FRESH-WATER ALGAE. 163
Taking as a type a very common alga in pools—(Edogonium, with long
green unbranched filaments, and well-marked cells—we find the entire
contents of a cell are used up to form a zoospore, which when mature is pear-
shaped, and at the smaller extremity fringed with cilia, by the rapid vibration
of which the zoospore is able to move through the water with great activity,
exactly like an animalcule. After swimming about freely for some time—an
hour or two perhaps, it comes to rest, and attaches itself to a solid body, and
then immediately starts growing into a new filament like the parent.
This mode of reproduction by motile spores capable of immediate
germination is extremely common among the algae, and is especially
characteristic of the Chlorophyceae, or pure green group. It may be taken as
a type of asexual reproduction, but it is needless to say that many modifica-
tions occur. The important point to note is that in its very earliest stage the
plant is endowed with the power of locomotion, and that in a very marked
degree.
The modes of sexual reproduction are extremely varied, and much less
easy to describe briefly, We may take as the type of one mode, the process
which occurs in Spirogyra. These plants are familiar objects to every one
who has made any study of pond life, for the species are very numerous and
abundantly distributed. And, moreover, every kind of Spirogyra is at once
distinguished by the beautiful arrangement of the chlorophyll, which takes
the form of narrow spiral bands winding through the whole length of the
filament. Another distinguishing character is that the filaments are always
free, and never attached at one end to some solid object, as in the case in
Oedogonium.
In Spirogyra reproduction takes place between two filaments lying side by
side, the cells of which send out lateral outgrowths which unite and coalesce,
and then the protoplasm passes from one cell, through the connecting band,
into the opposite cell, and there forms a spore ; and in this way the spore is
the result of the fusion of the protoplasm of both cells. This process is
termed conjugation, from the conjugating or yoking together of the cells, and
in like manner the resulting spore is called a zygospore. This method of
spore-formation is prevalent among the Desmids, which with Spirogyra and a
few other genera, constitute the family Conjugatae.
In Vaucheria, on the other hand, quite a different sort of operation takes
place. A small branchlet sprouts out upon a filament and develops a
multitude of excessively minute flask-shaped bodies, each furnished at one
end with a pair of cilia ; these are called spermatozoids, and they swim about
freely in the water. In the meantime on another part of the plant there
develops a somewhat globular outgrowth having a minute aperture at the
apex. Into this aperture some of the roving spermatozoids find their way,
and fertilization takes place; after which the fertilized cell is called an
oospore. This process is precisely similar to that which prevails in the
higher plants, The fertilized cell now becomes a resting-spore, and is able to
endure a period of drought with safety.
It would be easy to give examples of other modes of sexual reproduction,
but these two types must suffice. One point it will be well to bear in mind,
as it fixes an important physiological distinction. In the first example,
Spirogyra, we have an instance of conjugation, or the union of two similar cells,
and the result is the production of a zygospore. In the second example,