7.74 



Proceedings of the Royal Irish Academy. 



on to the upper end of a fine glass tube (see diagram). The porous 

 pot is enclosed in a bladder B, the narrow opening of which is bound 

 closely round the glass tube below the pot, so as to be water-tight. 

 The pot and tube are filled with water and the lower end of the latter 

 dips into a vessel of water, while the space between the bladder and 

 the pot is filled with a solution of potassium nitrate. It is evident that 

 in such an arrangement the potassium nitrate in the bladder will attract 

 water from the inside of the pot, and consequently give rise to an 

 upward motion of the water in the tube. After 

 some time the bladder will become distended 

 and tense and water will diffuse through its 

 wall and evaporate into the surrounding space. 

 This evaporation will maintain a certain con- 

 centration of the solution in the bladder, so 

 that a state of equilibrium will be attained in 

 which the amount of water evaporated from 

 the surface of the bladder will equal the 

 amount which passes from the pot into the 

 solution, and at the same time the bladder 

 will remain distended or turgescent. The 

 transference of water across the bladder will 

 be due to the difference of the state of satura- 

 tion obtaining inside the porous vessel at the 

 top of the tube from that obtaining outside the 

 bladder. We may suppose that a certain num- 

 ber of water molecules pass from the pot through the semipermeable 

 membrane into the bladder and a certain number back from the 

 bladder into the pot, but that the latter number will be less than the 

 former owing to the attraction of the potassium nitrate for water; 

 similarly at the outside of the bladder a certain number of water 

 molecules are leaving the bladder and a certain number are passing 

 from the surrounding water-vapour into it, and when evaporation is 

 taking place the former number will be greater than the latter. This 

 constant loss of water from the outer surface of the bladder would be 

 competent to set up a tension in the water in the tube if the latter 

 contained no free gas in it, and thus a constant current upwards in the 

 tube might be maintained even if the reservoir into which its lower 

 extremity dipped were cut off from atmospheric pressure. In this 

 experiment it is seen that the porous pot and tube, which correspond 

 to the upper portions of the tracheidal system of plants, must be able 

 to withstand the tendency to collapse due to the weight of the long 



