CLIMATIC CONDITIONS OF THE UNITED STATES. 247 



Vital activity is influenced by internal moisture-conditions that 

 mainly remain to be studied by physiological science. We are sure 

 that these internal conditions are largely dependent upon external 

 water-relations, and our task is to find ways of measuring and defining 

 the latter as they exist in nature, in such a way as to render our 

 description of the moisture conditions of the environment valuable 

 to those interested in geographical distribution. It has been already 

 noted that such a procedure is rather simple in the case of the tempera- 

 ture-relation, for the immediate and internal temperature conditions 

 effective in the control of plant activity are closely paralleled at all 

 times by the more remote conditions of the environmental temperature; 

 there is usually no great lag between the march of the external, or 

 ecological, and that of the internal, or physiological, temperature con- 

 ditions. This is, of course, simple because heat migrates with com- 

 parative readiness either into or away from the plant and hence equilib- 

 rium in this regard, between plants and their surroundings, is seldom 

 very far from being attained. Similarly, the environmental moisture 

 conditions are also effective to control the immediate, internal moisture 

 conditions, through the relative rates of the entrance and exit of water. 

 In this case we have to deal with material instead of energy, but the 

 general relations are the same. Therefore, it is with those conditions 

 of the environment that may influence the rates of entrance and exit 

 of water that the present section has to deal. A plant may suffer from 

 lack of water, (1) because of too slow a rate of entrance of this sub- 

 stance into its body during some previous time period, (2) because of 

 too rapid a rate of exit, or (3) because these two conditions have been 

 simultaneously effective. The environmental conditions to be here 

 considered for the area of the United States will be presented under the 

 two captions, the supply of water to the plant, and the removal of water 

 from the plant. 



2. SUPPLY OF WATER TO PLANT. 

 (A) PRELIMINARY CONSIDERATIONS. 



A discussion of the power of the surroundings to supply water to 

 plants should begin (for ordinary plants) with the power of the soil 

 to supply moisture to roots; it makes no difference, for this primary in- 

 quiry, what conditions may determine this power, for the only thing 

 directly affecting the plant in this connection is this power itself.^ 

 Since, however, ways and means for comparing the water-supplying 

 power of the soil at various times and places are still to be perfected, 



^ Livingston has emphasized the crying need for methods of measuring and comparing the 

 powers of soil to supply moisture to unit absorbing surface, and he and his co-workers have 

 suggested three methods for the quantitative measurement of this power, all of which appear 

 promising in this direction, but little has yet been done of a positive character. See, in this 

 connection, the following papers: Livingston, 1906, b. — Idem, 1909. — Idem, 1912, b. — Living- 

 ston and Hawkins, 1915. — Pulling and Livingston, 1915. 



