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XXVIII. 



OSMOTIC PRESSURES IN PLANTS. 



I. — Methods of Extracting Sap from Plant Organs. 



By henry H. DIXON, Sc.D., F.R.S., 

 University Professor of Botany, Trinity College, Dublin; 



AND 



W. R. G. ATKINS, M.A., A.I.O., 

 Assistant to the Professor of Botany, Trinity College, Dublin. 



[Eead December 17, 1912. Published February 8, 1913.] 



In several recent researches on the freezing-points of the sap of plants it has 

 been our practice (5, 10, 11, 12, 13), and that of several other investigators 

 (Sutherst, 21; Cavara, 6 and 7; Heald, 14; Nicolosi-Roncati, 19; 

 Trinchieri, 20; Marie and Gatin, 15), to press the sap from the living 

 untreated organ. The sap so obtained has been regarded as a fairly average 

 sample of the sap of the organ pressed. This seemed a reasonable view to 

 take, inasmuch as the pressures applied so completely crushed the cells of the 

 tissues tliat the sap expressed contained large quantities of protoplasmic 

 fragments, which in the case of green organs were particularly noticeable, 

 owing to the presence of chlorophyll corpuscles embedded in them. It 

 seemed allowable to assume that, where the component cells are so completely 

 disintegrated as is indicated by this observation, all the sap of their vacuoles 

 must be shed into the expressed fluid; or at least there would be no reason to 

 suspect a difference in composition between the latter and the sap which 

 remained behind in the organ. 



Fairly early in our work, however, we made observations which, in the 

 light of subsequent work, might have borne a different interpretation. For 

 example, when we exposed leaves to the vapour of chloroform, we found that 

 the sap was pressed out with much greater ease, and its freezing-point was 

 very much lower, than that of tlie sap coming from the untreated leaves. 



