in t r o d u c t io n .
cloud, which is dispersed by the slightest touch. When they
are left exposed by the evaporation of the water, this mucus
becomes denser, and is apparently secreted in larger quantities to
protect them from the effects of drought. I have observed
more especially that Tetmemorus granulatus and Penium Brebis-
sonii, under such circumstances, form a distinct mucous
stratum. With the exception of Sphcerozosma the side walls
only of the cells united in rows, or chains, are surrounded by
mucus, so long as they remain connected, but, when they
become free, the flat ends also secrete mucus.
A delicate primordial membrane, intimately joined to the
cellulose membrane, surrounds the cell-contents. I t appears
in most cases to be simply a homogeneous transparent cuticle,
rich in nitrogen, and generally bears a plasma stratum of a fine
granular appearance, which is deposited on its inner wall. In
this plasma stratum earlier observers noticed a visible movement,
through the locomotion of the grannies, sometimes
called rotation or circulation. De Bary, whom we have followed
rather closely in this “ Introduction,” has applied himself to the
investigation of these movements, which, he says, are found to a
most astonishing extent, particularly in CAostermm lunula and
Tetmemorus granulatus, when the cells are in very brisk vegetation
and division. He says,* “ The small granules of the stratum
are impelled very quickly in numerous small streamlets to and
fro. These glide in constant change, sometimes from the middle
of the cell to the end, at other times in the reverse direction,
irregularly gliding by one another, and by those granules which
are at rest. At one margin of the semi-cell we find a stream
going to the cell end, at the other one going from the end to the
middle, so that it appears at first sight to be a rotation analogous
to that of Chara cells, but we see by constant observation that the
movement is always transposed in different directions. Where
the plasma is collected in great plenty, we perceive most clearly
that the currents exist throughout the whole mass, not alone
either in the primordial membrane, or the stratum bounding the
watery cell-contents.” «W e see that the observed
s reams are caused by one constant movement of the whole
granular plasma-mass, which alternately accumulates at different
* De Bary, ■< Conjugateen,”
points, and again retreats. Whether, therefore, the change in
the direction of the streams is, in fact, as irregular as it appears
at first sight to be, or whether, on the contrary, one does not
follow the other in regular progress around the whole circumference
of the cell, has hitherto not been decided, on account of
the opacity of the chlorophyll contents.” The existence of
internal cilia, as maintained by Osborne, is denied, and the
appearances accounted for as optical illusions.
All the Desmids are tinted green by chlorophyll, and the
colouring matter is confined to bodies of regular form, such
special form and structure being very often characteristic of
separate genera or species. In some species the endoohrome is
accumulated in parietal bands, and in Spirotmnia and Geni-
cularia the elongated ribbons are arranged spirally as in
Spirogyra. Mr. Archer was the first to point out that in
Xanthidium the endochrome forms in each segment four parietal
quarters, rather than bands, these interrupted or separated by
as many narrow, nearly straight vacancies running down the
centre of each front, and each lateral aspect of the segment.
In Mesotesnium the simple laminated endoohrome either runs
directly through the longitudinal axis of the cell, or sometimes
slightly concentrically, as a sharply-defined chlorophyll plate.
In most species of Staurastrum the endoohrome is arranged in
plates radiating from a common centre, so that in an end view
it presents a more or less perfectly stellate appearance.
The elongated middle band in the cells of Closterium and
Penium generally exhibit several starch granules arranged in
one long row, very exceptionally only one. In other genera
the starch granules are fewer, and more irregularly disposed.
In rare cases starch granules of determined shape and structure
are not to be seen, but the application of iodine will usually
detect the amorphous starch which permeates the chlorophyll.
The spaces which remain free between the chlorophyll and
the wall plasma are filled with a watery fluid. Within this
fluid are suspended in plenty a great number of incommen-
surably small dancing granules in lively molecular motion.
Under this head come the vesicular spaces so often mentioned,
as found in the ends of Closterium. These terminal vacuoles
are often sphierical, or half oval in shape, containing either a