34 WATER-RELATION BETWEEN PLANT AND SOIL. 



23 to hour 9 of the following day. The average value for this period 

 is 1.23 and the maximum is 3.8 times this. 



For the later series of observations the maximum (5.54) occurred 

 with hour 14 and the fall was rather uniform thereafter, the night rate 

 being again attained with hour 23. The average night rate (hour 23 

 to hour 7 in this case) is 1.23 and the maximum is 4.5 times this. 



It is noticeable that the first maximum occurred three hours and the 

 second, one hour later than the corresponding maximum in the evaporat- 

 ing power of the air ; also that the low rate for the night hours was not 

 attained in the rate of irrigator loss until 2 or 3 hours later than in the 

 case of evaporation, at least for the February period. These condi- 

 tions surely have to do with a lag in the chain of cause and effect, from 

 the evaporating power of the air to the absorbing power of the soil; 

 some tune must elapse before an increased desiccating power of the air 

 becomes effective, through the plant and soil, upon the irrigator cup. 

 Alteration in transpiring power (due to stomatal movement, etc.) must 

 influence the form of this lag. 



It is likewise to be remarked that the maximum in rate of irrigator 

 loss for the second series occurred two or more hours earlier than that 

 for the first. This appears to mean that the effective power of the 

 aerial environment to tax the water-retaining and water-supplying 

 power of the plant was greater during the morning and early afternoon 

 hours of the second period. 



ABSOLUTE TRANSPIRATION (7> 



The transpiration rate is the prime controlling condition for the rate 

 of water absorption from the almost uniformly moist soil of our experi- 

 ments, provided no marked alteration occurs in the state of the absorb- 

 ing surfaces, aside from the change in their water content conditioned 

 by fluctuation in the tensile stress transmitted from the leaves above. 

 The graphs of transpiration rate thus furnish a picture of the propaga- 

 tion into the plant body of the environmental disturbance of altered 

 evaporating power of the air. In physiological terms, the rate of 

 transpiration is a measure of the intensity of the effective stimulus which 

 tends to alter the state of saturation deficit within the plant. This 

 first response (which might be paralleled by perception in such unana- 

 lyzed processes as tropistic bendings) becomes a condition determining 

 the propagation, inward and downward, in the plant body, of a wave 

 of tension which ultimately results in an alteration in the rate of 

 absorption from the soil. The latter may be paralleled by the process 

 of bending itself, in the case of tropisms. 



The transpiration maxima for the two series of observations occurred 

 with hour 14 (8.14) and hour 12 (8.00), respectively. Both graphs 

 show low rates for the night period, from hour 19 in both series to hour 

 7 in the first and to hour 5 in the second. The two average night rates 



