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A. C. Halket. 
the cell sap of plants. The great objection that can be brought 
against most of them is that a considerable quantity of liquid is 
required and this is difficult to obtain from many parts of plants. 
However, by one of the indirect methods mentioned above, utilising 
another physical property, osmotic pressures of cell sap have been 
determined. 
Freezing point method. In these cases the osmotic pressure 
of the cell-sap has been calculated from the amount the freezing 
point of the sap is depressed below that of pure water. The 
ordinary Beckmann’s thermometer generally used to measure these 
small differences in temperature is large and so necessitates the use 
of a considerable quantity of sap—10-c.c. to 20-c.c. A smaller form 
of thermometer requiring only 1‘5-c.c. of liquid has been described 
by Guye and Bogdan. 1 
Sutherst, 3 seeking to account for the fact that plants were not 
always injured by frost, found the depression of the freezing point 
of the sap of certain plants below that of pure water. He used 
somewhat watery plants, e.g., vegetable marrow, celery, cabbage, 
etc., reduced the tissues to a pulp by means of a grater and obtained 
the sap by filtering the mass. He found the freezing point using 
5-c.c. of the sap and a freezing mixture of Glauber salts and con¬ 
centrated hydrochloric acid. He did not, however, calculate the 
values of the osmotic pressures of the various saps. 
Atkins 3 gives a number of determinations of the values of the 
osmotic pressures of the sap of many fruits, tubers, etc., calculated 
from the depression of the freezing point found, using Beckmann’s 
thermometer. 
Cavara 4 has also made an extensive series of estimations of the 
osmotic pressure of the cell sap of the stems, leaves and fruits of 
many plants making the amount of the lowering of the freezing 
point the basis for calculating the osmotic pressures. 
Dixon and Atkins 5 have described a modification of this method 
1 “ M^thodes rapides pour l’analyse physico-chimique des liquids physio- 
logiques.” Journ. Chirn. Phys., 1903. 
2 “The Freezing Point of Vegetable Saps and Juices.” Chem. News 
Vol. 84, 1901, p. 234. 
3 “ Cryoscopic Determination of the Osmotic Pressures of Some Plant 
Organs.” Proc. Roy. Dub. Soc., Vol. 12, 1910; also Notes from the Bot. 
School Trim Coll. Dub., Vol. 2, 1910. 
4 “ Risultati di una serie di rceriehe crioscopiche sui vegetali.” Contrib. 
Biol. Veg., Vol. 4,1905, p. 41. (Abstract in Bot. Centr., Band 104, 1907, p. 547). 
6 “ On Osmotic Pressures in Plants, and on a Thermo-Electric Method of 
Determining Freezing Points.” Proc. Roy. Dub. Soc., Vol. 12, 1910; also in 
Notes from the Bot. School Trim Coll. Dub., Vol. 2, 1910. 
