i 4 2 TRANSPIRATION AND ASCENT OF SAP ch 



force applied to the sap in the trachea? during normal 

 transpiration does not exceed the osmotic pressure in the 

 leaf-cells ; and consequently, if we can determine the 

 osmotic pressure in the leaf-cells we shall have a measure 

 of the maximum stress which is applied to the sap during 

 normal transpiration. 



Until recently the most usual way of determining the 

 osmotic pressure in cells was the well-known plasmolytic 

 method. 



There are several reasons why the application of this 

 method is not suitable to leaf-cells. In the first place, 

 it is necessary to cut sections of the leaf in order to apply 

 the solutions and to allow of microscopic observation. 

 The injury involved in sectioning acts as a violent stimulus 

 to the tissues, which may in itself evoke a change in the 

 concentration of the vacuoles or a contraction of the 

 protoplasm. Secondly, accurate determination of the 

 plasmolysing concentration is very difficult, as the con- 

 traction of the protoplasmic membrane must be consider- 

 able before it can be observed microscopically. 



It was owing to these objections that the plasmolytic 

 method was abandoned -and other means for estimating 

 the osmotic pressures in the cells of leaves were sought. 

 The first method devised was the following : 



Osmotic pressure balanced against gas-pres- 

 sure. A branch bearing a number of leaves is enclosed in a 

 strong glass cylinder, capable of resisting high gas-pressure 

 {e.g., 50 to 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 carbon 

 dioxide. 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 

 Fig. 24. 



It is evident that when the gas-pressure in the glass 

 vessel surrounding the branch is raised and maintained 



