WATER-RELATION BETWEEN PLANT AND SOIL. 19 



It loses in water content when T is greater than A, and the condition 

 that growth and other water-consuming processes may go forward is 

 that A must be somewhat greater than T, in general. 



The movement of water from soil films into plant roots is dependent 

 upon a complex of internal conditions which may be taken together as 

 the absorptive power of the root system. This power operates, again, 

 against external resistance, the resistance offered by the soil to water 

 absorption by roots. The absorptive power of roots is in large part 

 ultimately dependent upon transpiration, through the rate at which 

 water is removed from the roots upward; the water columns are con- 

 sidered as continuous (though interrupted, in a sense, by imbibed cell 

 walls) from root periphery to leaf periphery and as capable of bearing 

 great strain.^ Root-absorbing power may also vary, however, because 

 of changes originating in the roots themselves. Such changes have not 

 been quantitatively studied as yet, no method being so far available 

 for this sort of investigation. Nevertheless, there can be no question 

 that the extent of root surfaces, ceteris paribus, is a very important fac- 

 tor in determining the absorptive power of a root system for water, 

 just as the extent of foliar surfaces has great influence in the control of 

 the plant's general transpiring power. Furthermore, Livingston^ has 

 suggested that plants growing in certain toxic soils and having the cor- 

 tical cells of the roots greatly swollen may be injured in their aerial 

 parts primarily through low water-absorbing power of the roots. Also, 

 Caldwell (1913) has pointed out that permanent wilting with low 

 evaporation rates, causing the death of root hairs, markedly decreases 

 the root absorbing power, apparently through the breaking of the 

 film continuity between plant water and soU water. The resistance 

 offered by the soil to water entrance into roots, that is, the attraction 

 of the soil for water, should of course be proportional to the reciprocal 

 of the power of the soil to deliver water to absorptive surfaces. 



If we wish to compare the diurnal marches of all these various powers, 

 we need first to measure them in some adequate way and with suitable 

 time intervals, and next to reduce each series of values obtained to a 

 uniform basis, considering the value for some particular hour as unity. 

 The same hour is of course to be employed in every case, thus present- 

 ing the complex data in a form such that the relative rates of change of 

 the different powers dealt with may be directly comparable. 



In our attempt in this direction, the desiccating power of the aerial 

 environment is considered as the evaporating power of the air, neglect- 

 ing the effect of radiant energy, as far as the absorption of the latter by 

 the plant foliage is more pronounced than its absorption by the white 

 porous cup of the atmometer. It would probably have been better if a 



'Dixon, 1909. 



^Livingston, B. E., Note on the relation between growth of roots and of tops in wheat. Bot. 

 Gaz. 41: 139-43. 1906. 



