[Illustration: Fig. 236. Agaricus campestris. Soil washed from “spawn” and “buttons,” showing the minute young “buttons” attached to the strands of mycelium.] =422. How the mushroom is formed.=—The mycelium of the mushroom lives in the ground, and grows here for several months or even years, and at the proper seasons develops the mature mushroom plant. The mycelium lives on decaying organic matter, and a large number of the threads grow closely together forming strands, or cords, of mycelium, which are quite prominent if they are uncovered by removing the soil, as shown in fig. 236. [Illustration: Fig. 237. Agaricus campestris; sections of “buttons” of different sizes, showing formation of gills and veil covering them.] =423.= From these strands the buttons arise by numerous threads growing side by side in a vertical direction, each thread growing independently at the end, but all lying very closely side by side. When the buttons are quite small the gills begin to form on the under margin of the knob. They are formed by certain of the threads growing downward in radiating ridges, just as many of these ridges being started as there are to be gills formed. At the same time, threads of the stem grow upward to meet those at the margin of the button in such a manner that they cover up the forming gills, and thus enclose the gills in a minute cavity. Sections of buttons at different ages will show this, as is seen in fig. 237. This curtain of mycelium which is thus stretched across the gill cavity is the veil. As the cap expands more and more this is stretched into a thin and delicate texture as shown in fig. 238. Finally, as shown in fig. 239, this veil is ruptured by the expansion of the pileus, and it either clings to the stem as a collar, or a portion of it remains clinging to the margin of the cap. When the buttons are very young the gills are white, but they soon become pink in color, and very soon after the veil breaks the spores mature, and then the gills are dark brown. [Illustration: Fig. 238. Agaricus campestris; nearly mature plants, showing veil still stretched across the gill cavity.] [Illustration: Fig. 239. Agaricus campestris; under view of two plants just after rupture of veil, fragments of the latter clinging both to margin of pileus and to stem.] [Illustration: Fig. 240. Agaricus campestris; plant in natural position just after rupture of veil, showing tendency to double annulus on the stem. Portions of the veil also dripping from margin of pileus.] [Illustration: Fig. 241. Agaricus campestris; spore print.] [Illustration: Fig. 242. “Fairy ring” formed by Agaricus arvensis (photograph by B. M. Duggar). The mycelium spreads centrifugally each year, consuming the available food, and thus the plants appear in a ring.] [Illustration: Fig. 243. Amanita phalloides; white form, showing pileus, stipe, annulus, and volva.]