PICTURESQUE EXPERIMENTS 213 



Porometer. Its essential part is shown in Fig. 7. 

 It consists of a funnel-shaped tube, having a broad 

 flange, which is cemented on to the stomata-bearing 

 surface of a leaf. The leaf is represented by the 

 obliquely shaded object and is enormously magni- 

 fied. To the upper orifice of the funnel is fixed a 

 rubber tube, and by means of it steady suction 

 can be supplied. The result is that a current of air 

 is drawn through the stomata into the leaf, and 



jO*M 11 ?2 1 2 



Fig. 8. 

 Curve of Porometer readings in light and darkness (black). 



then out of the leaf into the cavity of the poro- 

 meter. The rate of this current is an index of the 

 degree to which the stomata are open. With this 

 apparatus a number of interesting points can be 

 determined. 



Fig. 8 shows the effect of alternate periods of 

 light and darkness. The fall of the curve represents 



