mar., 1921] knudson and ginsburg — osmotic pressure 169 
Remarks 
As pointed out previously, Harvey (5) noted that the method of freezing 
the tissue affected the concentration of the sap. Assuming that the same 
pressure was used by him in expressing the sap from leaves frozen by the 
different methods, it follows that the investigator must exercise some 
discretion in the method employed for freezing the tissue. 
It seems to the writers, however, that the amount of pressure applied is 
more important than the method of freezing. What amount of pressure 
should be used would depend upon the character of the plant tissue under 
investigation. It would seem desirable to use that pressure which would 
yield a sap of the greatest concentration, and the investigator should state 
specifically the pressure employed. Furthermore, if tissues of unlike 
character are under investigation, as, for example, parenchymatous and 
woody tissues, it would seem that a greater pressure would be needed for 
the woody tissue than for the parenchymatous tissue in order to give com- 
parable results. In other words, with a parenchymatous tissue a pressure 
of 10,000 pounds might yield a sap containing 95 percent of the solutes of 
the original sap, while for a woody tissue the same percentage of solutes 
might be obtained only with a pressure of 50,000 pounds. It seems to the 
writers, therefore, that in any investigation involving the determination of 
osmotic pressures of plant tissues, preliminary experiments are essential 
in order to determine the most desirable methods of freezing and the amount 
of pressure to be applied. 
Summary 
1. A piston-cylinder apparatus is described for use in the expression of 
cell sap. 
2. Recommendation is made with respect to the use of standard ma- 
terials-testing machinery where definite pressures are available. 
3. Experiments show that a pressure of 50,000 pounds yields a more 
concentrated sap than a pressure of 10,000 pounds. 
4. No great differences were found in the concentration of the sap ex- 
pressed from leaves frozen with liquid air or with an ice-salt mixture. 
5. Considerable differences were observed between the osmotic pressure 
as determined by the plasmolytic and by the cryoscopic methods. 
Laboratory of Plant Physiology, 
Cornell University, Ithaca, New York 
BIBLIOGRAPHY 
1. Davis, W. A., Daish A. J., and Sawyer, G. C. Studies of the formation and trans- 
location of carbohydrates in plants. Jour. Agr. Sci. 7: 255-314. 1916. 
2. Dixon, H. H. Transpiration and the ascent of sap in plants. 1914. 
3. Findlay, A. Osmotic pressure. 1919. 
4. Gortner, R. A., and Harris, J. A. Notes on technique of determinations of freezing 
points of vegetable saps. Plant World 17: 49-53. 19 14. 
