44 WATER-RELATION BETWEEN PLANT AND SOIL. 



conditions (such as evaporation from the soil surface, precipitation and 

 the rise and fall of the subterranean water table), is of course a matter 

 for future determination. In our studies the only immediate condi- 

 tions markedly altering the soil-moisture content (and therefore its 

 resistance to root intake) were those furnished by the plant itself. 

 These conditions acted through the actual rate of root absorption, 

 which tends to dry the soil layers adjacent to the absorbing surfaces 

 and hence to lower the possible rate of water supply to the roots, by 

 the formation of a drier layer of soil between the roots, and the more 

 distant and more moist soil layers. Aside from growth and other 

 water-consuming processes of the plants themselves, the actual rate of 

 absorption by our roots was controlled by the evaporation rate, which 

 was partly controlled, in turn, by internal conditions, and partly by 

 the evaporating power of the aerial surroundings. But this external 

 influence was not direct, for the disturbance of altered evaporating 

 power never affected the absorptive rate directly, but always through 

 a propagated disturbance in moisture equilibrium traversing the plant 

 body. Internal variations altering the resistance to this propagation 

 of Renner's "suction wave" apparently play an important role in 

 determining the amount of work performed in the actual transfer of 

 water from soil to plant. Thus the rate of irrigator loss is not as 

 purely an environmental factor as is usually that of atmometer loss. 

 It is considerations of this sort which render satisfactory interpreta- 

 tion of many of the phenomena above recorded quite out of the ques- 

 tion at the present time. The unexplained phenomena depicted by 

 our graphs appear at present to be related mainly to fluctuations in 

 the internal conditions of the plants. 



The data from our measurements bring out one general and possibly 

 important feature of the daily march of the conditions considered. 

 All of the graphs exhibit a high region for the day and a low one for 

 the night, although there is a shifting, by a few hours, backward and 

 forward, of the time at which the various maxima occur and of the 

 beginning and end of the night period of low values. Although we 

 have not been able to study minimal values, we have found it possible 

 to compare the magnitude of each daily maximum with the correspond- 

 ing average value for the night period. The ratios of maximum to 

 mean night value have been given in the preceding section, and are 

 here brought together in table 5. 



The data of table 5 show that in all cases but two the ratio value is 

 greater for the March than for the February period. That is, outside 

 of the two exceptions, the degree of diurnal fluctuation in the magni- 

 tudes of the various conditions is shown to be greater at the later time. 

 The two exceptions are both for Vicia, one for irrigator loss and the 

 other for environmental aridity; the difference between the ratio 

 values, by which the February value is greater, is not large in the former 

 case, but is consideral)le in the latter one. The occurrence of these 



