that their body is composed of two cells, as seen in Euastrum, Cosmarium, Closterium (Figs. 89, 90, 92), or of many cells, as in Hyalotheca (Fig. 91), but they are really one-celled plants, as proved by the fact of all the green contents escaping if the cell-wall be broken, the indentations and constrictions being only superficial. The constriction of Cosmarium indicates the place where the body will divide into two halves, each half becoming a new individual. In some forms, as Pediastrum (Fig. 97), the green contents of the cell are transformed into ciliated bodies, or zoospores, which escape, the cell-wall breaking, and move about for some time (Fig. 98), then settle, coalesce, and finally take on the appearance of the parent plant. The green, thread-like plant of horse-troughs, etc. is generally composed of the Alga known as Spirogyra (Fig. 93). Each Spirogyra is a chain of cells, the green matter of the cells being disposed in the form of a spiral. At certain seasons of the year, adjacent Spirogyrae are seen to push out the walls of their cells towards each other until a communication is formed. (Fig. 94.) The green contents of one individual, leaving its cell, pass through the communicating process, and mix with the contents of its neighbor: this is later the beginning of a future Spirogyra. This kind of reproduction is known as conjugation, and is the simplest type of sexual reproduction, i.e., the union of two distinct germinal masses to form a spore. The Nostochaceae (Fig. 95) attached to stones are composed of a row of cells or beads, making filaments imbedded in a gelatinous kind of matter which is inclosed by a membrane; the membrane breaking, the gelatinous matter escapes into the water, carrying the filaments with it. The Nostochaceae have been considered by some botanists as an undeveloped form of Lichen. With the exception of the Ulvaceae (Fig. 96), which are marine in their habitat, the green Algae are confined to fresh water. The Ulva,
