118 Scientific Proceedings, Royal Dublin Society. 



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 conditions of 

 the solvent and of the dissolved substances, it is easy to con- 

 ceive 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 ; its 

 outer surface, being less extensible, will become concave, if the 

 cells of its tissues are distended by osmotic pressure. 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, 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 wash- 

 ing 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 use the peduncle of 

 Boronicum austriaeum), after soaking for several hours in well- 

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

 upper end, where there is a small bend made to receive it. The 

 J -tube is now set in a vertical position, and its short limb is 

 connected with an air-pump. By the action of the pump the 

 atmospheric pressure is removed from the lower end of the column 

 of water in the tube, and the weight of the lower parts of this 

 column, hanging from the upper parts, puts them in tension. As 

 the piece of tissue occupies the top of the tube, the water in it 

 and around it is in a tensile state. It will be noticed that, 

 although exposed to this tension for a considerable time, the tissue 

 will retain its curvature, indicating, as we have seen, an osmotic 

 pressure in its cells. I have exposed a piece of the peduncle of 

 Doronicuni austriaeum to a tension of 50 cm. of water for two 



