Grafts et al. 



86 — 



Water in Plants 



difficulties. After 30 minutes (or longer, if necessary) the sections are examined 

 microscopically for evidence of plasmolysis. The concentration range of the solutions 

 should be chosen so that no plasmolysis occurs in the more dilute while the highest 

 causes plasmolysis of all cells. Between the two extremes, sections will show plas- 

 molysis of some cells, but not of others. By carefully scrutinizing 100 cells of each 

 section under high power, and plotting the number of plasmolyzed cells against 

 the sugar concentration, a curve may be drawn from which the concentration effecting 

 plasmolysis of 50 per cent of the cells may be found. Figure 21 shows such curves. 

 This concentration value is taken as the Og value for the section. It should be clear 

 that it is an average value since each cell behaves somewhat differently. The degree 

 of precision obtainable by this method can be of the order of ± 0.1 atmosphere. 



The assignment of OP values to 

 sucrose solutions may be based on: 

 1) osmometric measurements (Morse, 

 1914; Berkeley and Hartley, 1916; 

 Frazer and Myrick, 1916) ; 2) vapor 

 pressure measurements (Berkeley, 

 Hartley, and Burton, 1919) ; 3) 

 freezing point data (Int. Crit. Tables, 

 1933) ; or 4) formulas corrected for 

 the anomalous behavior of aqueous su- 

 crose solutions (see discussion in 

 Chapter IV). Study of data from 

 these four sources shows that the va- 

 por pressure data most nearly agree 

 with the directly measured values. 

 Freezing point data are a close sec- 

 ond ; values calculated from concen- 

 trations require correcting, particu- 

 larly above 1 molal. At concentrations 

 below this, no great error is intro- 

 duced by any of these methods. Phys- 

 iologists owe a great debt of gratitude 

 to the workers listed above who pains- 

 takingly determined by direct meth- 

 ods highly accurate values for the os- 

 motic pressures of sucrose and other 

 solutions over so wide a range of con- 

 centrations. Tables of osmotic pres- 

 sure values of volume molar sucrose 

 solutions at 20° C. may be found in 

 MoLZ (1926) and Ursprung (1938, page 1275). It may be concluded from Morse's 

 work that while sucrose solutions do not obey the ideal gas law equation with respect 

 to concentration, they do for temperature, at least from 0° C. to 25° C. The correction 



293.1 _ „ 



factor for plasmolytic experiments carried out at 20 C. amounts to ^yj^ — l.U/J. 



De Vries in 1888 proposed the name isotonic coefficient to describe the relative 

 molar amounts of substances forming isotonic solutions. Thus a molal solution of 

 KNO3 exhibits a greater osmotic pressure than a molal solution of sucrose. This 



Table 24. — Isotonic coefficients of various salts calculated on a zvcight molar basis. 

 Sucrose = 1.00 {data of Fitting, 1917) : — 



O.Z 



o.d 0.4 o.s o.c 



M0I3 6ucrosQ 



0-7 



Fig. 21. — The relation between con- 

 centration of plasmolyzing solution and 

 degree of plasmolysis. Curves showing 

 the 50 per cent value. 



Salts to which Rhoeo ct:lls are 

 relatively permeable 



Salts to which Rhoeo c^ells are 

 relatively impermeable 



Salt 



By plasmolysis By cryoscopy 



Salt 



By plasmolysis By cryoscopy 



KNO3 1-69 



KCl 1.74 



KBr 1.14-5 



KCIO2 1.73 



K2SO4 2.27 



NaNOs 1.7 



NaCl 1.71 



LiNOs 1.83 (?) 



LiCl 1.8 



1.78 MgS04 105 



1.84 Mg(N03)2 2.54 



1.84 Ca(N03)2 2.43 



1.75 Ba(N03)2 2.23 



2.36 BaClj 2.42 



1.80-2 MgCl> 2.49 



1.84 Sr(N03)2 2.35 



1.77-8 SrCl2 2.49 



1.85-7 CaClo 2.46 



1.1 



2.SS 



2.4 



2.22 



2.46 



2.64 



2.43 (?) 



2.55 



2.59 



