Chapter VIII — 1 15 — Active Relations 



discrepancies would probably have been greater had he used a mean plas- 

 molytic value for mesophyll and epidermis. 



A somewhat different interpretation was offered by Buhmann (1935). 

 Investigating various types of tissues, discrepancies varying from a frac- 

 tion of an atmosphere to as much as 60 atmospheres were found. Plas- 

 molytic values were, on the average, in the order of 5 to 7 atmospheres 

 higher than the cryoscopic. The discrepancies were ascribed to adhesion 

 of the cytoplasm to the cell wall, producing a delay, and hence an over- 

 estimation of the OP at incipient plasmolysis. For a discussion of the ad- 

 hesion problem, see Chapter VII. Figure 36 shows a series of measure- 

 ments on four tissues of Bergenia cordifolia leaves showing values of POg, 

 DOg, and cryoscopic On. In Finns laricio where, in one instance, the dis- 

 crepancy attained a value of 60 atm., Buhmann attributed 13.5 to 27 atm. 

 to adhesion. 



Using Beta vulgaris root, and neglecting change in volume, Buhmann 

 found discrepancies in the plasmolytic and cryoscopic values ranging from 

 5.25 to 7.78 atm., the plasmolytic values being higher. Adhesion pressure 

 amounted to 1.07 to 3.26 atm. An unusually high shrinkage (Vn/Vg = 

 1.22 to 1.36) was reported for this tissue which, when applied as a further 

 correction, produced good agreement between plasmolytic and cryoscopic 

 values. 



In a study of the water relations of red beet root tissue one of the pres- 

 ent authors (Currier, 1944a) verified the occurrence of plasmolytic- 

 cryoscopic discrepancies (PCD's) in measurements of osmotic pressure. 

 However, there were unexpected variations among different lots of beets 

 tested. After volume corrections had been applied, the value at limiting 

 plasmolysis exceeded the cryoscopic value by as much as 5.9 atm. for two 

 of the lots tested ; for other beets the average discrepancy did not exceed 

 one atmsphere. In some instances the cryoscopic value was the higher one, 

 but here the difference rarely exceeded a fraction of an atmosphere 

 (Tables 31 and 32). 



Table 31. — Description of beets employed in PCD determinations, and periods during 



zvhich tests zvere made: — 



Age Stage 



Lot Variety in weeks of growth Time of tests 



The excessive plasmolytic values could not be attributed to any of the 

 explanations previously offered. Using a technique similar to that of 

 Buhmann (1935), no significant adhesion pressure could be detected 

 (Table 33). The method involved the use of plasmolyzing and deplas- 

 molyzing solutions having osmotic pressures about 4 atm. greater and less 

 respectively than the predetermined limiting plasmolysis value, which in- 

 sured complete plasmolysis and deplasmolysis without stretching or shrink- 

 ing the wall to any great extent. Adhesion pressure values lay within 0.5 

 atm., and were no greater for beets exhibiting a -f-PCD (plasmolytic value 

 > cryoscopic value corrected for volume) than for those having a — PCD 



