ike the protococcus, being of extreme
simplicity; others attaining a large size, and presenting the appearance
of a stout stem with branches and leaves.

The Algae are divisible into the green-spored[19] (_Chlorospermeae_), the
rose-spored (_Florideae_), and the olive-spored (_Melanospermeae_).

It is in the first division that the _Protococcus_ may be placed, as
also those microscopic plants called _Diatoms_ and _Desmids_. The
former, the _Diatomaceae_, are a very numerous group of minute organisms,
some of which are used as test objects for microscopes. They contain in
their outer coat or case a relatively large portion of silex, and their
remains here and there form deposits--vast beds many feet in
thickness--known as "tripoli," and used for polishing. The minute
particle of their protoplasm is contained within the siliceous case.
They may be entirely free, or cohere in aggregations, or be attached to
a supporting surface by a slender stalk, which may ramify and bear a
little siliceous case or "frustule" at the end of each branch.

The desmids (or _Desmidiaceae_) are green and devoid of silex, though
their protoplasm is enclosed in hard or flexible cases, often marked
with beautiful and characteristic patterns.

Both diatoms and desmids may cohere together, forming more complex
masses; but another creature allied to _Protococcus_ is noted for its
mode of cohesion. This is the microscopic plant _Volvox_, the
individuals of which cohere so as to form spheroidal aggregations, which
swim about by the action of filamentary prolongations of their
protoplasm, such prolongations reminding us of the pseudopodia of
radiolarians and other rhizopods.

Amongst these simplest plants may be also mentioned the curious
thread-like organisms, which, on account of their remarkable and as yet
unexplained movements, are called _Oscillatoriae_.

Another curious vegetable organism which may here be mentioned is
_Vaucheria_. It is a green, thread-like plant, which may be several
inches long, and which at one stage of its existence (when it is what is
called a "spore") swims about by pseudopodial prolongations of its
protoplasm.

Some few of the _Chlorospermeae_ are large and conspicuous organisms.
Such, _e.g._, is _Caulerpa_, which abounds on warm, sandy coasts, and on
which turtles browse. Though, as we shall hereafter see, it is really as
simple in structure as a particle of yeast, it yet presents a very
complicated external figur