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XVI. 



CHANGES IN THE OSMOTIC PRESSUIiE OF THE SAP OP 

 THE DEVELOPING LEAVES OF SYRINGA VULGARIS. 



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

 University Professor of Botany, Trinity College, Dublin ; 



AND 



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



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



[Read January 23. Published February 21, 1912.] 



Previous experiments in determining the osmotic pressures of the saps of 

 plants' showed that in one and the same plant the pressure may vary within 

 fairly wide limits. For example, the osmotic pressure of the sap of Syringa 

 vulgaris was found to vary between 26"87 atm. and 11'57 atm. In the cases 

 examined it was found that external conditions — especially those controlling 

 carbon assimilation, respiration, and the hydrolysis of carbohydrates — eifected 

 the most important changes in the osmotic pressure. 



In addition to the changes due to external conditions, it was found that 

 the age of the leaves from which the sap was taken also influences the osmotic 

 pressure — the younger leaves having a lower osmotic pressure than the more 

 mature. Observations of this rise in osmotic pressure with age have since 

 been extended to the leaves of some evergreens, viz.. Ilex AquifoUum and 

 Hedera Helix. 



The following experiments were carried out with the view of tracing the 

 changes in the osmotic pressures during the unfolding of the buds and the 

 maturing of the leaves. The buds and leaves of Syringa vulgaris were selected 

 for the work. All the samples used were taken from a group of plants growing 

 in a sheltered position in Trinity College Botanic Gardens. The osmotic 

 pressures, as in the previous work, were obtained from the freezing-points of 

 the saps determined by the thermoelectric method of cryoscopy^ already 



' H. H. Dixon and W. R. G. Atkins : " On Osmotic Pressure in Plants ; and on a Thermoelectric 

 Method of Determining Freezing-Points." Proc. R.D.S., vol. xii (N.S.), No. 25. 

 2 H. H. Dixon and W. R. G. Atkins, loc. cit. 



SOIENT. PEOC. E.D.S., VOL. XIII., NO. 3ail. 2 L 



