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! and a small “india-rubber plant,” ? each growing in a small 
flower-pot, and with the number 
Psp te of square inches of leaf-surface 
Aaa in the two plants not too widely 
ea different. Calculate the area of 
ey fa . 
Wits sos the leaf-surface for each plant 
NY / b 
pe) by dividing the surface of a piece 
i eg 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 
Fic. 122.— A Hydrangea pottedina —_ Jeaf, weigh this and calculate the 
Battery Jar for Exp. XXX. 3 
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 
1 The common species of the greenhouses, Hydrangea hortensia. 
2 This is really a fig, Ficus elastica. 
