44 EXPERIMENTAL PLANT PHYSIOLOGY 



be dispensed with and the leaf stalk inserted through 

 the cork (Fig. 21). 



The amount of air in the bottle is now reduced by 

 drawing it out by suction through the right-angled 



tube, and then the end 

 is closed by means of 

 the clip. 



Observations. — Bub- 

 bles appear in the water, 

 in one case at the end of 

 the drawn-out tube, and 

 at the end of the petiole 

 in the other. The bub- 

 bles continue to come off 

 for a considerable time. 



Inferences. — Since the 

 evolution of the bubbles 

 continues for some time the air thus given off cannot 

 all have been inside the leaf at the start. The bubbles 

 must therefore be air which has come into the flask 

 from the atmosphere to take the place of that which 

 has been drawn out. This air must have passed through 

 the leaf, since no other route exists. 



Therefore it is possible for the air to pass into the 

 tissues of a plant. 



Fig. 21 



Experiment 25 



Aim. — To find out whether air passes into a leaf 

 equally through both the upper and under surfaces. 



Method. — Apparatus similar to that set up for the 

 last experiment is required. Two leaves of the same 

 kind are used. One of these is smeared with vaseline 

 on the upper surface, and the other on the lower. 



Observations. — -Air is easily drawn through the leaf 

 which is vaselined on the upper surface, but it is ex- 

 tremely difficult, and, in some cases, impossible, to get 

 any bubbles from the leaf which has been vaselined on 

 the lower surface. 



