sed rocks, on
the ground, trunks of trees, fences, etc., and are found pretty much
the world over. Among the commonest of plants are the lichens of the
genus _Parmelia_ (Fig. 44, _A_), growing everywhere on tree trunks,
wooden fences, etc., forming gray, flattened expansions, with much
indented and curled margins. When dry, the plant is quite brittle, but
on moistening becomes flexible, and at the same time more or less
decidedly green in color. The lower surface is white or brown, and
often develops root-like processes by which it is fastened to the
substratum. Sometimes small fragments of the plant become detached in
such numbers as to form a grayish powder over certain portions of it.
These, when supplied with sufficient moisture, will quickly produce
new individuals.

Not infrequently the spore fruits are to be met with flat discs of a
reddish brown color, two or three millimetres in diameter, and closely
resembling a small cup fungus. They are at first almost closed, but
expand as they mature (Fig. 44, _A_, _ap._).

[Illustration: FIG. 44.--_A_, a common lichen (_Parmelia_), of the
natural size. _ap._ spore fruit. _B_, section through one of the spore
fruits, x 5. _C_, section through the body of a gelatinous lichen
(_Collema_), showing the _Nostoc_ individuals surrounded by the fungus
filaments, x 300. _D_, a spermagonium of _Collema_, x 25. _E_, a
single _Nostoc_ thread. _F_, spore sacs and paraphyses of _Usnea_,
x 300. _G_, _Protococcus_ cells and fungus filaments of _Usnea_.]

If a thin vertical section of the plant is made and sufficiently
magnified, it is found to be made up of somewhat irregular,
thick-walled, colorless filaments, divided by cross-walls as in the
other sac-fungi. In the central parts of the plant these are rather
loose, but toward the outside become very closely interwoven and
often grown together, so as to form a tough rind. Among the
filaments of the outer portion are numerous small green cells, that
closer examination shows to be individuals of _Protococcus_, or some
similar green algae, upon which the lichen is parasitic. These are
sufficiently abundant to form a green line just inside the rind if
the section is examined with a simple lens (Fig. 44, _B_).

The spore fruits of the lichens resemble in all essential respects
those of the cup fungi, and the spore sacs (Fig. 44, _F_) are much
the same, usually, though not always, containing eight spores, which