108 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



final reading then taken. This reading was considered as the freezing point of 

 the pulp tested. Correction for undercooling has not been applied, since the 

 undercooling was always the same. Since, as has been especially emphasized 

 by Shive, 14 the external air temperature exercises a marked influence on the 

 apparent depressions of the freezing point obtained by means of the Beckmann 

 apparatus, the freezing of the pulp or expressed juice must always be carried 

 out with aproximately the same temperature of the surrounding air as pre- 

 vailed during the determination of the freezing point of distilled water used 

 for comparison with that of pulp or juice. The simplest way to avoid possible 

 sources of error in this connection is to make a freezing-point determination on 

 distilled water for each external air temperature at which pulps or juices are 

 tested. Then the lowering for any test is considered as the difference between 

 its freezing point and that obtained on distilled water with the same room 

 temperature. 



The property of the solution upon which its maximum possible osmotic 

 pressure depends is approximately measured by its freezing point lowering. 

 and this property may be expressed in terms of pressure. Thus, according to 

 the formula of Lewis, 15 II=12.06A— 0.O21A 2 , where II is the maximum osmotic 

 pressure, in atmospheres, at the freezing point of the solutions, and A is the low- 

 ering of the freezing point in centigrade degrees, below that of distilled water. 

 With the aid of this formula Harris and Gortner 10 have prepared a table of 

 the values of II for the range, A=0.001° C. to A=5.999° C. This table has been 

 employed in our deductions. 



At the beginning of the work the material to be used was first ground and 

 then frozen, but it was difficult to prevent some loss of sap in this way, and 

 difficulty was also encountered in getting a perfect mixture of the material after 

 thawing, since much of the sap had left the cells on grinding and had settled 

 to the bottom of the mass. Consequently, it was found better first to freeze 

 the material and to grind it afterwards. In the earlier tests this preliminary 

 freezing was carried out in large test tubes immersed in a mixture of salt and 

 ice at a temperature of from —12° to — 17° C. Sometimes during cold weather 

 the material was placed out of doors overnight for the preliminary freezing. 

 In the remainder of the work it was frozen by carbon dioxide and ether. Car- 

 bon dioxide was obtained in the solid state by allowing the compressed gas to 

 escape from the supply cylinder into a small cloth bag. The material to be 

 frozen was placed in a beaker and completely covered with solid carbon dioxide. 

 A small amount of ether was then added, until complete freezing had taken 

 place. A temperature of approximately — 120° C. may be obtained in this way. 



The tissue is reduced to a finely divided condition by grating or grinding in a 

 food grinder. The ground material must be quickly and thoroughly mixed lie- 

 fore sampling, since as would be expected, and as has indeed been found by 

 other investigators, not all parts of a given organ give the same concentration 

 of sap. Unless great care is taken in mixing, two or more samples of the same 

 pulp do not have the same osmotic concentration. 



Samples are placed in the freezing tubes and allowed to thaw completely be- 

 fore the determination of the freezing point is made. When the tissue is 



M Shive, J. W-, The freezing-points of Tottingham's nutrient solutions. Plant World 

 17 ; 345-S53, 1914. 



* 15 Lewis, G. N., The osmotic pressure of concentrated solutions and the laws of the 

 perfect solution. Jour, Amer. Chem. Soc. 30 : 668-683, 1908. 



16 Harris, J. A. and R. A. Gortner. Notes on the calculation of the osmotic pressures 

 of expressed vegetable saps from the depression of the freezing point, with a table for. the 

 values of II for A=0.001° C. to A=2.999° C. Am. Jour. Bot. 1 : 75-78, 1914. 



Harris, J. A., An extension to 5.999° C. of tables to determine the osmotic pressures of 

 expressed vegetable saps from the depression of the freezing point. Amer. Jour. Bot. 

 2 : 418-419, 1915. 



