80 EVOLUTION OF LIFE. 
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 indenta- 
tions and constrictions being only superficial. The con- 
striction 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, coa- 
lesce, 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 Spirogyra 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 conju- 
gation, and is the simplest type of sexual reproduction, Ze, 
the union of two distinct germinal masses to form a spore. 
The Nostochacez (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 mem- 
brane; the membrane breaking, the gelatinous matter 
escapes into the water, carrying the filaments with it. 
The Nostochacez have been considered by some botanists 
as an undeveloped form of Lichen. With the exception 
of the Ulvacez (Fig. 96), which are marine in their habitat, 
the green Alga are confined to fresh water. The Ulva, 
