they may be single or united into filaments, and not
infrequently are attached by means of a delicate gelatinous stalk
(Fig. 25).

[Illustration: FIG. 25.--Diatoms attached by a gelatinous stalk.
x 150]

They are at once distinguished from the desmids by their color,
which is always some shade of yellowish or reddish brown. The
commonest forms, _e.g._ _Navicula_ (Fig. 24, _C_), are boat-shaped
when seen from above, but there is great variety in this respect.
The cell wall is always impregnated with large amounts of flint, so
that after the cell dies its shape is perfectly preserved, the flint
making a perfect cast of it, looking like glass. These flinty shells
exhibit wonderfully beautiful and delicate markings which are
sometimes so fine as to test the best lenses to make them out.

This shell is composed of two parts, one shutting over the other
like a pill box and its cover. This arrangement is best seen in such
large forms as _Pinnularia_ (Fig. 24, _A_ ii).

Most of the diatoms show movements, swimming slowly or gliding over
solid substances; but like the movements of _Oscillaria_ and the
desmids, the movements are not satisfactorily understood, although
several explanations have been offered.

They resemble somewhat the desmids in their reproduction.

THE TRUE BROWN ALGAE.

These are all marine forms, many of great size, reaching a length in
some cases of a hundred metres or more, and showing a good deal of
differentiation in their tissues and organs.

[Illustration: FIG. 26.--_A_, a branch of common rock weed (_Fucus_),
one-half natural size. _x_, end of a branch bearing conceptacles. _B_,
section through a conceptacle containing ooegonia (_og._), x 25. _C_,
_E_, successive stages in the development of the ooegonium, x 150. _F_,
_G_, antheridia. In _G_, one of the antheridia has discharged the mass
of spermatozoids (_an._), x 150.]

One of the commonest forms is the ordinary rock weed (_Fucus_), which
covers the rocks of our northeastern coast with a heavy drapery for
several feet above low-water mark, so that the plants are completely
exposed as the tide recedes. The commonest species, _F. vesiculosus_
(Fig. 26, _A_), is distinguished by the air sacs with which the stems
are provided. The plant is attached to the rock by means of a sort of
disc or root from which springs a stem of tough, leathery texture, and
forking regularly at intervals, so that the ultimate branches are v