producing their spore sacs in closed cavities. Some are
parasites; others live on dead wood, leaves, etc., forming very hard
masses, generally black in color, giving them their common name. Owing
to the hardness of the masses, they are very difficult to manipulate;
and, as the structure is not essentially different from that of the
_Discomycetes_, the details will not be entered into here.

Of the parasitic forms, one of the best known is the "ergot" of rye,
more or less used in medicine. Other forms are known that attack
insects, particularly caterpillars, which are killed by their attacks.

CHAPTER X.

FUNGI--_Continued_.

CLASS _Basidiomycetes_.

The _Basidiomycetes_ include the largest and most highly developed of
the fungi, among which are many familiar forms, such as the mushrooms,
toadstools, puff-balls, etc. Besides these large and familiar forms,
there are other simpler and smaller ones that, according to the latest
investigations, are probably related to them, though formerly regarded
as constituting a distinct group. The most generally known of these
lower _Basidiomycetes_ are the so-called rusts. The larger
_Basidiomycetes_ are for the most part saprophytes, living in decaying
vegetable matter, but a few are true parasites upon trees and others
of the flowering plants.

All of the group are characterized by the production of spores at the
top of special cells known as basidia,[8] the number produced upon a
single basidium varying from a single one to several.

[8] Sing. _basidium_.

Of the lower _Basidiomycetes_, the rusts (_Uredineae_) offer common and
easily procurable forms for study. They are exclusively parasitic in
their habits, growing within the tissues of the higher land plants,
which they often injure seriously. They receive their popular name
from the reddish color of the masses of spores that, when ripe, burst
through the epidermis of the host plant. Like many other fungi, the
rusts have several kinds of spores, which are often produced on
different hosts; thus one kind of wheat rust lives during part of its
life within the leaves of the barberry, where it produces spores quite
different from those upon the wheat; the cedar rust, in the same way,
is found at one time attacking the leaves of the wild crab-apple and
thorn.

[Illustration: FIG. 47.--_A_, a branch of red cedar attacked by a rust
(_Gymnosporangium_), causing a so-called "cedar apple," x 1/2. _B_,
spores of the same