THALLOPHYTES: ALG^ 



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. 20i), 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 inotoiylasm. 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 ^gg^ 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, cj'toplasm (C), and chloroplasts 

 (.1).— Caldwell. 



