few or a great number of dancing granules. I f we isolate the
separate granules while we crush a cell, they clearly show that
they have the form of a very small rhomboidal tablet, with sharp
corners and angles. They remain unchanged by burning, or by
treatment with concentrated acids and cold alkali. At all
events, they answer to inorganic bodies, and are probably small
crystals of gypsum, but, on account of their excessive minuteness,
the accuracy of this suggestion cannot be tested by closer
analysis. Other small granules in the cell fluids of Cosmarium
botnjtis (and perhaps of other species) have a different composition,
for they will dissolve readily in reagents, and are destroyed
by burning, but are insoluble in alcohol.
The increase of cells in the Desmids by ordinary cell division
is by no means uncommon, and may be seen illustrated on
several of the plates (PI. XXXV., f. 5, c d ; PI. XLVI., f. 2,
b; PL X LV II., f. 4, e ; PL L ., f. 9, e). The process has
been so often observed and described th a t it need not be
lepeated here. I t does not differ in its chief characteristics
from the similar process in other Algæ. The isthmus, or connecting
tube, between the two semi-cells elongates, a hyaline
lobe is formed at each retreating base, connate at the convex
extremity. These lobes continue to grow in size, becoming-
coloured, and by degrees attain the form and dimensions of the
original semi-cells; meanwhile the old segments are pushed
farther and farther apart, until at length separation takes place
between the two new semi-cells, and two individuals are free,
each with one old semi-cell and one new semi-cell, or daughter
cell, combined to form a complete individual.
As to the movements of Desmids very little can be added to
what has been stated by Ealfs,# for De Bary confesses that he
has nothing of importance to contribute, but that all conclusions
as to their animal or vegetable nature must be considered in
abeyance. Meanwhile, no one doubts of their truly vegetable
nature. Braunf only confirmed Ealfs. Alluding to Penitim.
curtum he says, “ The plant here named is remarkable for
exhibiting the peculiar movement of the Desmidlaceæ more
regularly, and more actively, than the other members of the
* Ealfs, «Desmidieæ,” p. 20.
t Braun, “ Rejuvenesoenoe,” p. 203, note.
family, a motion very different from that of the Diatomaceie. I t
is a remarkable sight to behold all the individuals in a dish of
water in a short time turn their long axes to the light, and
thus arrange themselves in beautiful streaks in the gelatinous
mass. Observation with the microscope shows that it is the
younger half of the cell which hero turns towards the light.”
"With this exception hardly any attention has been paid to
the influence of light on the movements of Desmids. Stahl
recently made some experiments in this direction,*' chiefly with
Closterium moniliferum, which he enclosed in glass tubes,
changing the direction of the light by means of mirrors. I t
soon became evident that the direction of the light exercised a
material influence on the position of the longer axis of the cell,
this axis having a tendency to place itself in the direction of
the rays of light, and that there is also a polarity between the
two halves of the cell, in consequence of which one is attracted
towards, and the other driven away from the source of light.
The direction is subject to alternations, in consequence of which
the cell is continually shifting its position through an angle of
180°, presenting each end alternately to the light. In one
experiment, at a temperature of 33° 0., the time occupied by
this reversal of position was from six to eight minutes ; in
another, where the temperature was 17° 0., from fifteen to
thirty-five minutes. In addition to this reversal there was also
a slow movement of the individual along the bottom in the
direction of the source of light. When the light is very intense
the conditions are reversed, and the cell places itself with its
longer axis at right angles to the direction of the light. Dr.
Gbbel has made similar experiments on Micrasterias rotata,
which was found to place itself with the plane of its disk at right
angles to the direction of the rays of light. The direction of
the chlorophyll band varies with that of the incident light.f
The conjugation of Desmids and formation of the zygospore
is very similar in all species. The cells lie surrounded by loose
and indistinctly circumscribed mucus in pairs together, always
crossed in Staurastrum, and parallel in Closterium. In Closterium
lunula the concave sides are turned towards each other ; in
* "Verb., Physikal Med. Gesell. Wurzburg,” xiv. (1880), p. 24.
t “ Journal of Royal Micros. Society,” iii. (1880), pp. 318.