Crafts et al. 



— 126 — 



Water in Plants 



there are as yet too many unknown factors related to protoplasmic struc- 

 ture, permeability, and imbibition to allow a simple osmotic explanation of 

 all cell water relations. 



Gryoscopy of Tissues and their Equilibrium Bathing Solutions: — 



If cells at limiting plasmolysis possess in their vacuoles a sap equal in 

 osmotic pressure to the external solution, the same should hold true where 

 the external solution is hypertonic. Results of such tests utilizing beet root 

 tissue (Currier, 1944a) indicated the expressed sap (after freezing) to 

 be somewhat more dilute than an external "isotonic" solution for tissue 

 exhibiting a +PCD (Og > On • Vn/Vg). For —PCD tissue (Og < On ■ 

 Vn/Vg), the expressed sap was slightly more concentrated. Where the 

 tissue cylinders were treated with weakly hyper- and hypotonic sucrose 



expressed sap O P 



ratio was 



solutions, there was also evidence that the .,., . , ,■ r. x. 



' equuibriu'.Ti solution O F 



higher in the case of the — PCD beets. The data are shown in Table 40. 

 Each value represents an average of five determinations. 



Eaton (1943) has suggested that the water secretion values of Bennet- 

 Clark are invalidated by the questionable assumption that plasmolytic 

 values measure the water retaining force of cells in the normal condition. 

 He believes that solute accumulation during the immersion of sections in 

 plasmolyzing solutions is a real possibility, and cryoscopy of tissues removed 

 from such solutions, rather than of tissues of normal volume, would have 

 led to different conclusions. The data in Table 40 do not bear this out 

 with respect to one type of tissue. It is doubtful that sucrose would be ab- 

 sorbed from near-isotonic solutions sufficiently to be troublesome. 



When equilibrium values were determined for dead tissues, there was no 

 great difference between the two kinds of beets mentioned above. How- 

 ever, in seven out of a total of eight tests, the expressed sap was from 0.4 

 to 1.7 atm. more dilute than the external solution. 



Some equilibrium studies on certain marine algae have been reported. 

 For some materials, the expressed liquid may be considered almost pure 

 vacuolar sap. Mosebach (1936) so views the sap obtained from the swim- 

 bladder of Sargassum Unifolium, where the expressed sap amounted to 



