The veil is not a constant feature in the Agaricini, at least it is not always visible. When present it consists of a membrane which extends from the margin of the cap to the stem, veiling or protecting the gills. This membrane, called the cortina, has given its name to a numerous and important class of mushrooms (the Cortinarias). It is generally white, soft, slightly spongy, cottony, at times fibrillose or even slightly fibrous, again in texture comparable to the spider's web, and may be even powdery or glutinous. It exists intact only in the youth of the plant. It is not visible in the developing mushroom, at least while the cap is closely pressed against the stem, but as the cap expands the membrane extends and finally breaks, leaving in some species its remnants upon the margin of the cap and upon the stem in the usual form of a ring or a mere zone. When the stem is not ringed the veil rises high upon the stalk, stretches across to meet the edges of the cap, and is afterwards reflected back over its whole surface.