are sometimes two-celled. The sterile filaments between the spore
sacs usually have thickened ends, which are dark-colored, and give
the color to the inner surface of the spore fruit.

In Figure 45, _H_, is shown one of the so-called "_Soredia_,"[7] a
group of the algae, upon which the lichen is parasitic, surrounded by
some of the filaments, the whole separating spontaneously from the
plant and giving rise to a new one.

[7] Sing. _soredium_.

Owing to the toughness of the filaments, the finer structure of the
lichens is often difficult to study, and free use of caustic potash is
necessary to soften and make them manageable.

[Illustration: FIG. 45.--Forms of lichens. _A_, a branch with lichens
growing upon it, one-half natural size. _B_, _Usnea_, natural size.
_ap._ spore fruit. _C_, _Sticta_, one-half natural size. _D_,
_Peltigera_, one-half natural size. _ap._ spore fruit. _E_, a single
spore fruit, x 2. _F_, _Cladonia_, natural size. _G_, a piece of bark
from a beech, with a crustaceous lichen (_Graphis_) growing upon it,
x 2. _ap._ spore fruit. _H_, _Soredium_ of a lichen, x 300.]

According to their form, lichens are sometimes divided into the bushy
(fruticose), leafy (frondose), incrusting (crustaceous), and
gelatinous. Of the first, the long gray _Usnea_ (Fig. 45, _A_, _B_),
which drapes the branches of trees in swamps, is a familiar example;
of the second, _Parmelia_, _Sticta_ (Fig. 45, _C_) and _Peltigera_
(_D_) are types; of the third, _Graphis_ (_G_), common on the trunks
of beech-trees, to which it closely adheres; and of the last,
_Collema_ (Fig. 44, _C_, _D_, _E_), a dark greenish, gelatinous form,
growing on mossy tree trunks, and looking like a colony of _Nostoc_,
which indeed it is, but differing from an ordinary colony in being
penetrated everywhere by the filaments of the fungus growing upon it.

Not infrequently in this form, as well as in other lichens, special
cavities, known as spermogonia (Fig. 44, _D_), are found, in which
excessively small spores are produced, which have been claimed to
be male reproductive cells, but the latest investigations do not
support this theory.

[Illustration: FIG. 46.--Branch of a plum-tree attacked by black knot.
Natural size.]

The last group of the _Ascomycetes_ are the "black fungi,"
_Pyrenomycetes_, represented by the black knot of cherry and plum
trees, shown in Figure 46. They are mainly distinguished from the cup
fungi by