162 ^4MINERAL SALTS ABSORPTION IN PLANTS 



*fT»» 



in turn is controlled by accumulation of salt, it is understandable 

 that excretion is reduced by metabolic inhibitors. What is less clear 

 is why the concentration of salts remains constant in these circum- 

 stances. If ..the mechanism operates in the manner suggested by 

 Arisz, a reduced flow of liquid should be associated with lower 

 concentration since the pressure developed, and hence the amount of 

 liquid exuded, apparently depends on concentration. A more 

 satisfactory explanation of Arisz's observations is that salt is actively 

 excreted at the pore surfaces and this is followed by osmotic with- 

 drawal of water. The reduced flow of more highly concentrated 

 exudate when the tissue is in contact with a solution of high osmotic 

 pressure can be adequately explained on the basis of either of the 

 proposed hypotheses. 



There are other kinds of glands on the leaves of some plants, for 

 example, Hypericum sp. which excrete fragrant oils, and in these it is 

 possible to see with a microscope that the oils appear first as 

 minute globules in the protoplasm of the excretory cells. Later they 

 can be seen as small droplets adhering to the outer surface of the 

 gland. These observations suggest by analogy that the salt glands 

 may function by the accumulation of salt in small vesicles within the 

 cytoplasm of the leaf tissue. These are transferred into the excretory 

 cells and discharged at the external protoplasmic surface in the 

 vicinity of the pore. A further investigation of the mode of action 

 of salt glands is urgently required, especially as it may shed important 

 light on the mechanisms of salt absorption and movement in less 

 specialized cells and tissues.* 



For further reading 



Arisz, W. H. (1953). Significance of the symplasm theory for transport across 



the root. Protoplasma, 66, 5-62. 

 Arnold, A. (1955). Die Beutung der Chlorionen fiir die Pflanze, insbesondere 



deren physiologische Wirksamkeit. Eine monographische Studie mil 



Ausblicken aiif das Halophytenproblem. Gustav Fischer, Jena. 

 Bernstein, L. and Hayward, H. E. (1958). Physiology of salt tolerance. 



Ann. Rev. Plant Physiol. 9, 25-46. 

 Helder, R. J. (1956). The loss of substances by cells and tissues (salt glands). 



Encyclo. Plant Physiol. 2, 468-88. 



* Note added in proof: An electron microscopic investigation undertaken by 

 Dr. R. Barton at the instigation of the author has confirmed the presence of 

 numerous vesicles in the cytoplasm of the excretory cells in salt glands of 

 Limonium. 



