164 FRESH-WATER ALGAE.
Vaucheria, we have an instance of fertilization, or the union of unlike cells, and
the result is the production of an oospore.
If we leave aside for the moment the section of the Blue-green Algae or
Cyanophyceae—a group of rather obscure plants, of which the real relation-
ships are open to doubt, it may be said that practically the whole bulk of
fresh-water species are Green Alga;, and there can hardly be a more fascinating
group of plants than this, whether to the strictly scientific botanist or to the
simple lover of nature, Because firstly, they are the most widely diffused as
well as the simplest of all plant forms ; secondly, they are of surpassing
variety and beauty, quite unlike any other members of the vegetable world ;
and then, thirdly, they claim our special interest by the fact that we. must
look among them not only for indications of the origin of all plant life, and of
the forms from which the whole of the higher plant world arose, but also for
aids and hints towards the solution of some of the most difficult fundamental
problems with which biologists are occupied ; such as the real nature and
origin of sexual reproduction, the functions of the nucleus and its relation to
the protoplasm, and many other questions of that kind.
In their lower forms the Green Algae are very closely allied to those
organisms — undoubted animals — which zoologists term the Flagellate
Protozoa. In fact there exists an unbroken series of forms connecting
unquestionable Protozoa, having mouths by which they eat solid food, with
unquestionable Green Algae depending entirely upon soluble inorganic food.
It is, therefore, quite clear that the dividing line between animals and plants
is entirely an artificial one, and is very naturally drawn by different authorities
at different points in the series. And then on the other side the Algae are
very intimately related to the Fungi and the Lichens, and here again the
boundary is obscure and ever-changing, various genera being shifted and
shunted about from time to time in the most perplexing fashion, until at last
it seems a hopeless task to endeavour to determine the limits of either of the
great sub-kingdoms.
The Classification of the Algae is in the main based upon the character
and modes of reproduction, just as it is in other plants. According to one of
the latest theories all the various forms of vegetable life are to be traced to
three lines of descent, represented by three distinct kinds of cell-contents : the
colourless, the pale blue, and the pure green.
The first series originates in the Bacteria, and through them are derived
the whole tribe of Fungi. The second primordial type consists of organisms
in which the cell contents are of a pale blue, or watery, bluish-green
colour, entirely devoid of true chlorophyll, starch-grains and nucleus.
These comprise the Phycochromophyceae, i.e., the Blue-green Algae of
which I have given as examples Nostoc and Oscillatoria. This group has
never developed into a high stage, but by what the Germans call "retro-
gressive development," a sort of evolution in a backward direction, in other
words, by a partial or complete suppression of either the reproductive or the
vegetative organs, they have been incapable of advance. The third and most
important series of all, the Chlorophyllophyceae, that is to say the pure
grass-green Algae, are the only stock which has developed into the higher
plants. This division is characterized from the outset by the cells possessing
a nucleus, starch grains, pure chlorophyll, and in certain states a true cell-wall
of cellulose.