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XVIII. 



VAEIATIONS IN THE OSMOTIC PEESSUEE OF THE SAP OF 



ILEX AQUIFOLIUM. 



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

 University Professor of Botany, Trinity College, Dublin ; 



AND 



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



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

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



Previous observations have brought to light considerable fluctuations in the 

 osmotic pressure of the sap of the leaves of plants during short intervals of 

 time. Further, it has been shown that, other conditions being equal, a 

 difference is observable between the osmotic pressure of the sap of leaves of 

 different ages.' The present research was undertaken to ascertain if there 

 was a periodic fluctuation in the osmotic pressure corresponding with seasonal 

 changes. 



The materials selected for the investigation were the leaves of two 

 evergreens Ilex Aquifolium and Hedera Helix, so that a continuous series of 

 observations might be made throughout the year. In the case of Ilex 

 Aquifolium the leaves examined were growing in the open in Trinity College 

 Botanic Garden, all taken from the same individual, and from off the upper 

 branches, 3-4 metres from the ground, so that they were exposed to as 

 uniform external conditions as possible. The leaves were gathered for 

 each experiment between 9 and 10 a.m. For the sake of comparison 

 measurements were also made on the sap of the roots. 



The observations on Hedera Helix are recorded in a subsequent note. 



Former observations had sliown that the sap from the young leaves of 

 various plants has as a rule a smaller depression of freezing-point than 

 that of the mature leaves. Experiments on Ilex showed that the same 

 rule held good in its case ; for usually the young leaves of this plant have 

 a sap with a smaller depression, and consequently a lower osmotic pressure 

 than that of the mature ones. 



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

 Pointa." Dixon and Atkins. Sci. Proe. Roy. DuW. Soc, vol. xii, pp. 275, et seq. 



SCIENT. PEOC. K.D.S., VOL. XIII., NO. XVIII. 2n 



