Grafts et al. 



— 90 — 



Water in Plants 



a 



b 



Fig. 27. — Vacuole contraction as shown 

 by leaf cells of Elodea canadensis after 

 staining with neutral red. Unplasmolyzed 

 cell, a; plasmolyzed, b. Redrawn from 

 Strugger (1935). 



whose volume may be determined with fair accuracy are suitable. Large 

 cells with a low protoplasm/vacuole ratio are recommended. The possi- 

 bility of injury and subsequent changes due to strong plasmolysis must 



be considered. Levitt and Scarth 

 (1936) used the plasmometric method 

 to investigate the bound water content 

 of hardened and unhardened cells. It 

 is a preferred procedure in permeabil- 

 ity studies, but may not necessarily in- 

 dicate permeability in the normal state. 



Gryoscopy : — The cryoscopic meth- 

 od has been widely used for indirect 

 measurement of osmotic pressure in 

 plants. It involves the determination 

 of the freezing point of expressed cell 

 sap, the osmotic pressure being directly 

 proportional to the freezing point low- 

 ering. The important investigations 

 of Dixon and Atkins (1913) in Eng- 

 land, Walter (1931^) in Germany, 

 and Harris (1934) in this country utilized this method and have con- 

 tributed to its improvement. 



The following advantages over the plasmolytic method may be noted: 1) measure- 

 ments may be made at normal cell volume; 2) no errors due to change in permeability 

 are encountered; 3) changes resulting from injury due to shock are avoided; 4) deter- 

 mination of average OP values of whole tissues or organs is possible; 5) freezing 

 point lowerings may be determined with a high degree of accuracy. 



Possible errors and limitations of the method include : 1 ) inherent difficulties con- 

 nected with expression of plant saps. Small samples from a single tissue type are more 

 significant than large mixed samples. With any sample, dilution of the vacuolar sap 

 by water liberated by alteration in physical organization of colloidal matter may con- 

 stitute a real limitation (Newton, Brown, and Martin, 1926; Meyer, 1928; Jaccard 

 and Frey-Wyssling, 1934 ; Roberts and Styles, 1939 ; Kerr and Anderson, 1944 ; 

 Currier, 1944a). Walter and Weismann (1935) consider this not to be a source of 

 error. — 2) If killing is accomplished by heat, water loss from the sample may result in 

 too great a depression. — 3) Chemical changes such as hydrolysis, mostly enzymatic in 

 nature, may increase the amount of osmotically active solutes. Condensation reactions 

 or production of volatile substances might lower the osmotic concentration. — i) Con- 

 tamination of sap by contents of vascular elements is a possible source of error. — 5) 

 Use of different pressures for expression of sap may give discordant results because 

 varying amounts of water of imbibition from protoplasm and walls are included in the 

 samples. — 6) Filtration, and adsorption of solutes by dead cellular membranes during 

 pressing may result in lower values. There is evidence to the contrary {e.g. Thren, 

 1934) but more information is needed. — 7) The press metal may have a catalytic action 

 on the sap. 



When these errors are all reduced to a minimum, valuable data can be obtained by 

 the cryoscopic method. In spite of its drawbacks it is the most practical method for 

 measuring the osmotic pressure of bulk tissue. 



Expression of Sap: — Dixon and Atkins (1913) recommended that sap be ex- 

 pressed from tissues that have been frozen ; if the tissue is not killed in some manner 

 filtration of solutes by the living cell membranes may lead to low values. The succes- 

 sive fractions of expressed sap from untreated tissue show a progressive increase in 

 OP. GoRTNER, Lawrence, and Harris (1916) showed that while this is true for 

 most plants, for some the concentration remains constant. 



Other means of rendering the cell membranes permeable prior to sap expression 

 are: grinding (Hibbard and Harrington, 1913; Newton, Brown, and Martin, 1926; 

 Sayre and Morris, 1932) ; and exposure to toxic vapors (Chibnall, 1923; Goldsmith 

 and Smith, 1926). Heating the samples (Thren, 1934; Doneen, 1934; Mallory, 



