TRANSPIRATION OF STEMS AND LEAVES 



205 



cient capacity, and which is accurate to .5 g. Set in bright sun- 

 light near a window or in a glass experiment room, and place a 

 thermometer, or better a thermograph, near the preparation. 

 Four hours later again weigh the preparation and note the total 

 loss of weight. This will represent the amount of water given off 

 by the stems and leaves during the period over which the experi- 

 ment extended. Still more reliable results may be reached if 

 the testis continued during 8 or 10 hours. Strip the leaves from 



Fig. 103. Balance for determination of amount of transpiration. After Giesenhagen. 



the stem and place together on a table, matching the edges to 

 form a regular rectangular figure, the area of which may be Easily 

 calculated. A more accurate method consists in tracing the 

 outlines of the leaves on a sheet of paper of known area. Weigh 

 the paper on a precision balance, then cut out the figures of the 

 leaves, weighing remainder. Comparison of the data will give 

 total area of leaves, or find the area of the leaf-tracings by the use 

 of a planimeter. Compute the area of transpiration per square 

 centimeter of leaf surface (Fig. 103). 



