Grafts et al. — 116 — Water in Plants 



(cryoscopic > plasmolytic). Nor was there visible evidence that the cyto- 

 plasm adhered to the wall. 



With one exception, the highest -|-PCD values were found in beets 

 yielding the most concentrated expressed sap (Lots D, E, G), the lowest 

 PCD's in beets with least concentrated saps (Lots B, C, F). Lot A, with 

 the highest average discrepancy, had a relatively dilute sap. 



Table 32. — Summarised results of plasmolytic and cryoscopic tests on various lots of 



beets: — 



" Cylinders of four roots of Lot F and of six roots of Lot G were soaked in isotonic sucrose 

 solution prior to sectioning, hence no volume change is involved. When these are not considered, 

 average changes in volume are somewhat higher, 3.3 and 4.4 per cent respectively. 



^ PCD is introduced as a convenient contraction of the term plasmolytic-cryoscopic discrepancy. 



The suggestion was made by Bennet-Clark, Greenwood and Barker 

 (1936) that water secretion may be a function only of potential growing 

 tissues. This does not appear to be true for beet root tissue. Lot F was 

 tested beginning when the plants were 3 weeks old, and continuing until 

 they were 25 weeks old. The youngest plant examined showed a PCD of 

 — 0.6 atm. Out of twenty determinations, nine showed negative PCD's 

 and an average discrepancy of near zero. During this growth period, both 

 Og and On increased at about an equal rate. 



Table 33. — Results of deplasmolysis and replasmolysis tests on beet root tissue. 



Atmospheres at 22.5° C: — 



Plants yielding the highest -j-PCD values (Lots A and D) were in a 

 prebolting or bolting stage of growth. These lots were fall planted, the 

 others spring planted. That temperature was not directly associated with 

 the discrepancy is shown by the fact that Lot A was subjected to winter 

 temperatures, while D grew under warm greenhouse conditions, yet both 

 yielded high PCD's. 



On the basis of this and other experimental evidence presented else- 

 where in this chapter, it was postulated that the discrepancies (PCD's) are 

 related to contamination of expressed sap by liquid held by and expressed 



