Crafts et al. — 138— Water in Plants 



of colloids to hold water varies greatly with changing environment such as 

 salt concentration, etc., it is proposed that water may be taken up at one 

 point and released at another. In a strict sense this cannot be considered 

 as active water transport. 



PiCKEN (1936) stated that the mviscles of higher invertebrates appear 

 to be "considerably less hydrated than would be the case if they were in 

 simple equilibrium with the surrounding protein solution" and visualized 

 some active regulation of hydration. This suggests that energy is expended 

 in decreasing the imbibition pressure of the protoplasm. In a recent per- 

 sonal communication this author suggests that "regulation" may probably 

 be achieved in the first place by the synthesis of structurally different pro- 

 teins which differ in their affinity for water. 



While ultimate explanations for the phenomena listed above are not at 

 hand, it seems that active processes involving water are numerous in the 

 animal organism. For a thorough discussion of active transport of solutes 

 and water in animals, see Hober (1945). 



Relation to Solute Accumulation: — Though solute accumulation 

 cannot receive detailed treatment in this volume, it seems appropriate to 

 attempt a reconciliation of the processes of water and solute absorption 

 since both may apparently be active processes. Since solute accumulation 

 implies absorption against a concentration gradient, the dynamic "steady 

 state" maintained by the plant is usually characterized by a higher solute 

 concentration within the cell than without. The analogy between this and 

 water secretion should be clear. 



The known facts concerning solute accumulation by plant cells may be 

 briefly stated : 



1) Plants under the appropriate conditions retain within their cells higher con- 

 centrations of certain ions than exist externally (Osterhout, 1922; Hoagland and 

 Davis, 1923a). Furthermore, the total ion concentration in the cell sap may exceed 

 the external concentration by as much as 25 times (Hoagland and Davis, 1929). 



2) The accumulation process has been demonstrated to depend on metabolic energy 

 (Steward, 1932; Hoagland and Broyer, 1936). This conclusion has resulted from 

 tests utilizing cyanide, oxygen deficiency, temperature variation, and limited respirable 

 food reserves. Light was found by Hoagland and Davis (19236) to be an important 

 factor, probably through its relation to food synthesis. 



3) Solute accumulation takes place in both aquatic and terrestrial plants. Many 

 investigators have used massive-celled algae such as Nitclla, Valonia, Chara, and 

 Halocystis since vacuolar sap in a fairly pure state may be obtained in sufficient quanti- 

 ties for analysis. For a review of such studies, see Osterhout (1936, 1947o). Barley 

 roots have been used with success, sap expressed from frozen and thawed roots being 

 employed for analysis (Hoagland and Broyer, 1936). Potato tuber tissue has been 

 used in studies on salt uptake by Steward and his associates. 



4) The preponderance of the ions determined in such studies are believed to exist 

 within the vacuoles of the cells almost entirely in the free state (Hoagland and Davis, 

 1923a; Osterhout, 1936). 



5) The process appears to be limited to growing tissues or to those potentially able 

 to grow. 



6) While most investigations have dealt with electrolyte absorption, other solutes 

 may be accumulated. 



7) The underlying mechanism is not yet clear. Steward and Harrison (1939) 

 distinguish between "primary" and "induced" absorption. The first refers to an active 

 process dominant in accumulation phenomena of living cells. The second implies ab- 

 sorption due to the physical and chemical properties of the tissue with no dependence 

 on metabolic energy. Absorption of ions by dead tissues comes in this category. The 

 forces involved in induced absorption are those of dififusion, complex formation, col- 

 loidal adsorption, ionic exchange, and Donnan membrane potentials. 



