Grafts et al. — 154 — Water in Plants 



cause of excessive sampling of the soils and the use of approximate methods 

 for determining osmotic pressure. Since the OP values usually found for 

 fertile irrigated soils range from 1 to 2 atm., it is apparent that the soils 

 reported above were actually saline. Crop production was poor in soils 

 having OP values of 10 or more atmospheres at PWP; growth was com- 

 pletely inhibited where OP values reached 47 atm. (M agist ad and 

 Reitemeier, 1943). 



Water Movement: — Experimental work has proved rather conclusive- 

 ly that the rapid longitudinal movement of water in plants takes place 

 through the nonliving tracheal elements of the xylem. The forces involved 

 in such movement are not as clearly understood. Both vitalistic and physical 

 theories have been propounded. Copeland's (1902) review presents the 

 early concepts. 



Vesque (1883), Strasburger (1891), and Dixon and Joly (1894) 

 showed that plants could conduct water through dead tissues, and physiol- 

 ogists have generally discounted purely vitalistic interpretations of water 

 movement; most, however, accept the view that living cells in some way 

 condition the xylem tubes, keeping them free of gum and obstructions. 

 MacDougal and Overton (1927) have pointed out that living cells in 

 contact with tracheal elements develop tyloses that plug them; anatomical 

 investigations, however, indicate that tylose formation occurs only in xylem 

 that is injured or that is approaching senility. The functioning of young 

 active xylem and the provision of new xylem through cambial activity de- 

 pend upon Hving cells ; this does not prove that such cells are actively en- 

 gaged in pumping water through tracheal elements. 



Bose (1923) has been the chief advocate in recent years of water trans- 

 port by vital activity. He claims that by very delicate instruments he has 

 been able to detect a pulsation of the protoplasm of living cells throughout 

 the plant and proposes that this action of the cells promotes conduction by 

 injecting liquid into the xylem thus setting up an intra-vascular pressure 

 and producing a mechanical transport. Molisch (1928, 1929) supported 

 this theory but the experimental evidence has not been generally accepted 

 (Dixon, 1924; Benedict, 1927; MacDougal and Overton, 1927). 



It has been proposed (Priestly, 1935) that water movement into the 

 expanding leaves is associated with the processes of growth and differentia- 

 tion and should be interpreted on an anatomical basis. The living xylem 

 elements are assigned a major role in conduction ; dead trachael elements are 

 thought to act chiefly as storage tissue. That the living cells of the xylem 

 are involved in the ascent of sap and that the dead elements are incidental 

 and only function as reservoirs was suggested again in 1939 by Handley. 



Cohesion Theory: — Despite the objections cited, the cohesion theory 

 of water movement in plants has gained almost universal acceptance by plant 

 physiologists. This theory is based on the classical researches of Dixon 

 and Joly (1894), Askenasy (1895), and Dixon (1914, 1924). As gen- 

 erally accepted the cohesion theory proposes that water is able to move 

 through the xylem of plants, even to the tops of tall trees, by reason of its 

 continuity and cohesion, and that such movement results from differences in 

 diffusion pressure of water between the soil, the plant, and the aerial en- 

 vironment. 



Cohesion and Adhesion: — Forces of cohesion between the water 

 molecules and of adhesion between water and the cell walls permit the main- 



