CH. IV] LOSS OF WEIGHT. 97 



SECTION B. Loss of Water by Transpiration. 



(113) Loss of weight. 



To get a general idea of the amount of loss due to 

 transpiration it is well to take a series of weighings of a 

 plant growing in a flower-pot. Select a 

 plant 1 with a large leaf-surface, in a small 

 flower-pot, so that it may not be too heavy 

 for the balance 2 . In order to confine the 

 loss by evaporation to the plant, the sur- 

 face of the earth must be covered \vith a 



FIG. 17. Exp. 113. 

 divided disc of sheet-cork painted over 



with wax-mixture. The pot is wrapped in sheet india- 

 rubber, which may be held in its place as shown in 

 fig. 17 ; the glass vessel c grips the rubber sheet, and 

 also serves to prevent evaporation from the bottom of the 

 pot. Since it may be necessary to water the plant during 

 the course of the experiment, a corked tube must be fitted 

 into a hole in the cork plate. 



(114) Transpiration compared with evaporation of a 



surface of water. 



To estimate the transpiration from a given leaf- 

 surface it will be necessary to take a plant small enough 

 to be placed on a more delicate balance. Detmer recom- 



1 Helianthus tuberosus or Chrysanthemum. 



2 We use a French druggist's balance capable of carrying 4 or 5 

 kilograms, and of turning with 0'5 gram when loaded with 1000 

 grams in each pan; it is a useful form of balance for the purpose in 

 question. The beam, etc. being below in the box, and the pans there- 

 fore free to take a tall plant. 



D. A. 7 



