to a
single row of cells, or even reduced to single cells. Indeed,
Thallophytes are so multifarious, so numerous in kinds, so protean in
their stages and transformations, so recondite in their fructification,
and many so microscopic in size, either of the plant itself or its
essential organs, that they have to be elaborately described in separate
books and made subjects of special study.

[Illustration: Fig. 545, 546. Two plants of Riccia natans, about natural
size. 547. Magnified section of a part of the frond, showing two
immersed spore-cases, and one emptied space. 548. Magnified section of a
spore-case with some spores. 549. Magnified spore-case torn out, and
spores; one figure of the spores united; the other of the four
separated.]

504. Nevertheless, it may be well to try to give some general idea of
what Algae and Lichens and Fungi are. Linnaeus had them all under the
orders of Algae and Fungi. Afterwards the Lichens were separated; but of
late it has been made most probable that a Lichen consists of an Alga
and a Fungus conjoined. At least it must be so in some of the ambiguous
forms. Botanists are in the way of bringing out new classifications of
the Thallophytes, as they come to understand their structure and
relations better. Here, it need only be said that

505. Lichens live in the air, that is, on the ground, or on rocks,
trunks, walls, and the like, and grow when moistened by rains. They
assimilate air, water, and some earthy matter, just as do ordinary
plants. Algae, or Seaweeds, live in water, and live the same kind of life
as do ordinary plants. Fungi, whatever medium they inhabit, live as
animals do, upon organic matter,--upon what other plants have
assimilated, or upon the products of their decay. True as these general
distinctions are, it is no less true that these orders run together in
their lowest forms; and that Algae and Fungi may be traced down into
forms so low and simple that no clear line can be drawn between them;
and even into forms of which it is uncertain whether they should be
called plants or animals. It is as well to say that they are not high
enough in rank to be distinctively either the one or the other. On the
other hand there is a peculiar group of plants, which in simplicity of
composition resemble the simpler Algae, while in fructification and in
the arrangements of their simple cells into stem and branches they seem
to be of a higher order, viz.:--

[Illustration: Fig. 55