FRESH-WATER ALGAE :
A SKETCH OF THEIR STRUCTURE, DISTRIBUTION
AND RELATIONSHIPS.
By E. D. MARQUAND, Kew Museum.
[Being an Address delivered at the Muting in Epping Forest, on April 10th, 1897.]
[After some introductory observations Mr. Marquand continued:—]
THE Algae comprise the lowest forms of vegetable life, the very earliest
rudiments as it were of the great world of plants. Here we find the
very simplest of all possible conditions of existence, the single separate cell,
leading a perfectly independent existence.
The most unobservant person cannot fail to have noticed how in shady
places walls, wooden fences, and tree-trunks are coated with a vivid-green
powdery layer which comes off on one's clothes at the slightest touch. This
is Pleurococcus, one of the very lowest of unicellular Algae, and probably in
temperate climates the most abundant of all plants. You will find this
powdery coating consists of an immense multitude of small round cells,
uniformly green in colour, and often adhering together in little clusters of
from 2 to 8. Multiplication takes place by simple division ; each cell divides
in the middle and each half becomes a perfect cell. This process of cell-
division is the most simple of all conceivable modes of increase, and in
Pleurococcus it takes place with amazing rapidity. An American botanist
records that under his own observation one single cell produced in the course
of 10 days a colony of more than 600,000 cells. This accounts for the sudden
appearance of a green coating on palings and damp woodwork during warm
moist weather.
Porphyridium, the alga which forms the blood stains on damp ground, that
I spoke of in my introductory observations, multiplies in the same way, but
not quite so rapidly, and partly for that reason is not quite so common.
The belief is now commonly entertained, though at present the theory is
not supported by direct proof, that these very simple unicellular algae (of
which there are a large number of forms) are not perfect plants at all, but are
merely growth stages or arrested conditions of development of higher forms ;
these simple cells divide and re-divide and so serve to form a stratum or bed
upon which in course of time, and under the requisite favorable conditions of
temperature and moisture, the original filamentous type of the parent plant
may be reproduced. This is what is called the "polymorphism theory "and
like many other things, it has come over to us from Germany. There is,
however, a good deal of evidence in support of it, and the theory gains ground.
In the days of the old naturalists, and indeed within the memory of many
persons still living, motion was regarded as the distinctive attribute of animal
life—an organism capable of spontaneous movement was by that very fact an
animal, whilst one which remained throughout its existence incapable of free
motion, was a plant. It is now well known, however, that spontaneous