mosses growing on the ground or low down on
the trunks of trees, etc. They differ principally from the common
mosses in having the capsule open irregularly and not by a lid. The
commonest forms belong to the genus _Phascum_ (Fig. 65, _A_).

The vast majority of the mosses the student is likely to meet with
belong to the last order, and agree in the main with the one
described. Some of the commoner forms are shown in Figure 65.

CHAPTER XII.

SUB-KINGDOM V.

PTERIDOPHYTES.

If we compare the structure of the sporogonium of a moss or liverwort
with the plant bearing the sexual organs, we find that its tissues are
better differentiated, and that it is on the whole a more complex
structure than the plant that bears it. It, however, remains attached
to the parent plant, deriving its nourishment in part through the
"foot" by means of which it is attached to the plant.

In the Pteridophytes, however, we find that the sporogonium becomes
very much more developed, and finally becomes entirely detached from
the sexual plant, developing in most cases roots that fasten it to the
ground, after which it may live for many years, and reach a very large
size.

The sexual plant, which is here called the "prothallium," is of very
simple structure, resembling the lower liverworts usually, and never
reaches more than about a centimetre in diameter, and is often much
smaller than this.

The common ferns are the types of the sub-kingdom, and a careful study
of any of these will illustrate the principal peculiarities of the
group. The whole plant, as we know it, is really nothing but the
sporogonium, originating from the egg cell in exactly the same way as
the moss sporogonium, and like it gives rise to spores which are
formed upon the leaves.

The spores may be collected by placing the spore-bearing leaves on
sheets of paper and letting them dry, when the ripe spores will be
discharged covering the paper as a fine, brown powder. If these are
sown on fine, rather closely packed earth, and kept moist and covered
with glass so as to prevent evaporation, within a week or two a fine,
green, moss-like growth will make its appearance, and by the end of
five or six weeks, if the weather is warm, little, flat, heart-shaped
plants of a dark-green color may be seen. These look like small
liverworts, and are the sexual plants (prothallia) of our ferns
(Fig. 66, _F_). Removing one of these carefully, we find on the lower
side numerous