_Nostoc._—On wet rocks and damp soil dark, semitransparent irregular or spherical gelatinous masses about the size of a pea are often found. These consist of a colony of contorted filamentous algæ embedded in the jelly-like mass. The chain of cells in the filament is necklace-like. Each cell is homogeneous, without apparent nucleus, and blue-green in colour, except one cell which is larger and clearer than the rest. The plant therefore belongs to the group of _blue-green algæ_. The jelly probably serves to maintain a more even moisture and to provide mechanical protection. Multiplication is wholly by the breaking up of the threads. Occasionally certain cells of the filament thicken to become _resting-spores_, but no other spore formation occurs. [Illustration: FIG. 264.—FILAMENT OF OSCILLATORIA, showing one dead cell where the strand will break.] * * * * * _Oscillatoria._—The blue-green coatings found on damp soil and in water frequently show under the microscope the presence of filamentous algæ composed of many short homogeneous cells (Fig. 264). If watched closely, some filaments will be seen to wave back and forth slowly, showing a peculiar power of movement characteristic of this plant. Multiplication is by the breaking up of the threads. There is no true spore formation. * * * * * [Illustration: FIG. 265.—STRAND OF SPIROGYRA, showing the chlorophyll bands. There is a nucleus at _a_. How many cells, or parts of cells, are shown in this figure? ] _Spirogyra._—One of the most common forms of the green algæ is spirogyra (Fig. 265). This plant often forms the greater part of the floating green mass (or “frog spittle”) on ponds. The thread-like 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 centre 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 eosine they become clear. [Illustration: FIG. 266.—CONJUGATION OF SPIROGYRA. Ripe zygospores on the left; _a_, connecting tubes. ]