THE FOOD OF THE PLANT 25 



Two leafy twigs are placed on glass plates. By the 

 side of one is put a small dish containing calcium 

 chloride. The twigs are then covered with separate 

 bell-jars, the edges of which have been well vaselined 

 (Fig. 10). 



Observations. — The twig under the bell-jar contain- 

 ing calcium chloride withered much the more quickly 

 of the two. 



Inference. — Since the withering is caused by loss of 

 water, the twig that withered tirst must have trans- 

 pired most freely; that is, the rate of transpiration 

 increases with the dryness of the atmosphere. 



Experiment 13 



Aim. — To determine the rate at which transpiration 

 takes place and the general conditions that affect it. 



Method. — An apparatus similar to that used in 

 Experiment 3 is set up, a leafy branch being substituted 

 for the whole plant. There is no need, of course, to 

 cut a wedge out of the cork ; the stem of the branch 

 is inserted through a hole. 



In Experiment 3 the rate of absorption was measured. 

 The amount of water given off in transpiration in that 

 case was less than the amount absorbed by the quantity 

 of water retained as the seedling continued its growth. 

 In the case of a cut twig the weight, for a short period 

 at any rate, will remain practically constant, and the 

 amount of water taken up by the cut end of the stem 

 can be taken as a measure of the water given off by the 

 leaves. 



Observations. — Keadings should be taken under the 

 following conditions : 



1. While the apparatus is under the normal atmos- 

 pheric conditions of the room. 



2. While the branch is being fanned or is in a 

 windy place. Transpiration is increased. 



3. When the apparatus is moved to a colder place, 

 or is packed round with ice. Transpiration is decreased. 



