campestris. View of under side showing stem, annulus, gills, and margin of pileus.] [Illustration: Fig. 233. Agaricus campestris. Longitudinal section through stem and pileus. _a_, pileus; _b_, portion of veil on margin of pileus; _c_, gill; _d_, fragment of annulus; _e_, stipe.] [Illustration: Fig. 234. Portion of section of lamella of Agaricus campestris. _tr_, trama; _sh_, subhymenium; _b_, basidium; _st_, sterigma (_pl._ sterigmata); _g_, basidiospore.] [Illustration: Fig. 235. Portion of hymenium of Coprinus micaceus, showing large cystidium in the hymenium.] =421. Fruiting surface of the mushroom.=—The surface of these gills is the fruiting surface of the mushroom, and bears the gonidia of the mushroom, which are dark purplish brown when mature, and thus the gills when old are dark in color. If we make a thin section across a few of the gills, we see that each side of the gill is covered with closely crowded club-shaped bodies, each one of which is a _basidium_. In fig. 234 a few of these are enlarged, so that the structure of the gill can be seen. Each basidium of the common mushroom has two spinous processes at the free end. Each one is a _sterig′ma_ (plural _sterig′mata_), and bears a gonidium. In a majority of the members of the mushroom family each basidium bears four spores. When mature these spores easily fall away, and a mass of them gives a purplish-black color to objects on which they fall, so that a print of the under surface of the cap showing the arrangement of the gills can be obtained by cutting off the stem, and placing the pileus on white paper for a time.