Chapter VIII — 1 17 — Active Relations 



from the protoplasm ; that the discrepancy may be positive or negative de- 

 pending on whether the contamination was less or more concentrated than 

 the vacuolar sap ; that the nature of the contaminating protoplasmic sap 

 depends on the state of hydration of the cytoplasm ; and that where the dis- 

 crepancy is positive (protoplasmic sap less concentrated than vacuolar) the 

 cells may be in a cold hardy condition. The possibility of dilution by wall 

 imbibed liquid is not precluded; however, morphological differences (cell 

 size, wall thickness) sufficient to account for significant variation in PCD's 

 were not observed. 



While this monograph was in press a paper by Levitt (1947) appeared, 

 in which he throws doubt on the possibility of plant cells maintaining water 

 secretion pressures in excess of about one atmosphere. This conclusion is 

 based on calculations which indicate that there is insufficient energy re- 

 lease, judging from maximum observed respiratory rates, to make such 

 work thermodynamically possible. 



In summary, it is evident that with few exceptions, osmotic pressures 

 of plant cells measured plasmolytically exceed those determined cryoscopic- 

 ally, even when volume changes are taken into consideration. Explanation 

 of the discrepancy based on active secretion of water by the protoplasm 

 into the vacuole, adhesion of the cytoplasm to the wall, the presence of 

 colloids in the vacuole, various errors in method, and a high variable im- 

 bibitional capacity of the protoplasm, have been suggested. 



While admitting that both of these methods of measuring the OP of 

 plant tissues are beset with numerous errors, it is possible that where com- 

 parative values are secured from similar plants in different states of growth 

 many errors will cancel out. Additional data are needed. 



Interpretation of Sap Expression Data : — The meaning of results of 

 osmotic pressure measurements, where plasmolytic and cryoscopic methods 

 are compared, is confused by the questionable origin of expressed sap. If 

 plasmolytic values could be compared with osmotic pressure determinations 

 on sap known to have originated in the vacuole, the discrepancies, if any, 

 would have greater significance. 



Several methods have been devised in an attempt to obtain expressed 

 sap of known origin from plant tissue (cf. Chapter VI). Knowledge as to 

 the distribution of solutes and water between the protoplasm and vacuole 

 is greatly needed, for it would be of assistance in many kinds of physiologi- 

 cal investigations. 



The problem has been attacked by refining the pressing technique. The 

 usual method has been one in which frozen and thawed plant organs, or 

 parts of organs, are wrapped in cheesecloth and pressed, with no particular 

 attention to the arrangement of the individual pieces. Mason and Phillis 

 (1939) found that if cotton leaves were carefully stacked, and pressure 

 applied slowly so as to prevent shearing forces, a very dilute sap could be 

 obtained. They postulated that such sap was vacuolar in origin, and that 

 it was expressed through "fissures" produced in the protoplasm. On this 

 basis, in one experiment a difference between plasmolytic and cryoscopic 

 osmotic pressure determinations of 20.4 atm. was found (OP of plas- 

 molyzing solution = 22.8 atm., OP of press sap == 2.4 atm.). This led 

 them to suspect that the water secretion force was "much greater than any- 

 thing contemplated by Bennet-Clark, Greenwood and Barker." At 

 the same time they agreed that no actual evidence has been produced to 



