t of them form mere
threads, either simple or branched. To this division belong almost all
the Fresh-water Algae, such as those which constitute the silky threads
or green slime of running streams or standing pools, and which were all
called Confervas before their immense diversity was known. Some are
formed of a single row of cells, developed each from the end of another.
Others branch, the top of one cell producing more than one new one (Fig.
564). Others, of a kind which is very common in fresh water, simple
threads made of a line of cells, have the chlorophyll and protoplasm of
each cell arranged in spiral lines or bands. They form spores in a
peculiar way, which gives to this family the designation of conjugating
Algae.

[Illustration: Fig. 564. The growing end of a branching Conferva
(Cladophora glomerata), much magnified; showing how, by a kind of
budding growth, a new cell is formed by a cross partition separating the
newer tip from the older part below; also, how the branches arise.]

[Illustration: Fig. 565. Two magnified individuals of a Spirogyra,
forming spores by conjugation; a completed spore at base: above,
successive stages of the conjugation are represented.]

512. At a certain time two parallel threads approach each other more
closely; contiguous parts of a cell of each thread bulge or grow out,
and unite when they meet; the cell-wall partitions between them are
absorbed so as to open a free communication; the spiral band of green
matter in both cells breaks up; the whole of that of one cell passes
over into the other; and of the united contents a large green spore is
formed. Soon the old cells decay, and the spore set free is ready to
germinate. Fig. 565 represents several stages of the conjugating
process, which, however, would never be found all together like this in
one pair of threads.

[Illustration: Fig. 566. Closterium acutum, a common Desmid, moderately
magnified. It is a single firm-walled cell, filled with green
protoplasmic matter.]

[Illustration: Fig. 567. More magnified view of three stages of the
conjugation of a pair of the same.]

513. Desmids and Diatomes, which are microscopic one-celled plants of
the same class, conjugate in the same way, as is shown in a Closterium
by Fig. 566, 567. Here the whole living contents of two individuals are
incorporated into one spore, for a fresh start. A reproduction which
costs the life of two individuals to make a single new one would be
f