Chapter VII 



— 91 



Osmotic Quantities of Cells 



1934) gives satisfactory results, and in some cases may be preferred. Freezing is the 

 method commonly employed, and the use of liquid air or dry ice is recommended. 

 While for some plants an ice-salf bath at — 15° to — 20° C. is sufficient (Gortner and 

 Harris, 1914), for others such as the needles of Pinus rigida in the winter condition 

 the lower temperature of dry ice is required (Meyer, 1929). Broyer (1939) has re- 

 viewed methods of sap extraction. 9 



Hydraulic presses are commonly employed in connection with press cylinders of 

 various design. Figure 28 shows a semi-micro press cylinder useful in expressing sap 

 in volumes of 0.5 to 5.0 ml. In order to gain the desired precision, the magnitude of 



Top v/ev)/ of di\3C C 



Top view of baje D 



1 z d cm 



Fig. 28. — Press cylinder outfit used to express small amounts of sap from plant 

 tissue. From Currier (1943), modified after Broyer and Furnstal (1941). 



the pressure used should be known within narrow limits. This has special significance 

 in dealing with cells where the proportion of cell wall and protoplasm to vacuole is 

 high. The possibility of imbibed water diluting the expressed sap was considered by 

 MosEBACH (1936) to be a major error in cryoscopic investigations of certain brown 

 and red marine algae. Pressures should be expressed in common units, as pounds per 

 square inch, based on the area of the press cylinder. Pressures of 10,000 to 16,000 

 pounds per square inch are recommended. 



The Cryoscope: — Various cryoscope designs are available. The German workers 

 have preferred the Burian-Drucker (1909) apparatus, which accommodates 1.5 ml. 

 of sap, and the Drucker-Schreiner (1913) adapted for small volumes (about 0.005 

 ml.). The latter is less accurate and not as generally useful as the first. Figure 29 

 shows a semi-micro cryoscope accommodating 0.5 to 1.0 ml. of liquid. Mosebach 

 (1940) designed an apparatus which employs a fraction of a drop of sap obtained 

 from succulent tissue without pressing. The sap is frozen in a capillary tube, and the 

 melting point is accurately observed by means of a horizontal microscope. Where 

 larger amounts of sap are available (15-25 ml.) the standard method described in 

 manuals of physical chemistry is suitable. With this a differential thermometer of 

 the Beckman type, or a Haidenhain thermometer with a fixed zero point may be em- 

 ployed. 



All of the above apparatus depends upon a mercury thermometer for determination 

 of the freezing point. Another method employs the thermoelectric principle, and sev- 

 eral designs are available (Dixon, 1911; Herrick, 1934). In this procedure a fine 

 wire thermocouple consisting of two kinds of metal is inserted into the solution, an- 

 other is held in a reference liquid at constant temperature. The difference in tem- 

 perature causes current to flow through the circuit and this is measured by a sensitive 

 galvanometer. 



Where freezing points of "living" tissues are to be determined the thermocouple 

 has been preferred. However, because the thermocouple point requires rupture of 

 cells, whereby the needle is flooded with sap from a limited number of cells which 

 freezes ahead of the uninjured cells, Walter and Weismann (1935) claim that a 

 mercury thermometer inserted into a relatively dry cavity in the tissue gives a truer 

 freezing point value. Regardless of which method is employed, the data obtained in 

 the freezing of "living" tissues is difficult to interpret. This problem is further con- 

 sidered in Chapter VIII. 



