THE FOOD OF THE PLANT 45 



Inferences. — Air is taken into the leaf chiefly, and, 

 in some cases, entirely, through the lower surface. 



The observations made in "the above experiment 

 recall the results obtained in the experiments on tran- 

 spiration (page 28), when it was found that transpira- 

 tion takes place most actively from the lower surface 

 of the leaf. In this case, as in that of transpiration, the 

 number of stomata is the determining factor. 



The use of the stomata is thus twofold : they provide 

 a means whereby the water-vapour can pass out of the 

 leaf, and, further, it is through the stomata that the 

 air enters. 



The two foregoing experiments have shown that it is 

 possible to draw air through a leaf. The presence of 

 air in the leaf under natural conditions will now be 

 demonstrated. 



Experiment 26 



Aim. — To show the presence of air in a leaf. 



Method. — Some water is boiled until it is free from 

 air. A leaf is then put into the water and gently 

 heated. If air is present in the leaf it will expand on 

 heating ; as a result there will not be room for it all in 

 the leaf and some must escape into the water. 



Observations. — Bubbles of air come out into the 

 water. They are specially noticeable from the lower 

 surface of the leaf. 



Inferences. — Air is present in a leaf, and, when forced 

 out, it escapes chiefly from the lower surface. 



By these preliminary experiments it has thus been 

 shown that air is actually present in the tissues of a 

 plant, and, further, that air can be drawn into the 

 plant from the surrounding atmosphere. 



Starch Formation in the Plant. — The main question 

 can now be dealt with : it must be determined whether 

 the plant really feeds on the air ; that is, Avhether it uses 

 any part of the air to build up its own plant-tissues. 



