. 19, _B_, _D_), in
which the single chloroplast has the form of a thin median plate. (B
shows the appearance from in front, _C_ from the side, showing the
thickness of the plate.) _Mesocarpus_ and the allied genera have the
spore formed between the filaments, the contents of both the uniting
cells leaving them.

[Illustration: FIG. 20.--Forms of Desmids. _A_, _B_, _Closterium_.
_C_, _D_, _D'_, _Cosmarium_. _D_, and _D'_ show the process of
division. _E_, _F_, _Staurastrum_; _E_ seen from the side, _F_ from
the end.]

Evidently related to the pond scums, but differing in being for the
most part strictly unicellular, are the desmids (Fig. 20). They are
confined to fresh water, and seldom occur in masses of sufficient size
to be seen with the naked eye, usually being found associated with
pond scums or other filamentous forms. Many of the most beautiful
forms may be obtained by examining the matter adhering to the leaves
and stems of many floating water plants, especially the bladder weed
(_Utricularia_) and other fine-leaved aquatics.

The desmids include the most beautiful examples of unicellular
plants to be met with, the cells having extremely elegant outlines.
The cell shows a division into two parts, and is often constricted
in the middle, each division having a single large chloroplast of
peculiar form. The central part of the cell in which the nucleus
lies is colorless.

Among the commonest forms, often growing with _Spirogyra_, are
various species of _Closterium_ (Fig. 20, _A_, _B_), recognizable at
once by their crescent shape. The cell appears bright green, except
at the ends and in the middle. The large chloroplast in each half is
composed of six longitudinal plates, united at the axis of the cell.
Several large pyrenoids are always found, often forming a regular
line through the central axis. At each end of the cell is a vacuole
containing small granules that show an active dancing movement.

The desmids often have the power of movement, swimming or creeping
slowly over the slide as we examine them, but the mechanism of these
movements is still doubtful.

In their reproduction they closely resemble the pond scums.

ORDER IV.--_Siphoneae_.

The _Siphoneae_ are algae occurring both in fresh and salt water, and
are distinguished from other algae by having the form of a tube,
undivided by partition walls, except when reproduction occurs. The
only common representatives of