WATER-RELATION BETWEEN PLANT AND SOIL. 41 



(though not wholly) dependent upon the rate of transpiration, which 

 is, in turn, largely (though not wholly) dependent upon the evaporating 

 power of the aerial surroundings. Thus the internal conditions of the 

 plant (such as transpiring power, conducting power, absorbing power) 

 may greatly influence the effect of altered evaporating power of the 

 air upon the power of the soil to resist or retard absorption of water 

 by roots. 



PELARGONIUM. 



(See table 4 and fig. 3.) 

 EVAPORATING POWER OF THE AIR (£). 



The data of this feature are repeated in table 4 and figure 3, from 

 table 2, figure 1. 



ABSORBING POWER OF THE SOIL (/). 



The graph of this power for the February series shows a maximum 

 (3.27) with hour 16, 3 hours later than the occurrence of the maximum 

 in evaporation. The period of low night values may be taken as 

 beginning with hour 23 (3 hours later than the beginning of the low 

 night rate of evaporation) and ending with hour 7 for both series. The 

 ratio of the maximum value of the earlier series to the night average 

 (0.92) is 3.2. In the case of the later series the maximum (4.91) 

 occurs with hour 14, only a single hour later than the occurrence of the 

 evaporation maximum. The February ratio of the maximum rate of 

 irrigator loss to its average night value (0.68) is 7.2. It is noticeable 

 that the lag between evaporation and the drying of the soil adjacent 

 to the cup is greater for the earlier series, which is no doubt related 

 to a greater transpiring power and absolute transpiration, possibly also 

 to lower absorbing power of roots in the February series. As is fre- 

 quent in such graphs, the higher maximum is followed by more rapid 

 fall in this rate, perhaps due to phenomena within the plant as well as 

 to the retardation of root absorption by drying of the adjacent soil, 

 which must accompany high absorption rates. The March graph 

 rises earlier, to a higher maximum, and falls earlier to a lower night 

 rate than does the February one. 



ABSOLUTE TRANSPIRATION (f). 



The earlier graph shows a maximum (7.45) with hour 14, an hour 

 later than occurs the maximum of evaporation and 2 hours earlier than 

 occurs that of irrigator loss. Evidently incipient drying of leaves was 

 not effective in this case. The low night rate is attained with hour 

 18 and is maintained till hour 7. The ratio of maximum to average 

 night rate (0.53) is 14.1 in this February series. 



The March graph shows the maximum (7.00) with hour 12, an hour 

 earlier than the occurrence of the evaporation maximum, thus suggest- 

 ing some incipient drying in the late forenoon hours. The low night 

 period extends from hour 17 to hour 6 of the next day, its average value 

 being 0.37. The ratio of the maximum to this average is 18.9. 



