CH. IV] SUN AND WIND. 77 



than that used in exp. 92, and also 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/ 

 Ao drive it back along h. 



(94) Sunshine. 



Take a branch of Portugal laurel" which has been 

 cut and placed in water for at least 6 hours. This pre- 

 caution 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 potometer 

 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. 



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

 absorbing power of the specimen. In the figure ft is represented shorter 

 than it actually is. 



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

 the potometer is recommended, and vice versa. 



