The cell 



24 PLANT GROWTH 



Cell wall (Cellulose) 



{Cytoplasm (water, proteins, carbohydrates, fats, and mineral 

 Nucleus, which contains the chromatin (similar to cytoplasm, 

 but the protein has a high phosphorus content) 



Non-livine 1 Water solutions of salts, sugars, and other foods 

 content i Insoluble foods as starches and proteins 

 L Wastes 



The cell wall varies in thickness with the function of the 

 cell. In some strengthening cells such as those in wood, the 

 cross-sectional area of the wall may be as great as the re- 

 mainder of the cell. Even succulent vegetables such as 

 asparagus may become woody as the cell walls thicken. 

 Rapid continuous growth produces more desirable vegetables 

 because in rapid growth the cell walls stretch thin as the 

 cell increases in size. 



The function of a cell wall is comparable to an automobile 

 tire that resists the expansion of the inner tube, since it re- 

 sists the expansion of the protoplasm and gives shape to the 

 cells and to the plant. The pressure of the protoplasm 

 against the cell wall often exceeds one hundred pounds per 

 square inch; the air pressure in a tire is from twenty-five to 

 fifty pounds. When excessive loss of water from a plant re- 

 duces this protoplasmic pressure against the cell walls to 

 zero, the plant wilts, in other words water pressure prevents 

 wilting. 



The portion of protoplasm exclusive of the nucleus is 

 known as cytoplasm. In addition to its complex chemical 

 nature it appears to have very definite structural nature or 

 physical complex. Chemists can analyze protoplasm and 

 tell us its elements, but the physicist has been unable to give 

 us the details of their arrangement, which is generally be- 

 lieved to be the "key" to life. The cytoplasm absorbs and 

 holds water with an enormous force, and the withdrawal of 

 large amounts of water may kill it. An exceedingly thin 

 layer of cytoplasm, with different properties from the rest, 

 lies next to the cell wall and seems to control the substances 



