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XIX. 



VAEIATIONS IN THE OSMOTIC PRESSUEE OF THE SAP OF 

 THE LEAVES OF HEDEBA HELIX. 



By HENRY H. DIXON, Sc.D., F.E.S., 

 University Professor of Botany in Trinity Oollege, Dublin, 



AND 



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



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



[Read Februauv 27. Published April 9, 1912.] 



Simultaneously with the observations on Ilex AquifoUum recorded in a 

 previous paper,' observations were made on the depression of freezing-point 

 of the sap of leaves of Hedem Helix. The leaves which supplied the sap 

 were taken from prostrate Hedera plants. Two series of measurements were 

 made, one on the sap from the leaves of plants growing in a north aspect, and 

 the other on the sap of leaves gathered from plants growing in a south 

 aspect. Those in the north aspect were so sheltered, that they at no time 

 were exposed to direct sunlight, while those in the south aspect were exposed 

 when the sky was clear to direct sunlight, except when the sun was near the 

 east and west horizon. This selection of plants for examination was made 

 in order to determine the effect of direct sunlight on the osmotic pressure. 

 It had been previously noticed^ in the case of deciduous plants, that exposure 

 to sunlight raised the osmotic pressure. The leaves for each experiment 

 were gathered between 9 and 10 a.m. 



As in the case of Ilex aquifol'mm, the age of the leaves was found to have 

 a marked influence on the osmotic pressure. This will appear from the 

 figures recorded in Tables I and IT, relating to north and south aspect leaves 

 respectively. 



' "Variations in the Osmotic Pressure of the Sap of Jto Aquifolmm.'^ Dixon and Atkins. 

 Sci. Proc. Eoy. Dubl. Soc, 1912, vol. xiii, No. xviii, p. 229. 



2 " On Osmotic Pressures in Plants, and on a Thermo-electric Methed of Determining Freezing- 

 Pointa." Dixon and Atkins. Sci. Proc. Roy. Dubl. Soc, 1910, vol. xii, pp. 275, et seq. 



SCLENT. PROC, E.D.S., VOL. XIII., NO. XIX. 2 P 



