Crafts etal. —146— Water in Plants 



tending the experiments to include six other storage tissues, concluded that 

 water intake as observed in potato is a general phenomenon. Especially 

 striking behavior was shown by tuber tissue of Helianthus tuberosus. 

 While confirming the correlation with respiration, it seemed " . . . . most 

 probable that water-intake is only a secondary phenomenon, a consequence 

 of other metabolic processes, by which osmotically acting substances arise." 

 These substances would more logically be formed in the protoplasm than 

 in the vacuole, according to Reinders. 



Information relative to the hydration of protoplasmic proteins has been 

 obtained by Levitt (1946). Isolation of the proteins from potato tubers 

 was apparently accomplished without denaturing them. The protein solu- 

 tions were introduced into microcups where their osmotic pressures were 

 determined manometrically. There was some indication that proteins from 

 external tissues, allowed to form new periderm, exhibited greater hydration 

 than those from internal tissues that were dormant. This is a new method 

 of investigating the water relations of protoplasm ; it should lead to other 

 interesting results. 



Movement of water from cell to cell could conceivably result from ac- 

 tive processes causing inequalities in osmotic pressure within the cell, in- 

 equalities of membrane permeability, or changes in imbibition pressure. 

 Reference is made to these possibilities in chapters which follow. 



In conclusion, it seems evident that if active water movements, meta- 

 bolically controlled and energized, are demonstrated beyond doubt in plants, 

 it will be necessary to revise the basic concepts underlying many plant 

 processes. Though momentarily seeming to confirm vitalistic ideas on 

 water relations of plants, such a demonstration will require, in the long run, 

 a deeper search into the physical and chemical mechanisms involved. And 

 although certain classical views may seem already to be outmoded, it must 

 be admitted that the evidence for active water movement is still far from 

 complete as are many other aspects of protoplasmic activity. 



Summary: — In the classical concept of cell water relations the protoplasm is 

 passive with respect to the passage of water, and consequently water movement and 

 retention are determined by forces of osmosis, imbibition, etc. 



Recent work indicates that the protoplasm may play some vital role in water dis- 

 tribution. 



There is a fairly consistent discrepancy between tissue osmotic pressures deter- 

 mined plasmolytically and cryoscopically. This has been interpreted by some as in- 

 dicating that water may accumulate in vacuoles due to a secretory action of the proto- 

 plasm. Other explanations proposed for the discrepancy are : the presence of colloids 

 in the vacuole ; dilution of expressed sap by protoplasmic water, producing cryo- 

 scopic values which are too low ; adhesion of the protoplasm to the wall, resulting in 

 excessive plasmolytic values ; and methodical errors in both procedures. 



Refinement of sap expression technics has produced saps termed "protoplasmic" 

 and "vacuolar" by some investigators. The results have been interpreted as support- 

 ing the secretion hypothesis. It seems questionable if any clear distinction of this type 

 can be made on sap fractions obtained by pressure. 



While plasmolytic methods by classical theory should provide an accurate picture 

 of vacuolar concentration, many anomalous results have been obtained which may re- 

 flect some kind of active control of water by the protoplasm. 



When expressed saps are compared with equilibrium bathing solutions by cryo- 

 scopic methods, discrepancies appear that indicate possible imbibitional or secretory 

 activity of cells. 



Auxin has been shown to increase water uptake by living cells. This has been 

 considered as evidence for active secretion of water into the vacuole, but other ex- 

 planations are possible. 



The freezing point of living tissue is usually lower than that of dead tissue. The 

 most plausible explanations are : difficulties exhibited by living tissue toward freezing 



