18 WATER-RELATION BETWEEN PLANT AND SOIL. 



content if two or more irrigator cups had been used instead of a single 

 one. It is in this direction that first progress toward a still more 

 sensitive instrument may be expected. The device may be improved 

 in many other ways. 



It will have been noted that we have tacitly assumed the water- 

 content of our soils to be uniformly distributed throughout the soil 

 mass. As has been stated, however, the moisture content may always 

 be expected to be somewhat greater in the near vicinity of the supplying 

 surface than elsewhere, and it is also clear that it must be exceptionally 

 low in the near vicinity of absorbing surfaces, such as those of plant 

 roots. It follows that the fluctuations shown above do not really 

 apply to the entire soil mass in question, but that they are merely like 

 mathematically derived averages, which may find no actual physical 

 expression in any case. There seems no ground for questioning the 

 supposition that the first few layers of soil particles adjacent to the 

 absorbing root hairs of our experiments must have undergone far 

 greater fluctuations in moisture content than those indicated above. 

 The problems of water movement through soils and those of root 

 absorption, when the latter shall be seriously attacked, require to be 

 approached from the point of view suggested by the question just 

 raised, but nothing has so far been done to furnish the kind of infor- 

 mation here needed; yet this whole matter is surely approachable 

 along relatively simple lines of physical study. 



PRELIMINARY ANALYSIS OF THE PROBLEM. 



The numerical results of our two series of measurements are shortly 

 to be presented, but since the important features of the tabulations 

 and graphs involve arithmetical manipulation of the observational 

 data, it will be expedient first to set forth the general interpretation 

 which we have attempted. 



As in all profitable physiological analysis, the organism and its 

 surroundings should here be treated as a dynamic system. The move- 

 ment of water from plant to air represents icork, in the physical sense, 

 and the complex of conditions determining the time rate of doing this 

 work is to be measured as a power — the desiccating power of the aerial 

 environment (see Livingston, 1911). This power operates against a 

 resistance in accomplishing the work done, which is the resistance 

 afforded by the plant surfaces to transpirational water loss (see Living- 

 ston, 1913). Loss of water by transpiration must, in ordinary plants, 

 be balanced by entrance of water through root surfaces and the water 

 efficiency of a plant, under any set of environmental conditions, is 

 dependent upon the relation of entrance to outgo. If A denote the 

 rate of absorption and T the rate of transpiration, then the plant 

 body gains in water content for any period when A is greater than T. 



