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 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.

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).

, 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.