Chapter VIII — 113 — Active Relations 



Studies dealing with growth processes in tomato have led Went (1944) 

 to admit the possibility of a non-osmotic force contributing to the DPD of 

 cells. Sap from killed leaves had an OP (determined cryoscopically ) of 

 slightly over 9 atm., while a sucrose solution of about 25 atm. was required 

 for plasmolysis of mesophyll cells. 



The presence of hydrophilic colloids in the vacuole has been advanced 

 as an explanation of discrepancies between the plasmolytic and cryoscopic 

 methods for determining osmotic pressures. For ten species of conifers 

 whose expressed cell sap was low in mucilaginous colloids Roberts and 

 Styles (1939) found fairly good agreement between the two methods; 

 the average discrepancy amounted to 2.6 atm. in favor of the plasmolytic 

 determination. On the other hand, plasmolytic values w^ere around 14 atm. 

 higher, or over 40 per cent of the DPD for seven species possessing much 

 colloid. The colloid content was measured by the somewhat questionable 

 method of comparing outflow times for sap through a capillary tube. Other 

 substances, for example sugars, also have marked effects upon viscosity and 

 further analyses of the saps would have aided in interpreting the data. 

 Although this method possibly involves an error, the fact that cryoscopic 

 values of the two groups of conifers were of the same order of magnitude 

 is evidence that colloids are probably responsible for the viscosity differ- 

 ences. While recognizing water secretion as a possibility, Roberts and 

 Styles favor the presence of colloids in the sap as an explanation for the 

 differences obtained. They believed that the plasmolytic method embraces, 

 in addition to osmotic pressure of the cell sap, swelling pressures developed 

 by vacuolar colloids which the cryoscopic procedure fails to measure. The 

 difficulty of distinguishing between osmotic and imbibitional forces in com- 

 plex solutions was emphasized in Chapter III. Although Roberts and 

 Styles did not make it clear, it should be emphasized that in order for the 

 above explanation to be valid, water must have been removed from the 

 colloid during preparation of sap for the cryoscopic determination. 



Using leaves of several tree species, Maximov and Lominadze ( 1916) 

 compared osmotic pressure values of pressed sap determined by the Barger- 

 Halket capillarimetric method with plasmolytic values. The latter values 

 averaged 1.7 atm. higher than those of the pressed sap and this discrepancy 

 accounts for a force representing about 8 per cent of the water absorbing 

 power (DPD) of the cells at limiting plasmolysis. Though one may ques- 

 tion the use of whole leaves for expression of sap while only epidermis is 

 used for plasmolytic measurements, the fact shown by Ursprung and Blum 

 (1916a), BuHMANN (1935), and others that the osmotic pressure of the 

 mesophyll exceeds that of the epidermis, indicates that the discrepancy 

 found was too low. 



Comparisons of the plasmolytic and Barger methods in studies on 

 Nitella, where volume changes were considered, gave almost identical values 

 (WiLDERVANCK, 1932). Similar results were obtained by comparing 

 plasmolytic and cryoscopic methods (Currier, 1943). It is perhaps sig- 

 nificant that in this case sap may be obtained without pressing, so that it 

 is almost identical with vacuolar sap in the intact cells. 



Oppenheimer (1932a) thought that good agreement between plas- 

 molytic and cryoscopic osmotic pressure values could be obtained if suitable 

 plant materials were employed, and when the various sources of error were 

 accounted for. He investigated twelve plants, employing mostly leaf tis- 

 sues. With a few exceptions his values agreed within 1.5 atm. In most 

 instances the plasmolytic value was the higher of the two. 



