84 SUN AND WIND. [CH. IV 



longer ^ When no readings are being taken the bent free 

 end of h dips into the vessel of water v ; when v is removed 

 a column of air enters and travels along h, its rate of 

 movement being noted by timing it over intervals of 5 or 

 10 mm. For this purpose h may be graduated, or a 

 millimeter scale may be set up behind it. When the 

 column of air has nearly reached h it can be brought to 

 the zero of the scale by opening the clip and allowing the 

 water from the funnel /to drive it back along h. 



(94) Sunshine. 



Take a branch of Portugal laurel (Priuius lusitamca) 

 which has been cut and placed in water for at least 6 

 hours. This precaution is necessary to satisfy the negative 

 pressure in the branch ; if this is not effected, variations in 

 the rate of absorption will by no means represent variations 

 in rate of transpiration. Fit up the branch in the poto- 

 meter- and take readings until the rate of absorption is 

 fairly constant. Then place the plant in sunshine and 

 observe the increased rate of absorption, and finally replace 

 it in shade. 



(95) Wind. 



When the rate is once more steady, open a door 

 and a window so that the plant is exposed to a draught. 

 The rate of absorption may easily increase by 50 per cent. 



1 The bore of the tube must be varied according to the size and 

 absorbing power of the specimen. In the figure, h is represented shorter 

 than it usually is. 



2 Kohl's apparatus will answer for any of the experiments for which 

 the potometer is recommended, and vice versa. 



