46 THE RELATION OF DESERT PLANTS TO 



same period and in the same position from a single square centimeter 

 of leaf surface of this plant. This ratio will be termed the rate of 

 relative transpiration, the term being used to denote that this ratio 

 shows the relation of transpiration to evaporation. 



For the whole period of the experiment, from 12 h 30 m p.m., August 16, 

 to 6 p.m., August 19, the average hourly rate of transpiration for the 

 entire plant was 0.420 gram., and the same rate per square centimeter 

 of leaf surface was 0.00105 gram. In order to bring out clearly the 

 manner in which the rates per hour vary during the day, curves have 

 been constructed for them and for their ratio, and these are presented 

 in figure 8. The curves are marked at the left with the symbols which 

 head the corresponding columns in Table VII. Abscissas denote time, 

 dates and two-hour periods being marked on the lower horizontal axis, 

 which is drawn as a broad line for the night periods, from 6 p.m. to 

 6a.m. The ordinates are the figures from the table and are placed 

 directly upon the curves. They are plotted at the middle of their 

 respective periods. The two rate curves are plotted on the same 

 horizontal axis and on the same scale for the abscissas. In order to get 

 the curve of evaporation rate into the space allowed, the ordinates for 

 this curve are plotted on a scale only one-fourth as great as that used 

 in the curve df transpiration rate. The horizontal axis for the ratio 

 curve is placed above the other two curves in order to avoid intersec- 

 tions. The scale of the abscissas for this curve is identical with that 

 for the other curves, but the scale for the ordinates is merely one of 

 convenience. A curve of temperatures, arranged by plotting the 

 average temperature for each partial time period at the middle of that 

 period is given with the curve of evaporation rate, this curve being 

 marked T. Since weather records are usually made with the Fahrenheit 

 scale a thermometer of this type was used for these observations. The 

 data are given without reduction to the centigrade scale. 



It is to be noted at once that the rate of transpiration rises to a 

 maximum in the day period and falls to a minimum in the night, and 

 that the rate of evaporation has similar maxima and minima. This 

 illustrates the commonly observed phenomenon that the rate of 

 transpiration is higher in the day than in the night, and points to the 

 fact that this is largely due to variations in the evaporating power of 

 the air and not mainly, at least, to physiological regulation. It is plain, 

 however, that the two sets of ordinates do not vary at the same rate. 

 This is brought out clearly in the ratio curve, which shows that the 

 rate of transpiration approaches most nearly that of evaporation in 

 the day time and departs farthest from it in the night, although the 

 periods do not coincide exactly with those of light and darkness. This 



