Mushroom gills, or lamellÆ, anatomically considered, are composed, first, of a central portion, a prolongation of the hymenophore or flesh of the cap, more or less dense, sometimes so thin as to be scarcely perceptible; second, the hymenium or spore-bearing membrane covering the surfaces of this prolonged hymenophore. They are vertical, simple, equal, respectively, or more frequently alternating with shorter gills. They are often evanescent and putrescent, sometimes liquefying altogether. Their color is usually different from the upper surface of the cap, not always similar to that of the spores borne upon them, at least in youth; with age, however, they usually assume the color of the mature spore. The change of color of the gills according to the age of the plant is very important in the study of the Agaricini; it accounts for the white gills of certain species in youth, the pink in maturity, and the brown when aged.

The end of the gill nearest the stalk of the plant is termed the posterior extremity; the opposite end, the anterior extremity. In most of the Agaricini the gills are unequal. Some extend from the margin to about half the space between it and the stem; others are still shorter.