ABSORPTION OF WATER AND MINERAL SALTS 45 



larger amounts than can be explained by diffusion. Potas- 

 sium goes from a more dilute soil solution to a cell with a 

 higher concentration of the ion. It appears to be absorbed 

 several times more rapidly when the roots have readily oxi- 

 dizable food material and plenty of oxygen, and may be a 

 case of actual plant energy used in absorption. If this is 

 true, and the evidence is most convincing, salt absorption is 

 the result of work by the plant and can be controlled by 

 enzyme and hormone regulation. This research is in prog- 

 ress and may suggest methods of stimulating these growth 

 regulators to speed up the development of the plant. Other 

 elements appear to be absorbed with difficulty and a wide 

 difference in their concentration is always found. In some 

 cases the cell sap has a very low concentration of the ion. 

 This is spoken of as selective absorption. Much research 

 work has been done on this problem, but we have many 

 unanswered questions. The following table shows one of the 

 cases of unequal diffusion of elements by a pondweed: 



Analysis of the sap of Nitella and of the fond water in which it was growing, 



by Hoagland and Davis 



Ion Concentration in sap 



Concentration in pond 

 water 



Ca 13.0 1.3 



Mg 10.8 3.0 



Na 49.9 1.2 



K 49.3 0.51 



In the case above all the elements are more concentrated 

 in the cell sap than in the pond water, but note that the Mg 

 is three and one half times as concentrated, while the K is 

 almost a hundred times as concentrated. These are not 

 uncommon results. 



REFERENCES 



Hill, J. Ben, L. 0. Overholts, and H. W. Popp, Botany, McGraw-Hill Book Co., 1936. 

 Meyer, B. S., "The Water Relations of Plant Cells," The Botanical Review, vol. 4, pp. 



531-547, 1938. 

 Miller, E. C, Plant Physiology, McGraw-Hill Book Co., 1938. 



