SOIL MOISTURE AND TO EVAPORATION. 



45 



planting. The pot was sealed and the experiment was begun at 12''30 m 

 p.m., August 17, the plant standing in bright sunshine. Weighings 

 and readings of the evaporimeter were made from time to time until 

 6 p.m., August 19. No wilting or drying of the leaves had yet taken 

 place when the record was discontinued. The data are tabulated in 

 Table VII. In the first column are given the times of observation. 

 Column It gives the increment of water loss during the time period 

 just ending, column Rt gives the rate of water loss in grams per hour 

 for that period, and column Rt a gives the same rate per square centi- 



T ABLE VII . Data from Experiment /. Euphorbia . 

 [Total leaf area 398.4 sq. cm.] 



meter of leaf surface. Thus Rt is It divided by the number of hours in 

 the period, while Rta is Rt divided by the total leaf area. Column I e 

 gives the increment of water loss from the evaporimeter (approximately 

 equivalent to 100 sq. cm. of free water surface) for the period, column 

 Re gives the rate per hour, and column R ca gives the same rate per 

 square centimeter of free water surface. In the last column the figures 

 denote the ratio between the rate per hour for unit leaf surface and 

 the same rate for unit water surface. In other words, this ratio shows 

 the fractional part of a square centimeter of water surface which would 

 be required to give off as much water as would evaporate during the 



