OSMOSIS AND IMBIBITION 237 



(2) a differentially permeable membrane. As we have seen, these 

 conditions are both met, the first by the more concentrated cell 

 sap as compared with the watery solution in the soil and the second 

 by the plasma membrane. As Thoday (1918) has pointed out and 

 earlier Ursprung (1916), if P is the osmotic pressure within the cells 

 and T the turgor pressure, then the amount of water entering the 

 cell at any given moment depends upon P-T. While the turgor 

 pressure increases owing to the intake of water, the osmotic pressure 

 decreases as the solution becomes diluted. This value of P-T is 

 known as the suction force. Consequently, in considering the 

 possible exchange of water between two adjacent cells, one must 

 consider both the osmotic and the turgor pressure. 



The osmotic pressures found in plant cells are sometimes very 

 high. De Vries found the following values for the expressed juice: 



Potato leaves (Solarium) 5.5 atmospheres 



Mountain ash berries (Sorbus) 9.0 



Beetroots (Beta) 21.0 



While plant cells in general range from 5-10 atmospheres, desert 

 plants have been found to possess pressures exceeding 100 at- 

 mospheres. Some of the molds when grown in concentrated 

 solutions develop pressures as high as 150 atmospheres, and Atri- 

 plex confertifolia growing near Great Salt Lake has an osmotic 

 pressure of 153 atmospheres! This is the world record for Angio- 

 sperms. Even 10 atmospheres means a pressure of 150 lbs. per 

 square inch, and it is not to be wondered at that plants can split 

 rocks, lift sidewalks, and perform similar feats of strength. 



Absorption by Plants. — Most of the water and salts absorbed 

 by plants are taken in by the roots, but some water is absorbed 

 also by aerial parts, especially leaf hairs and other uncutinized 

 portions of the aerial organs. Such absorption takes place in 

 Eriospermum, Massonia, and Gethyllis by the leaf hairs (Marloth, 

 1926), and in the Australian salt bush (Atriplex) by the uncutinized 

 leaves, which are especially rich in accumulations of sodium 



chloride. 



The chief absorbing region of the root lies directly back of the 

 root cap and is enlarged many fold by the formation of the root 

 hairs, the suction force of which plays the chief role in determining 

 whether water will be absorbed under a given set of osmotic condi- 

 tions. The intake of salts is largely determined by the laws of 

 diffusion. For this reason a plant often takes up salts which have 



