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VI. 



OSMOTIC PRESSURES IN PLANTS. 



VI. — On the Composition of the Sap in the Conducting Tracts of 

 Trees at Different Levels and at Different Seasons of the 

 Tear. 



By HENRY H. DIXON, Sc.D. (Duel.), F.R.S., 

 University Professor of Botany, Trinity College, Dublin ; 



AND 



W. R. G. ATKINS, Sc.D. (Dubl.), F.I.C., 

 Assistant to the University Professor of Botany, TriDity College, Dublin. 



[Read December 21, 1915. Published March 21, 1916.] 



In a previous paper of this series (1) it was shown that the sap centrifuged 

 from the wood of trees always contains sugars and salts, the former being in 

 preponderating quantities as a rule. Furthermore, it was proved that the 

 sugars are present during early spring in larger amounts than at other times. 

 Certain views were also put forward regarding the functions of the living 

 elements of the wood, and root-pressure was explained with due regard to the 

 quantitative measurements of the osmotic pressure of the wood-sap. 



In the above-mentioned research attention was mainly focussed on the 

 constituents of the sap at constant levels in the various trees investigated. 

 The aim of the present paper is to study the composition of the sap at 

 different levels in the same tree, and to repeat the examination during the 

 seasons of the year upon closely similar trees. 



With this object nine trees were investigated — three of Acer mac- 

 rophyllum growing out of one old stump, and very much alike in their 

 dimensions ; two each of Ilex aquifolium and Cotoneaster frigida ; and one each 

 of Arbutus wnedo and Ulmus campestris. Thus the list includes the evergreens 

 Ilex and Arbutus, the sub-evergreen Cotoneaster frigida, and two typical 

 deciduous trees, Acer and Ulmus. It would be preferable to include a larger 

 variety of trees, and a greater number of each species ; but up to the present 

 we have had opportunities of investigating only a limited number. 



As in former papers, A denotes the depression of freezing-point of the 

 sap as ascertained by the thermo-electric method of eryoscopy, and P the 

 osmotic pressure in atmospheres, calculated from A. Under C x 10 5 are given 



SCIENT. PKOC. R.D.S., VOL. XV., NO. VI. I 



