62 



Proceedings of the Royal Irish Academy. 



forces of the cell-sap ; and we may conclude that the pressure in the 

 cell, if it is freely supplied with water, is equal to the osmotic pressure 

 which this solution could exert. Such a solution is said to be isotonic 

 with cell-sap. Another way of determining when a solution is isotonic 

 with the cell-sap, and so finding the osmotic pressure exerted by the 

 cell-sap, is to observe tentatively what concentration is necessary in a 

 solution which will cause no alteration in form in a piece of turgescent 

 tissue. If the tissue expands in the solution, the latter is too dilute ; 

 if it contracts, the solution is too concentrated. 



By these methods various osmotic pressures have been determined, 

 3^ to 21 atmospheres in various tissues ; but, so far as I am aware, the 

 pressures obtaining in the tissues of the leaf have not yet been ascer- 

 tained. 



The method I have adopted in this research for estimating the 

 osmotic pressures existing in the leaves is the following : — A branch 

 bearing a number of leaves is enclosed in a strong glass cylinder, 

 capable of resisting high gas-pressure {e.g. 50-100 atmospheres), and 

 the pressure is raised in this vessel by means of an air compression- 

 pump, or by attaching it directly to a cylinder containing liquid CO2. 

 The lower portion of the branch projects from the cylinder and dips 

 into a glass vessel containing a weighed quantity of water. These 

 arrangements are shown in the above figure. 



