L 445 ] 



XXXIV. 



OSMOTIC PEESSURES IN PLANTS. 



V. — Seasonal Variations in the Concentration of the Cell-Sap of 

 SOME Deciduous and Evergreen Trees. 



By henry H. DIXON, Sc.D. (Dubl.). F.R.S., 



University Professor of Botany, Trinity College, Dublin, 



and 



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



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



[Eead January 26. Published March 26, 1915.] 



A knowledge of the concentration of the cell-sap of plants is essential for 

 the solution of several fundamental problems of vegetable physiology. 

 Several years ago we (2, 3, 4) attempted to investigate the periodic changes 

 during the year in the concentration of the sap of the leaves of 

 Syringa vulgaris, Ilex aquifolium, and Hedera helix. At the time, in common 

 with other investigators, we experimented on sap pressed from the tissues 

 not subjected to any previous treatment. Subsequent investigation showed 

 us (6) that the sap tlius obtained is not so concentrated as that in the 

 vacuoles of the uninjured cells, and that the unaltered sap may be obtained 

 by pressing tissues previously killed by freezing. This discovery rendered 

 necessary the revision of our previous work on the seasonal variations in the 

 concentration of the sap ; and in the present paper the results of this revision 

 are recorded, together with certain additions. 



Method. 

 The sap for freezing-point determinations (1, 5) and conductivity 

 measurements was pressed from tissues previously frozen in liquid air. 

 The depression of freezing-point A is defined by the concentration of all the 

 dissolved substances, and hence gives a measure of the total osmotic pressure. 

 These depressions and the calculated pressures in atmospheres are recorded 

 in columns under A and P in the tables, and in the ordinates of the graphs. 

 Under C are given the conductivity measurements in mhos. These are 

 proportional to the quantities of electrolytes dissolved in the sap. From 

 these figures are calculated those under Ae, the depressions of freezing- 

 point of solutions of potassium chloride having the same conductivity as 

 that observed for the sap. Under A - A^ are the depressions due to the 



SCIENT. PROC. R.D.S., VOL. XIV., NO. XXXIV. 4 B 



DEC 14 1915 ] 



