Grafts et al. 



— 112 



Water in Plants 



measured plasmolytically (Pg) with that of the expressed cell sap measured 

 cryoscopically (On). From such experiments Bennet-Clark, Green- 

 wood, and Barker ( 1936) were led to the conclusion that the protoplasm 

 of certain cells, using metabolic energy, was able to "secrete" water into 

 their vacuoles. It follows from this conclusion that such cells could retain, 

 within their vacuoles, more w^ater than would be expected from purely 

 osmotic forces. Using root tissues of Beta vulgaris and Brassica napobras- 

 sica, and petioles of Begonia rex, Rheum rhaponticum and Caladium hicolor 

 they measured cryoscopically the osmotic pressure of the expressed sap. 

 After correcting these values for change in volume (Vn to Vg) they com- 

 pared them with values obtained plasmolytically. As shown in Table 30, 

 marked discrepancies occurred, particularly in Beta, Brassica, and Begonia. 



Thus Og was greater than On 



Vn 



v^ 



by significant amounts. This dis- 



crepancy Bennet-Clark, Greenwood, and Barker attributed to a force 

 termed "water secretion." In place of the classical equation DPD = OP — 

 TP they proposed DPD :=r OP — TP -f X, and X they described as "an 

 unknown additional pressure sending water into the cell." 



Table 30. — Cryoscopic and plasmolytic measurement of osmotic pressure in various 

 plants (data of Bennet-Clark, Greenwood and Barker, 1936). Values are atmos- 

 pheres at 20° C, and in most instances represent averages of several tests: — 



OP FROM 



freezing point op from 



depression plasmolysis x 



Plant (On) (Og) (Og — On • Vn/Vg*) 



Beta vulgaris A 15.5 23.4 7.1 



Beta vulgaris B 9.5 12.6 2.6 



Beta vulgaris C 12.0 18.4 5.8 



Brassica napobrassiea A 11.4 17.8 5.9 



Brassica napobrassiea B 11.85 17.0 4.6 



Begonia rex A 5.3 8.0 2.4 



Begonia rex B 5.5 8.0 2.2 



Rheum rhaponticum A 7.5 8.1 0.2 



Rheum rhaponticum B 8.6 9.5 0.5 



Caladium hicolor A 5.8 5.5 — 0.5 



Caladium bicolor B 5.8 6.4 0.4 



* A constant factor of 1.05 was employed in calculations to correct for shrinkage from Vn to Vg. 



The discrepancies for Caladium and Rheum range from — 0.5 atm. to 

 -f 0.5 atm., values that probably are not significant. For Begonia, Brassica 

 and Beta on the other hand the discrepancies are all positive and range from 

 2.2 to 7.1 atm., values that are highly significant and constitute from 25 to 

 30 per cent of the DPD at limiting plasmolysis. These results are of im- 

 portance in pointing out the magnitude of errors that may occur in the usual 

 interpretation of such experiments ; they are of greater importance in that 

 they challenge the accepted views of the mechanics of water absorption and 

 retention by cells and call for re-examination of the data upon which these 

 views were built. In a later paper (Bennet-Clark and Bexon, 1940) 

 the nature of the secretion process is treated in more detail, the proposition 

 being made that it does not constitute an exception to the physico-chemical 

 laws of diffusion but rather that water is transported into the vacuole by 

 the concurrent diffusion of another component which acts as a "carrier." 

 Discussion of the theoretical considerations of the secretion theory is pre- 

 sented later. 



