32 WATER-RELATION BETWEEN PLANT AND SOIL. 



March period, the former drawn as a full line, the latter as a broken 

 one. The data for the second day of the March period, for hours 

 later than the ninth, have not been included in the graphs. The lowest 

 of the five axes of abscissas, at the base of each figure, is represented 

 by a broad Une for the night period, from hour 19 of one day to hour 6 

 of the next, inclusive; for the remainder of the day, from hour 7 to hour 

 18, this line is double, with the same total width as the broad one. 



The lowest double graph shows the march of the evaporating power 

 of the air (E), the second shows that of the rate of loss from the irri- 

 gator (7), the third is for absolute transpiration (T), the fourth is for 

 transpiring power or relative transpiration (T-^-E), and the fifth is for 

 what we have termed environmental desiccating power or aridity 

 (Ex I). It is to be noted that the evaporating power of the air was 

 assumed to be the same for all three plants, hence the lowest double 

 graph of figure 1 is simply repeated for figures 2 and 3. All values for 

 Coleus and Vicia (figs. 1 and 2) are stated in terms of the corresponding 

 value for hour 3, February 24, considered as unity. For Pelargonium 

 (fig. 3) the basis for comparison is the datvmi for hour 23, February 23. 



The three plants will be considered in the order of the tables. In 

 each case the five double graphs will be given attention in the order in 

 which they have just been characterized. Only the most obvious points 

 will be mentioned. 



COLEUS. 



(See table 2 and fig. 1.) 

 EVAPORATING POWER OF THE AIR (£). 



As is usual, the evaporation rates were generally relatively high 

 during the day hours and much lower during those of the night. The 

 rise in this rate began about hour 8 and attained a maximum (3.17 and 

 3.96) with hour 13, for both periods of observation. The low rates 

 characteristic of the night hours were first clearly attained with hour 

 20 for both periods. The pronounced temporary rise for hour 19 on 

 both these graphs is almost certainly due to some manipulation of the 

 general conditions of the greenhouse, though this matter was not inves- 

 tigated. The mean rate of water loss from the atmometer was 1.59 

 for the earlier period and 2.21 for the later one (relative to the rate for 

 hour 3, February 24), and the graph for the March day is seen to be 

 placed generally a little higher or farther from the horizontal axis than 

 is that for February. 



The minima for these graphs are not to be satisfactorily located, but 

 we may average the rates for the night period, from hour 20 to hour 7, 

 in each case, thus obtaining 1.11 and 1.47 for the February and March 

 series, respectively. Using these average night rates we find that the 

 maximum is 2.9 times the night average for the eariier series and 4.6 

 times the corresponding average for the later series. This state of 



