ALGAE 103 
two daughter cells have not yet rounded off or separated; 
so that they appear as two halves of the parent cell. Even 
before they sepa- 
rate they may di- 
vide again, and 
thus a group of 
cells may be 
formed.  Pleuro- 
coccus, therefore, 
is another illustra- 
tion of an extreme- 
ly simple plant, in 
that it consists of 
one cell and repro- Fic. 95.—Pleurococcus: A, the adult plant, with its 
nucleus; B-E, various stages of division in pro- 
duces by cell-divi- ducing new cells; F, colonies of cells that have 
sion. remained in contact. 
It would be hard to imagine a simpler plant, and the 
plant kingdom can be thought of as beginning with individ- 
uals consisting of one green cell and reproducing by divi- 
sion. This one cell, however, absorbs material, makes food, 
assimilates it, conducts respiration, etc.; in fact, does all the 
work of living carried on by plants with roots, stems, and 
leaves, although they may contain millions of cells. 
64. The plant cell.—Pleurococcus may be used to illus- 
trate the conspicuous features of a living plant cell. Bound- 
ing the cell there is a thin, elastic cell-wall, composed of a 
substance called cellulose. The cell-wall, therefore, con- 
stitutes a delicate sac, which contains the living substance 
known as protoplasm. It is the protoplasm that has formed 
the wall about itself, in the same sense that a snail deposits 
the shell about its body. The protoplasm is organized into 
various structures which are called organs of the cell. One 
of the most conspicuous protoplasmic organs is the nucleus, 
a comparatively compact and usually spherical body, and 
generally centrally placed within the cell (Fig. 95, A), 
8 
