Parts of the Mushroom
To serve as a means of comparison, the common pasture mushroom, or cultivated form (_Agaricus campestris_), is first described. Figure 1 illustrates well the principal parts of the plant;the cap, the radiating plates or gills on the under side, the stem, and the collar or ring around its upper end.
The Cap.
The cap (technically the _pileus_) is the expanded part of the mushroom. It is quite thick, and fleshy in consistency, more or less rounded or convex on the upper side, and usually white in color.
It is from 1–2 cm. thick at the center and 5–10 cm. in diameter. The surface is generally smooth, but sometimes it is torn up more or less into triangular scales.
When these scales are prominent they are often of a dark color. This gives quite a different aspect to the plant, and has led to the enumeration of several varieties, or may be species, among forms accredited by some to the one species.
The Gills.
On the under side of the pileus are radiating plates, the gills, or _lamellæ_ (sing. _lamella_). These in shape resemble somewhat a knife blade.
They are very thin and delicate. When young they are pink in color, but in age change to a dark purple brown, or nearly black color, due to the immense number of spores that are borne on their surfaces.
The gills do not quite reach the stem, but are rounded at this end and so curve up to the cap. The triangular spaces between the longer ones are occupied by successively shorter gills, so that the combined surface of all the gills is very great.