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



reading was considered as the freezing point of the pulp tested. Correction for under- 



ol j ng has een applied, since the undercooling was always the same. Since, 



illy emphasized by Shive 14 the external air temperature exercu 



■ 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 



I out with approximately the same temperature of the surrounding air as pre- 



Iled during the determination of the freezing point of distilled water used for 



>n wiili that of pulp or juice. The simplest way to avoid possible sources of 



n 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 



l,,v 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 pres- 

 sure depends is approximately measured by its freezing point lowering, and this 

 property may he expressed in terms of pressure. Thus, according to the formula of 

 Lewis, 15 11=12. 06A— 0.021 A, 2 where His the maximum osmotic pressure, in atmos- 

 pheres, at the freezing point of the solutions, and A is the lowering of the freezing 

 point in centigrade degrees, below that of distilled water. With the aid of this formula 

 Harris and Gortner 16 have prepared a table of the values of II for the range, A =0.001° 

 ( '. 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 sett lei 1 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° < '. 

 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. Carbon 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 before sam- 

 pling, since as would he 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 m mixing, two or more samples of the same pulp do not have the same osmo' 

 concentration. 



nil pie- are placed in the freezing tubes and allowed to thaw completely before 



the determination of the freezing point is made. When the tissue is ground and 



II iefore the preliminary freezing many serious changes may take pla 



• he liberated, many new chemical reactions may be brought about, and the 



solutions may change in various physical ways. After thawing, the material should 



ve, J W The freezing-points of Tottingham's nutrient solution. Plant World 17 14. 



Lewis, G. N., The osmotic pressure of concentrated solutions and the laws of tl olution. 



Jour. Amer. Chem. Soe. 30: 668-683, 1908. 



t a hi n l rriS V T ' A r d , R " A " G0rtner ' Noteson the calculation of the osmotic pi j of expressed vege- 



^P sfro ^ *e depression of the freezing point, with a tal the values of II for 



■ ' < ■ Am. Jour. Bot. 1: 75-78, 1914. 



taS rr !^ttn;n n r eUSi0nt0 ( 5 f 9 ° C,onalr,,> '" determi ™ 0» osmoti ot expressed v. 



table sapsjfram the d »n of the freezing point. Amer. Jour. Bot. 2: 418 119,1915. 



