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ON THE OSMOTIC PRESSURE IN THE CELLS OF LEAVES. 

 By HENRY H. DIXON, B.A., Assistant to the Professor of 

 Botany, Trinity College, Dublin. 



[Read June 8, 1896.] 



[COMMTINICATED BT DE. E. P. WEIGHT.] 



In a Paper in the Proceedings of this Acadeniy,^ I have advocated the 

 view that the sap is drawn up in trees in a state of tension, and that 

 under normal conditions this tension is established by means of the 

 osmotic attraction of the cell-sap in the parenchymatous cells of the 

 leaf, exercised on the water in the upper terminations of the water 

 conduits. 



Accordingly, it seemed to me of interest to investigate the osmotic 

 pressures actually existing in the cells of the leaves of plants, in order 

 to discover if these pressures are sufficient to account for the raising of 

 the sap in the conduits by the attraction exercised by the solutions 

 which give rise to these pressures. 



Yarious methods have been adopted in estimating the osmotic pres- 

 sures in cells. The most usual is to immerse the cell or group of cells 

 to be investigated in solutions of varied concentration, and finding what 

 concentration is necessary to balance the attractive forces of the cell- 

 sap. This may be done by direct examination of the cells, which, 

 when the surrounding solution is too dilute, will expand ; because the 

 amount of water attracted into the more concentrated cell-sap will be 

 greater than the amount drawn from it into the surrounding liquid 

 which is more dilute. If, however, the surrounding solution is too 

 concentrated, more water is drawn from the cell-sap than it can attract 

 to itself, and consequently the vacuoles in the cells diminish in size. 

 This leads to a contraction of the protoplasm of the cell, leaving the 

 cell-wall as it contracts, till finally it will form a small ball lying 

 within the cell-wall. It is evident that when the concentration of the 

 surrounding solution is such that it neither causes extension nor plasmo- 

 lysis, the attractive forces of the solution are equal to the attractive 



\" Eole of Osmosis in Transpiration," vol. iii., ser. 3, p. 767, Jan., 1896. 



