The Processes of Roots 201 



pounds to the square inch. This is readily appreciated when one 

 sees cement sidewalks broken and large rocks moved by the 



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FiG. 117. Fern leaves pushing upward through a cement sidewalk. 

 Growth pressure may amount to hundreds of pounds to the square 

 inch. {After G. E. Stone.) 



growth of roots under them. Growth pressure is just as powerful 

 in stems and other growing parts. Fleshy roots like those of 

 the radish and turnip sometimes force themselves partly out of 

 the ground by the thickening of the upper portion. How is it 

 possible for the cells of plants to withstand internal pressure of a 

 thousand pounds to the square inch without bursting? They 

 must have pressure equal to this amount or they could not move 

 rocks or break cement walks, which they do. The explanation 

 lies in the fact that they are so small that the pressure exerted 

 by a single cell is trifling compared with the strength of its cellu-. 

 lose wall. In a cell mass expanding against a pressure of 1000 

 pounds to the square inch, if the cells are of average size, say .03 

 mm. in diameter, the wall of each cell will not have to resist a 

 pressure of more than .13 of an ounce. 



Food conduction. The transfer of food takes place in the food- 

 conducting tissue of roots in the same way as in stems and leaves. 

 Substances that are transferred are in a soluble form, and they 

 are usually in a comparatively simple form. 



The movement of a substance in^o or out of a cell depends upon 

 the permeability of the cell protoplasm to that particular sub- 

 stance ; if the cytoplasm is impermeable to a substance, it can- 

 not enter or leave a cell. That the direction of the movement of 

 foods may change from time to time is shown by the fact that 



