i 4 o TRANSPIRATION AND ASCENT OF SAP ch. 



tension on the sap, and this tension is transmitted through 

 the leaf cells to the sap in the conducting tracts. 



Pressure and tension in leaf-cells. The simul- 

 taneous presence of pressure and tension within these 

 cells, at first sight, appears paradoxical ; but a moment's 

 consideration will show that it is quite possible for 

 the solvent, water, to be in a state of tension, i.e., at 

 a negative pressure, while the dissolved substances may 

 be at a positive pressure and be active as a distending 

 force in the cell. 



Although, by thus distinguishing the pressure con- 

 ditions of the solvent and of the dissolved substances, 

 it is easy to conceive how the water in a turgid cell may 

 be in a state of tension, it appeared of interest to show 

 experimentally in the following way that this peculiar 

 state of affairs is possible. 



It is well known that when a small piece is cut from 

 the young stem of an herbaceous plant, and immersed in 

 water, its curvature will show if its cells are distended by 

 osmotic pressure or not ; for the outer surface, being less 

 extensible, will become concave, if the cells of its tissues are 

 distended by osmotic pressure, and it will remain straight, or 

 become convex, in the absence of these pressures. If, then, 

 such a piece of tissue assumes and retains this concavity 

 when immersed in a tensile water column, we may be 

 assured that an osmotic pressure is exercised by the solute, 

 while at the same time the solvent is in a state of tension. 



The experiment may be carried out as follows : A long 

 piece of glass-tubing bent into a J -form is carefully cleaned 

 by washing with caustic potash solution, followed by 

 methylated spirit. Its upper end is then sealed, and it 

 is nearly filled with water which has been boiled for 

 some time. A piece of tissue cut from the stem of some 

 suitable plant (I used the peduncle of Doronicum 

 austriacum), after soaking for several hours in well- 

 boiled water, is introduced into the J -tube, and passed 



