THE ROOT AND ITS FUNCTIONS 



45 



plant substance cellulose. This is firm in texture but easily 

 penetrated by water. The protoplasm of the cell has two distinct 

 portions — the nucleus, a dense, somewhat spherical body which 

 appears to be the directive center for the cell's activities; and the 

 cytoplasm, thinner and more watery in texture, which lines the 

 inner surface of the wall in a tenuous laj^er and is bounded, 



Fig. 27. — A typical plant cell. Diagrammatic drawing .showing the cen- 

 tral sap-cavity, surrounded by a layer of cytoplasm which lines the cell-wall. 

 Embedded in the cytoplasm is the nucleus. Parts of the cell-walls of six adjacent 

 cells are also shown. 



without and within, by a very delicate membrane. Embedded 

 in the cytoplasm frequently appear small, somewhat denser 

 bodies, the plastids. These perform special functions, such as 

 carrying on the manufacture of fr )d, building up starch-grains or 

 producing colors. In many case^ it has been shown that the 

 cytoplasm is not passive and immobile, but that within it a slow, 

 streaming movement often takes place. In mature cells, the entire 

 central portion of the cell is occupied by a sap-cavity or vacuole, 

 filled with water in which various substances are dissolved, and 

 surrounded externally by the layer of cytoplasm. A typical 

 plant cell may thus perhaps be likened roughly to an inflated 

 football or basketball, the firm leathery covering corresponding 

 to the cell-wall; the thin inner bladder of rubber, tightly pressed 

 against it, to the layer of cytoplasm; and the air-space to the 

 sap-cavity. A comparison to an automobile tire, with its stout 

 shoe or casing, its delicate inner tube, and its central air-cavity, 

 might also be made. 



Cells are normally very small objects, averaging about .01 

 mm. in diameter, and varying widely in shape and character 

 according to the function which each performs, whether this be 

 support, absorption, conduction, storage, protection, food-manu- 



