160 



FOUNDATIONS OF BOTANY 



knowledge to an exact form, to learn how much water 

 a given plant transpires under certain conditions. It is 

 also desirable to find out whether different kinds of plants 

 transpire alike, and what changes in the temperature, the 

 dampness of the air, the brightness of the light, to which 

 a plant is exposed, have to do with its transpiration. 

 Another experiment will show whether both sides of a 

 leaf transpire alike. 



EXPERIMENT XXX 



Amount of Water lost by Transpiration. Procure a thrifty hydran- 

 gea 1 and a small "india-rubber plant," 2 each growing in a small 



flower-pot, and with the number 

 of square inches of leaf-surface 

 in the two plants not too widely 

 different. Calculate the area of 

 the leaf-surface for each plant, 

 by dividing the surface of a piece 

 of tracing cloth into a series of 

 squares one-half inch on a side, 

 holding an average leaf of each 

 plant against this and counting 

 the number of squares and parts 

 of squares covered by the leaf. 

 Or weigh a square inch of tinfoil 

 on a very delicate balance, cut 

 out a piece of the same kind of 

 tinfoil of the size of an average 

 leaf, weigh this and calculate the 

 leaf -area from the two weights. 

 This area, multiplied by the number of leaves for each plant, will 

 give approximately the total evaporating surface for each. 



Transfer each plant to a glass battery jar of suitable size. Cover 



FIG. 122. A Hydrangea potted in a 

 Battery Jar for Exp. XXX. 



1 The common species of the greenhouses, Hydrangea hortensia. 



2 This is really a fig, Ficus elastica. 



