MEASUREMENT OF TRANSPIRATION 



163 



which is shown in Fig. 39. The severed stem of a plant is immersed in water 

 in the reservoir of the potometer and the rate of water loss determined by 

 the rate at which the volume of water in the apparatus shrinks. This is 

 usually followed by noting the rate of movement of an air bubble in the water 



V_7 



Fig. 39. Potometer as set up for measurement of the rate of absorption of water. 

 The full length of the capillary tube is not shown. 



in the capillary sidearm of the instrument. Some potometers, as shown in the 

 figure, are so constructed that the entire root system of plants which have been 

 specially grown for the purpose in solution cultures can be immersed in the 

 reservoir of the instrument. A potometer actually measures the rate of ab- 

 sorption of water rather than the rate of transpiration. While under many 



