206 



EXPEBIMENTS WITH PLANTS 



in Fig. 119, is now forced through an opening in the 

 stopper and the stopper is slowly forced into the neck 

 of the bottle, w^hich is filled to the brim with water. 

 Enough water will usually run into the tube during this 

 operation to fill it completely; should this not happen, 

 fill the tube with water before forcing the stopper 

 into the bottle and keep the water from running out 

 of the tube during the insertion by 

 stopping one end with the finger. To 

 test whether the stopper is tight, dry 

 the joints and blow at the free end of 



the tube to see 

 whether water 

 can be forced 

 out. If the 

 joints are 

 tight, set the 

 apparatus in 

 a cool, shady 

 place, and let 



it stand until a little of the water has been absorbed 

 by the leaf and a small bubble of air (one -fourth to 

 one-half an inch long) has been drawn into the end of 

 the tube. When this has occurred, dip the end of the 

 tube into water, as shown in the figure, attach a 

 ruler to the glass tube and, by means of a watch, time 

 the progress of the bubble along the tube (which 

 should rest in a horizontal position) . When the bub- 



119. Apparatus for measuring 

 the transpiration of a leaf 

 and the degree in which it 

 is affected by sunlight, 

 wind, rolling the leaf, etc. 



