ALG^E 



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- Flo> ^ pleurococcui: A > * h adult plant, with its 



nucleus ; B-E, various stages of division in pro- 



duceS^by Cell;jdiyi- ducing new cells; F, colonies of cells that have 



'ned in contact. 



It woiilcLea-^e^imagine a simpler plant, and the 

 pTanTTdngdom 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). 



