SALT RELATIONS OF VASCULAR PLANTS 133 



referred to the recent reviews by Crafts (1951), Arisz (1952), Esau 

 et al. (1957), Swanson (1959) and Zimmerman (1960). 



G. Excretion of Salts 



Salts are lost from various organs of the plant by both passive 

 and active processes. Appreciable amounts are leached from roots, 

 for example, when they are transferred from salt solutions to distilled 

 water. Most of this salt is presumably washed from the free space 

 of the tissue, but some is lost more slowly from cytoplasm and 

 vacuoles. Such losses are probably due entirely to passive processes 

 — at least no evidence of active excretion by roots has yet been 

 obtained. 



Salts are likewise leached from leaves and an investigation by 

 Tamm (1951) revealed that certain forest mosses get most of their 

 cations from the leaves of trees which are leached in situ. Xylem 

 sap is exuded under pressure via stomata (guttation), and washed 

 away by rain. If the rain water has an acid reaction, metallic cations 

 may also be removed from the leaf through the cuticle by exchange 

 for hydrogen ions. The well-known "chalk glands" of Saxifragaceae 

 consist of modified stomata, from which a calcium-rich guttation 

 fluid is exuded. This evaporates in air and a deposit of calcium 

 carbonate remains. The term "gland" is more correctly restricted to 

 structures in halophytic plants which excrete salts actively. The 

 functioning of such glands is discussed in Chapter 9. 



For further reading 



BmouLPH, O. (1959). Translocation of Inorganic Solutes. In Plant Physiology— 

 A Treatise. (Ed. F. C. Steward). (Vol. II, Chap. 6 pp. 553-603). Academic 

 Press, New York. 



HoAGLAND, D. R. (1944). Upward movement and distribution of inorganic 



solutes in the plant. In Lectures on tlie inorganic nutrition of Plants, Chapter 



4, pp. 72-103. Chronica Botanica, Waltham, Mass. 

 Russell, R. S. and Barber, D. A. (1960). The relationship between salt uptake 



and the absorption of water by intact plants. Ann. Rev. Plant Physiol. 11, 



127-40. 



Stenlid, G. (1958). Salt losses and redistribution of salts in higher plants. 

 Encycl. Plant Physiol. 4, 615-37. 



Steward, F. C. (1954). Salt accumulation in plants. A. Reconsideration of the 

 role of growth and metabolism. B. Salt accumulation in the plant body. 

 Symp. Soc. Exp. Biol. 8, 393-406. 



10 



M.S. A. p. 



