THALLOPHYTES 



9 



To live in water as a medium means that all the structures 

 and habits of such plants must be adjusted to water. 

 Such plants can be explained only by remembering this 

 fact. That plants living in the water may be relatively 

 simple is illustrated by the fact that when plants live in 

 the air they must be protected against drying, and this in- 

 volves protective structures that water plants do not need. 



10. The cell (Fig. 1). The living 

 substance of plants and animals is 

 called protoplasm. It is the only sub- 

 stance that lives and works, and all 

 the structures and work of plants 

 are results of the activity of pro- 

 toplasm. This protoplasm is organ- 

 ized into definite units, which may 

 be thought of as protoplasm indi- 

 viduals, and these units or individuals 

 are the cells. The simplest plants 

 are single cells, while large and com- 

 plex plants consist of millions of cells. 

 It is in this sense that a cell may FlG - 1. Ceiis.of a moss 



leaf: in each of the two 



be called the unit of structure, and 

 that a plant consisting of one such 

 unit may be regarded as the simplest 

 kind of plant. 



Since a cell always includes sub- 



*. stances that are not protoplasm, the term protoplast is 

 used to indicate the living substance of the cell. The 

 protoplast, therefore, is the living, individual unit, and 

 protoplasm is the material of which it is composed. We 

 shall use the term protoplast, therefore, for the living, 

 protoplasmic individual. 



Among plants, the semifluid protoplast usually sur- 



C rounds itself with a wall (Fig. 1). This cell-wall is com- 

 posed of material called cellulose, which is manufactured 



complete cells the single 

 large nucleus is seen, the 

 numerous chloroplasts, 

 and the granular-looking 

 cytoplasm ; the cell-wall 

 surrounding each cell is 

 very distinct. 



