Dixon and Atkins — On Osmotic Pressure in Plants, c^c. 277 



In 1907 E. and H. Drabble^ investigated the osmotic pressure of the cells 

 of many plants, mostly herbaoeons, by the plasmolytic method. They 

 obtained results varying from 4.45-19.68 atm. In addition to many 

 interesting observations, they claim that their results show in general that 

 physiological drought is the principal determining factor in the osmotic 

 pressure of plants, and they hold that the plasmolytic method is, with certain 

 precautions, reliable. 



By quite a different method one of us carried out a number of observa- 

 tions on tiie osmotic pressures in leaves. The metliod consisted in balancing 

 the internal osmotic pressure against high external gas-pressures.- The 

 danger attending the observations render the method unsuitable for a 

 detailed investigation of a large number of leaves under varied conditions. 

 The results, however, which were obtained by this method were consistent, 

 and indicated that the osmotic pressures in the leaves are at least as great as 

 the tensions developed in the sap. On the whole, the results obtained at that 

 time were a good deal higher than the generally accepted values for leaves 

 which were based on plasmolytic observations ; but, as will appear later, these 

 gas- pressure results have received a general confirmation in the present 

 research. 



Sutherst,^ by squeezing the juice out of certain leaves, showed that the 

 sap so obtained has a freezing-point considerably below 0° 0. This author's 

 work was designed to explain why leaves do not freeze when exposed to 

 temperatures even below 0° 0. Sutherst's results were obtained with easily 

 expressed saps — e.g., celery, cabbage, and carrot, etc., amounting to seven in all. 



Beyond stating that he used a small quantity — viz., 5 c.c. — and a 

 freezing-mixture of Glauber's salts and hydrochloric acid, Sutherst did not 

 describe what precautions he took in making these determinations : conse- 

 quently his results must be looked upon as of a preliminary nature. 



The determination of the freezing-point affords a most convenient method 

 of measuring the osmotic pressure, which is constantly made use of in all 

 kinds of physical and physiological work. Livingston* used Sutherst's 

 determinations, and calculated the osmotic pressures they indicated. 



With suitable apparatus and proper precautions the freezing-points of 

 various solutions may be determined with great accuracy. From this the 

 osmotic pressure of the solution may be deduced, inasmuch as the depres- 



'E. and H. Drabble, The Kektion between the Osmotic Strength of Cell Sap in Plants and their 

 Physical Environment. Biochemical Journal, 1907, vol. ii., p. 11? et seq. 



^ H. H. Dixon, On the Osmotic Pressure in the Cells of Leaves. Proc. Eoy. Irish Acad., vol. iv., 

 ser. 3, 1S96, p. 61, and On the Physics of the Transpiration Current, Notes irom the Botanical 

 School, Trinity College, Dublin, No. 2, 1897, pp. 81 and 82. 



SW. F. Sutherst, Chemical News, 1901, p. 234. 



■■ B. E. Livingston, The Kole of Diffusion and Osmotic Pressure in Plants. Chicago, 1903. 



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