arger, but by far the greater proportion are of very much
smaller dimensions.

[Illustration: FIG. 3.--_Podosphenia Lyngbyii._]

[Illustration: FIG. 4.--_Pleurosigma balticum._]

[Illustration: FIG. 5. A-C, _Tetracyclus lacustris._ D and E,
_Tabellaria fenestrata._ F and G, _Tabellaria flocculosa._]

Diatoms are unicellular plants distinguished from kindred forms by the
fact of having their soft vegetative part covered by a siliceous case.
Each individual is known as a frustule, and the cell-wall consists of
two similar valves nearly parallel to each other, each valve being
furnished with a rim (or connecting-band) projecting from it at a right
angle.

One of these valves with its rim is slightly smaller than the other,
the smaller fitting into the larger pretty much as a pill-box fits into
its cover. This peculiarity of structure affords ample scope for the
growth of the protoplasmic cell-contents, for as the latter increase in
volume the siliceous valves are pushed out, and their corresponding
siliceous rims become broader. The connecting-bands although closely
fitting their respective valves are distinct from them, and together the
two bands form the girdle.

An individual diatom is usually described from two aspects, one in which
the surface of the valve is exposed to view--the valve view, and one in
which the girdle side is exposed--the girdle view. The valves are thin
and transparent, convex on the outside, and generally ornamented with a
variety of sculptured markings. These sculptures often present the
aspect of striae across the face of the valve, and the best lenses have
shown them to consist of a series of small cavities within the siliceous
wall of the cell. The valves of some of the marine genera exhibit a
beautiful areolated structure due to the presence of larger chambers
within the siliceous cell-wall. Many diatoms possess thickenings of the
cell-wall, visible in the valve view, in the centre of the valve and at
each extremity. These thickenings are known as the nodules, and they are
generally connected by a long median line, the raphe, which is a cleft
in the siliceous valve, extending at least some part of its length.

The protoplasmic contents of this siliceous box-like unicell are very
similar to the contents of many other algal cells. There is a living
protoplasmic layer or primordial utricle, connected either by two broad
bands or by a number of anastomosing threads with a central mass of
p