162 FRESH-WATER ALGAE.
appearance on shady gravel paths in gardens. If a small portion of the
plant be placed in water and examined under the microscope it will be found
to consist of a dense mass of interwoven necklace-like filaments, embedded in
colourless jelly.
Although abundant everywhere and in great variety, very little is known
about the life histories of the Blue green Algae, and in none of them has any
kind of sexual reproduction been observed. Sometimes they appear suddenly
in enormous quantities in some place, and then just as mysteriously and
suddenly disappear, and are seen no more. They are classed among the
Algae from their habit and mode of life, but some genera have much in
common with the bacteria and other simple fungi. Many authorities con-
sider that they are not autonomous forms at all, but merely intermediate
life-conditions or arrested growths of higher plants.
The large genus Oscillatoria is one of the most curious and interesting in
the entire family, on account of the remarkable animal-like movement of the
filaments. In this respect Oscillatoria appears at first sight even more wonder-
ful than the free Diatoms, because it is so much more plant-like in character
and appearance. The movements of the filaments are of three kinds : Firstly,
there is a pendulum-like swinging of the filament from side to side, one end
remaining quiescent ; secondly, there is a peculiar flexure of the filament
itself, which can only be compared to the twisting of a worm ; and
thirdly, there is the simple onward movement of progression. Some
species move much more rapidly than others ; in some, a very small fragment
of the plant, taken up on a needle point—a little bundle of filaments in fact—
will in a few hours spread over a comparatively large surface of water.
How these movements are caused no one knows, though probably the
attraction of light has something to do with it.
In the section called the Siphoneae we find a curious group of plants
distinguished by non-cellular structure. Although consisting of branched
filaments several inches in length, the protoplasm is not partitioned off by
walls into distinct cells, as in other plants, but is perfectly continuous
throughout both stem and branches. The whole plant in fact consists of a
long slender sac filled with chlorophyll. Some of them—especially species of
the genus Vaucheria—occur abundantly on the margins of ponds and ditches,
and on damp earth, where they form a dense layer of green velvet ; one or
two species are common on the surface of the soil in fern-pots, where they
form a tangled matting often troublesome to get rid of. Curiously enough
though one of the simplest of its order in vegetative structure, Vaucheria is more
highly differentiated as regards its reproductive system than any member of
the family at present known.
The reproduction of the Algae presents a greater variety of phenomena
than is to be found in perhaps any other of the large divisions of plants. It
would not be possible to go fully into the matter within the limits of one short
paper ; moreover the subject fairly bristles with technicalities, enveloped in 
uncouth Greek and Latin compounds by no means familiar to Lord
Macaulay's proverbial schoolboy Now, roughly speaking, all the varied
phenomena of reproduction may be classed under two heads : the asexual and
the sexual. In the former case the species is reproduced without the inter-
vention of fecundation, by the development of zoospores, which are active,
and as their name implies, animal-like in their character.