THALLOPI1YTES : ALG^E 
227 
plex plants consist of very many cells. It is necessary to 
know something of the ordinary living plant cell before 
the bodies of Algae or any other plant bodies can be under- 
stood. 
Such a cell if free is approximately spherical in outline 
(Fig. 204), but if pressed upon by contiguous cells may be- 
come variously modified in 
form (Fig. 200). Bounding 
it there is a thin, elastic 
wall, composed of a sub- 
stance called cellulose. The 
cell wall, therefore, forms a 
delicate sac, which contains 
the living substance known 
as protoplasm. This is the 
substance which manifests 
life, and is the only sub- 
stance in .the plant which 
is alive. It is the proto- 
plasm which has organized 
the cellulose wall about it- 
self, and which does all the 
plant work. It is a fluid 
substance which varies much in its consistence, sometimes 
being a thin viscous fluid, like the white of an egg, some- 
times much more dense and compactly organized. 
The protoplasm of the cell is organized into various 
structures which are called organs of the cell, each organ 
having one or more special functions. One of the most con- 
spicuous organs of the living cell is the single nucleus, a com- 
paratively compact and usually spherical protoplasmic body, 
and generally centrally placed within the cell (Fig. 200). 
All about the nucleus, and filling up the general cavity 
within the cell wall, is an organized mass of much thinner 
protoplasm, known as cytoplasm. The cytoplasm seems to 
form the general background or matrix of the cell, and the 
FIG. 200. Cells from a moss leaf, showing 
nucleus (B) in which there is a nucle- 
olus, cytoplasm ((7), and chloroplasts 
(A). CALDWELL. 
