Chapter VIII 



123 — 



Active Relations 



external solution, increasing the osmotic pressure of the vacuolar sap. At 

 equilibrium there would be a balance with respect to DPD, but not to OP, 

 and the cell will exhibit some turgor. 



Experimentally, the vacuolar concentration of contiguous cells may be 

 assumed equal to that in the cells under observation. Providing sap were 

 expressed from the tissue in the condition of limiting plasmolysis, and pro- 

 viding the section to be immersed in the sap is also in this condition, fifty 

 per cent of the cells should plasmolyze. It is necessary that the volume of 

 external juice be large enough to preclude error due to dilution by water 

 leaving the cells. 



There are few reports in the literature of such a procedure. To show 

 that sap expressed from living tissue is not isotonic with respect to living 

 cells, Dixon (1914, p. 184) mounted sections of beet root and leaves of 

 Chamaerops humilis in whole sap expressed from frozen tissue. Absence 

 of plasmolysis under these conditions was taken to substantiate the claim 

 that freezing had no concentrating effect on the cell sap. Sap from the 

 living tissue gave a freezing point depression of 0.599° C, while that from 

 frozen tissue was 1.517°. 



Bennet-Clark, Greenwood and Barker (1936) made use of this 

 procedure as an argument for water secretion. Cells of plants which dis- 

 played a marked discrepancy between plasmolytic and cryoscopic OP 

 values, failed to plasmolyze in sap expressed from frozen contiguous tis- 

 sue, while those in which the two measurements agreed well were approxi- 

 mately 50 per cent plasmolyzed. 



That there may be different responses to this type of treatment in rela- 

 tion to the magnitude of the plasmolytic-cryoscopic discrepancy (PCD) 

 was suggested by tests on red beet root (Currier, 1944a). It was shown 

 that where the plasmolytic OP was the higher, fewer cells mounted in ex- 

 pressed sap became plasmolyzed than where the reverse was true (Table 

 39). 



Table 39. — Plasmolysis of beet cells in their ozvn sap: — 



Although the above investigators make no mention of any injurious 

 effect, this type of experiment may not be valid because of the alleged toxic 

 action of expressed sap on living cells (Prat, 1927; Ernest, 1935). Ex- 

 pressed beet root sap is reported to markedly increase the respiratory ac- 

 tivity of discs of tissue immersed in it (Bennet-Clark and Bexon, 1943). 

 Diluting, boiling and filtering, and sterilizing the juice failed to significantly 

 retard the effect. The active constituents present were found to be organic 

 acids, malic especially. Just how an increased respiration would affect the 

 plasmolytic response is not known. The malate effect resembles the ability 



