=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.]
=424. Beware of the poisonous mushroom.=—The number of species of mushrooms, or toadstools as they are often called, is very great. Besides the common mushroom (Agaricus campestris) there are a large number of other edible species. But one should be very familiar with any species which is gathered for food, unless collected by one who certainly knows what the plant is, since carelessness in this respect sometimes results fatally from eating poisonous ones.