THE LEAF 89 



of the tin-foil and water the plant ; weigh again, and 

 once more record the weight Then by weighing after 

 another interval you can obtain once more the amount 

 of water transpired in this time. Vary the experiment 

 by placing the plant in dull light, or cool places, and see 

 what difference this makes in the weight of water given 

 off in a certain time. 



There is one point we have not yet determined, 

 namely, is the water transpired by the leaves given off 

 equally from both surfaces of the leaf, or does one sur- 

 face transpire more than the other ? A simple way of 

 testing this is to place some leaves flat, between two 

 pieces of glass, and notice whether one piece of glass 

 becomes bedewed quicker than the other, or whether both 

 become moist at equal rates. In making these experi- 

 ments, place the leaves between the glasses, and fasten 

 the glasses together with a string or elastic bands, and 

 then stand them upright so that both sides are equally 

 lighted. Unless this is done you would not be certain 

 that any differences noted were not due to one side 

 getting more light than the other, and as our earlier 

 experiments have taught us, transpiring quicker. 



Having found in the previous experiment some leaves 

 which transpire from one surface and not from the other, 

 plunge them into boiling water, and watch carefully for 

 small bubbles of air coming from them. These should 

 appear on the side from which the water is given off, 

 the bubbles coming out through the little pores or stomata, 

 owing to the air inside the leaf getting hot and expand- 

 ing. In many leaves the bubbles come only from the 

 lower side. 



