Crafts et al. — 122 — Water in Plants 



of bound water in the summer leaves, attributed to a somewhat greater 

 total water content. Nor could a clearly significant difference be shown by 

 pressure-dehydration tests on living (ground) leaves in the summer as 

 compared to the winter condition. Amounts of sap expressible were greater 

 from summer leaves, but a part of this may be accounted for by the greater 

 total hydration of those leaves. The disrupting effects of grinding should 

 not be overlooked. Difficulties met in interpretation of bound water data 

 obtained by such methods as the calorimetric are indicated in Chapter III 

 (see also Weismann, 1938). Levitt (1941) interprets such results as 

 obtained by the pressure method, employed also by Newton (loc. cit.) and 

 by Martin (1927), as a reflection of the degree of injury suffered by the 

 protoplastic membranes from the application of pressure. Yet the behavior 

 of living leaves in freezing and pressing studies must reflect properties 

 other than a variable resistance of membranes. If in the hardened condi- 

 tion the permeability of the plasma membranes is increased, as indicated by 

 studies in frost resistance, one should expect that expression of sap from 

 living tissue should proceed more easily than from unhardened leaves. 

 This is not true, as Newton and others have shown. 



The difference in osmotic pressure between sap expressed from living 

 and dead (frozen and thawed) beet root was found to be 0.1 atm. for tissue 

 exhibiting a higher plasmolytic than cryoscopic value. For another beet 

 root where the cryoscopic value was the higher, the difference was 3.1 atm. 

 (Table 38). Sap in the first instance was believed to have been diluted by 

 protoplasmic water to a greater extent than was true for the latter. 



Table 38. — Osmotic pressure of sap pressed from liz'ing as against dead (frosen) 

 tissue, comparing two types of beets. Values in atmospheres at 22.5° C. 



(data of Currier, 1943) : — 



Consideration of the data and discussion presented in this section brings 

 out the following necessary conclusions : 



i) No method that is above criticism has been devised for obtaining pure vacuolar 

 sap. 



2) The unknown quantities are filtration of solutes, especially by the semi- 

 permeable membranes of living cells ; the diluting effect of coUoidally held water, 

 especially as may occur on freezing or otherwise killing the tissue followed by pressing ; 

 and other changes which might decrease or enhance the osmotic pressure, e.g., enzymatic 

 reactions. 



3) It would appear that plants vary markedly with respect to differences in con- 

 centration of saps expressed from living and dead tissue. 



4) The potential errors involved appear to be serious enough to throw doubt on 

 much of the mass of osmotic pressure data obtained by the cryoscopic method. 



Plasmolytic Behavior of Cells : — In conformity with the classical 

 view of plant water relations, a cell in the condition of limiting plasmolysis 

 immersed in a sap identical with that contained in its own vacuole would be 

 expected to maintain a condition of limiting plasmolysis. In the case of a 

 turgid cell such a result would not be expected for while the osmotic pres- 

 sures may be equivalent within and without the cell, the diffusion pressure 

 deficit is less within. Hence water would tend to move outward into the 



