WATER-RELATION BETWEEN PLANT AND SOIL. 37 



at the same time the aerial desiccating power was 39 per cent, the 

 subterranean was 4 per cent, and the general environmental aridity- 

 was 53 per cent greater during the second period. It appears probable 

 that the absorbing power of the roots may also have been greater during 

 the later period, as might be the case if the absorbing surface were 

 larger. These values illustrate the manner in which external and 

 internal conditions may be expected to influence the plant in its general 

 water-relations. 



VICIA. 



(See table 3 and fig. 2.) 

 EVAPORATING POWER OF THE AIR (£). 



These data are the same as those used for Coleus (table 2, fig. 1). 

 They are simply repeated for Vicia (table 3, fig. 2) . 



ABSORBING POWER OF THE SOIL (/). 



For the February period the maximum of this rate (6.65) occurred 

 with hour 13, but nearly as high a rate was maintained tUl hour 15. 

 The period of low night values began with hour 22 and continued to 

 hour 7 of the next day. The average value for this period is 1.22 and 

 the maximum is 5.5 times this. 



For the March series the maximum (4.75) occurred with hour 14, 

 but the value for hoiu- 13 was almost as large. The fall after hour 14 

 was more rapid than in the earlier series and the night period began 

 with hour 19. It continued till hour 7, as in the February series, and 

 the night average was 0.94. The maximum is 5.1 times this average. 



Both maxima may be said practically to have occurred with the 

 same hour as the maximum in the evaporating power of the air, but 

 the high rate was continued, two hours in the first series and one hour 

 in the second, later than in the case of evaporation. This corre- 

 sponds with the stUl more pronounced lag in this feature which was 

 noted for Coleus. It is also noticeable that the February graph for 

 Vicia agrees with the corresponding graphs of this feature for Coleus 

 in that the low night rate in soil-absorbing power is not attained till 

 3 hours later than in the case of atmospheric evaporating power. In 

 the March graph for Vicia the soil-absorbing power attains its night 

 rate even an hour earlier than does the evaporation rate, which sug- 

 gests that the general low position of the March graph is not due to 

 drying of the soil immediately about the roots, but to low absorbing 

 power of these organs. It is also possible that this difference between 

 the two series is due to the alteration in the height of the irrigator 

 water column, which occurred between the two periods. 



Apparently the power of the plant to transmit the disturbance of 

 high evaporation to the soil about the roots and that of these soil 

 layers to transmit it to the irrigator cup were not greatly taxed in either 



