plant often forms the greater part of the floating green mass (or "frog spittle") on ponds. The threadlike character of the thallus can be seen with the naked eye or with a hand lens, but to study it carefully a microscope magnifying two hundred diameters or more must be used. The thread is divided into long cells by cross walls which, according to the species, are either straight or curiously folded (Fig. 266). The chlorophyll is arranged in beautiful spiral bands near the wall of each cell. From the character of these bands the plant takes its name. Each cell is provided with a nucleus and other protoplasm. The nucleus is suspended near the center of the cell (a, Fig. 265) by delicate strands of protoplasm radiating toward the wall and terminating at certain points in the chlorophyll band. The remainder of the protoplasm forms a thin layer lining the wall. The interior of the cell is filled with cell-sap. The protoplasm and nucleus cannot be easily seen, but if the plant is stained with a dilute alcoholic solution of eosin they become clear.
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Fig. 266.—Conjugation of Spirogyra. Ripe zygospores on the left; a, connecting tubes.
Spirogyra is propagated vegetatively by the
breaking off of parts of the threads, which continue
to grow as new plants. Resting-spores,
which may remain dormant for a time, are formed by a process
known as conjugation. Two threads lying side
by side send out short projections, usually from
all the cells of a long series (Fig. 266). The
projections or processes from opposite cells
grow toward each other, meet, and fuse, forming
a connecting tube between the cells. The
protoplasm, nucleus, and chlorophyll band of
one cell now pass through this tube, and unite
with the contents of the other cell. The entire
mass then becomes surrounded by a thick
cellulose wall, thus completing the resting-spore,
or zygospore (z, Fig. 266).
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Fig. 267.—Strand, or Filament of Zygnema, freed from its gelatinous covering.
Zygnema is an alga closely related to spirogyra
and found in similar places. Its life
history is practically the same, but it differs
from spirogyra in having two star-shaped chlorophyll bodies (Fig. 267) in each cell, instead
of a chlorophyll-bearing spiral band.
