Chapter VIII 

 ACTIVE CELL WATER RELATIONS 



Introduction : — Recent developments, most within the last decade, 

 necessitate re-examination of certain aspects of cell water relations. Spe- 

 cifically considerable doubt has been cast on the classical concept of the 

 mechanism of uptake and retention of water by plant cells. This classical 

 view has been fully discussed in the two preceding chapters. Simply stated, 

 it holds that water is retained in the vacuole osmotically by reason of the 

 presence of solutes in the sap ; that at limiting plasmolysis the osmotic 

 pressure of the sap is equal to that of the bathing solution ; and that, at full 

 turgor, the hydrostatic pressure developed in the vacuole is equal to the 

 osmotic pressure of the vacuolar sap. In simpler terms, for the range from 

 full turgor to incipient plasmolysis. DPD = OP — TP. The protoplasm 

 is considered to be passive in its behavior to water ; it constitutes a thin 

 layer, differentially permeable, retaining solutes within the vacuole but per- 

 mitting ready passage of water. 



Most interpretations of cell water studies have rested upon these assump- 

 tions. However, sufificient evidence has now accumulated to cause serious 

 doubt concerning the passivity of protoplasm with respect to water move- 

 ment and retention. Plant physiologists are asking themselves i) do some 

 living cells retain in their vacuoles more water than can be accounted for 

 by their solute content; 2) what role does metabolism play in water ex- 

 changes; 3) exactly how does protoplasm function in the water balance of 

 cells ; 4) how do certain alleged vital water functions relate to other physio- 

 logical processes — namely absorption, movement, and loss of water? Con- 

 cisely, we would like to know if protoplasm can act differentially toward 

 both water and solutes. Secretion of water is particularly difficult to visual- 

 ize in view of the high content of water in the protoplasm itself. 



Some of the early workers in plant physiology attempted to explain cer- 

 tain aspects of water absorption and translocation on the basis of a vital 

 control of water (Hales, 1738; Knight, 1801 ; Godlewski, 1884; Janse, 

 1913; BoSE, 1923), but the evidence has never been considered valid. In 

 the present chapter current research leading to evidence for active control 

 of water will be presented and discussed. 



The water specifically referred to in the phrase "active water relations" 

 we consider to be that water which may be transported or retained against 

 an apparent diffusion pressure gradient, by processes requiring the expendi- 

 ture of metabolic energy. It includes water held in the protoplasm by forces 

 involved in living processes which may bring about a separation of water 

 and solutes. Practically, it is reflected in an apparent inequality between 

 the DPD and the quantity OP — TP. A distinction is thus attempted be- 

 tween active forces and passive (osmotic) forces, a distinction which is 

 difficult in the present state of knowledge, and which may of necessity be 

 somewhat arbitrary. 



Evidence from Plasmolytic and Gryoscopic Measurements: — To 



determine the nature of the forces accounting for turgor, promising results 

 have been obtained by comparing the osmotic pressure of the living cells 



