branching arms of which are divided into
cells. Its greenness is due to its chlorophyll, and it undoubtedly has
the manufacturing power in full degree, decomposing carbonic acid and
setting oxygen free, under the influence of sunlight. But the
protoplasmic contents of some of the cells of which the plant is made up
occasionally divide, by a method similar to that which effects the
division of the contents of the _Peronospora_ spore; and the severed
portions are then set free as active monad-like zoospores. Each is oval
and is provided at one extremity with two long active cilia. Propelled by
these, it swims about for a longer or shorter time, but at length comes
to a state of rest and gradually grows into a _Coleochaete_. Moreover, as
in the _Peronospora_, conjugation may take place and result in an
oospore; the contents of which divide and are set free as monadiform
germs.

If the whole history of the zoospores of _Peronospora_ and of
_Coleochaete_ were unknown, they would undoubtedly be classed among
"Monads" with the same right as _Heteromita_; why then may not
_Heteromita_ be a plant, even though the cycle of forms through which it
passes shows no terms quite so complex as those which occur in
_Peronospora_ and _Coleochaete_? And, in fact, there are some green
organisms, in every respect characteristically plants, such as
_Chlamydomonas_, and the common _Volvox_, or so-called "Globe
animalcule," which run through a cycle of forms of just the same simple
character as those of _Heteromita_.

The name of _Chlamydomonas_ is applied to certain microscopic green
bodies, each of which consists of a protoplasmic central substance
invested by a structureless sac. The latter contains cellulose, as in
ordinary plants; and the chlorophyll which gives the green colour enables
the _Chlamydomonas_ to decompose carbonic acid and fix carbon as they do.
Two long cilia protrude through the cell-wall, and effect the rapid
locomotion of this "monad," which, in all respects except its mobility,
is characteristically a plant. Under ordinary circumstances, the
_Chlamydomonas_ multiplies by simple fission, each splitting into two or
into four parts, which separate and become independent organisms.
Sometimes, however, the _Chlamydomonas_ divides into eight parts, each of
which is provided with four instead of two cilia. These "zoospores"
conjugate in pairs, and give rise to quiescent bodies, which multiply by
division, find eventually pass