n the larger species are preferable.
When one of these is magnified (Fig. 18, _A_, _C_), the unbranched
filament is shown to be made up of perfectly cylindrical cells, with
rather delicate walls. The protoplasm is confined to a thin layer
lining the walls, except for numerous fine filaments that radiate
from the centrally placed nucleus (_n_), which thus appears
suspended in the middle of the cell. The nucleus is large and
distinct in the larger species, and has a noticeably large and
conspicuous nucleolus. The most noticeable thing about the cell is
the green spiral bands running around it. These are the
chloroplasts, which in all the _Conjugatae_ are of very peculiar
forms. The number of these bands varies much in different species of
_Spirogyra_, but is commonly two or three. These chloroplasts, like
those of other plants, are not noticeably different in structure
from the ordinary protoplasm, as is shown by extracting the
chlorophyll, which may be done by placing the plants in alcohol for
a short time. This extracts the chlorophyll, but a microscopic
examination of the decolored cells shows that the bands remain
unchanged, except for the absence of color. These bands are
flattened, with irregularly scalloped margins, and at intervals have
rounded bodies (pyrenoids) imbedded in them (Fig. 18, _C_, _py._).
The pyrenoids, especially when the plant has been exposed to the
light for some time, are surrounded by a circle of small granules,
which become bluish when iodine is applied, showing them to be
starch. (To show the effect of iodine on starch on a large scale,
mix a little flour, which is nearly all starch, with water, and add
a little iodine. The starch will immediately become colored blue,
varying in intensity with the amount of iodine.) The cells divide
much as in _Cladophora_, but the nucleus here takes part in the
process. The division naturally occurs only at night, but by
reducing the temperature at night to near the freezing point (4 deg. C.,
or a little lower), the process may be checked. The experiment is
most conveniently made when the temperature out of doors approaches
the freezing point. Then it is only necessary to keep the plants in
a warm room until about 10 P.M., when they may be put out of doors
for the night. On bringing them in in the morning, the division will
begin almost at once, and may be easily studied. The nucleus divi