STRUCTURE AND GROWTH.

Mushrooms consist wholly of cells. These cells do not contain either
starch or the green coloring-matter, called chlorophyll, which exists in
other plants. They are either parasites or scavengers, and sometimes
both. The food of fungi must form a part of some animal or plant. When
they commence to grow it is by the division of cells, not laterally, but
in one direction, upward. As the mushroom grows the stem lengthens, the
cap expands and bursts the veil that surrounds it, and gradually gains
its perfect shape.

Every mushroom has a spore-bearing layer of cells, which is called the
hymenium. This hymenium is composed of a number of swollen, club-shaped
cells, called basidia, and close to them, side by side, are sterile,
elongated cells, named paraphyses. In the family called Hymenomycetes
there are mixed with these, and closely packed together, one-celled
sterile structures named cystidia.

The basidia are called mother-cells because they produce the spores.

There is one great group of fungi called Basidiomycetes, so named from
having their stalked spores produced on basidia.

The basidia are formed on the end of threadlike branched bodies which
grow at the apex, and are called hyphæ. On top of the basidia are minute
stalk-like branches, called sterigmata (singular sterigma), and each
branch carries a naked spore. They are usually four in number. This
group of Basidiomycetes is divided into (1) Stomach fungi
(Gasteromycetes), (2) Spore sac fungi (Ascomycetes), and (3) Membrane
fungi (Hymenomycetes).