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FRESH-WATER ALGAE.
movement is a power common to all protoplasm, whether it belongs to a plant
or to an animal. And so it happens that there is no longer a vain endeavour
made to discover the separating line between the two great kingdoms. Why it
should have been supposed in later times that a hard and fast line exists, is
difficult to comprehend, because the more we learn the clearer we perceive
that Nature despises a dividing line as much as she does a straight Unc.
Groups everywhere, large and small, merge into each other by insensible
gradations, boldly defying anything like rigid mathematical precision.
Locomotion is very general among the lower algae at some stage or other
of their existence, but soma of them are motile throughout their whole
career. Perhaps the most beautiful example of a purely free-swimming alga
is Volvox. Surely nothing can surpass this, for exquisite grace and beauty.
Conceive a tiny spherical globe, mathematically perfect in form, as light and
delicate and translucent as a soap-bubble, dotted all over with brilliant
emerald beads set at regular distances apart. Let there be three or four
smaller, but exactly similar pale green globes enclosed within the larger one ;
then imagine this tinted glassy sphere rolling and revolving on its axis, and
gently sailing through the water in all sorts of graceful curves—and you have
Volvox.
There are two very extensive groups which have received a great deal
of attention from persons who have not otherwise studied the algae ; indeed
they are at once so surpassingly interesting and so infinite in variety and
numbers that they are commonly treated of by themselves ; I refer to the
Desmids and the Diatoms, well-known to everyone who has intelligently used
the microscope. They are all exceedingly minute, the giants among them
being hardly distinguishable by the naked eye, and although possessing a
certain superficial resemblance the two groups are easily separated by constant
and well-marked characters.
In Diatoms the outer envelope or shell is composed entirely of pure flint,
so that they will resist the action of fire or strong acids. These flinty shells
are beautifully striated and sculptured, and sometimes marked with lines of
such extreme fineness that they are used as tests for the highest powers of the
microscope. In Desmids on the contrary the outer case is never siliceous,
and seldom bears markings of any kind. Again, in Diatoms the colouring-
matter of the cells is a rich golden brown, whilst in Desmids it is a bright
grass green, so that in the living plants the colour alone is sufficient to
distinguish them.
As regards habitat, Diatoms are probably the most widely distributed of
all organized beings, for they occur in a living state wherever there is standing
water—fresh, salt, or brackish, from the Equator to the Poles ; and large
tracts of country are composed entirely of their fossil remains. Desmids on
the other hand are much more restricted in the range, being confined exclu-
sively to freshwater ; and very few traces of them have been found in
geological deposits.
Any person observing for the first time the peculiar spontaneous motions
of the free Diatoms might well be excused for assigning them a place in the
animal kingdom, as the old naturalists did. Some species are incessantly
moving, steering their way with great precision and rapidity among the
objects surrounding them. For some time they may hold a straight course,