248 ENVIRONMENTAL CONDITIONS. 



it is obviously impossible to deal here with this fundamental factor. 

 We turn, therefore, to the conditions next in order of remoteness from 

 the plant itself, and recognize at once that the water-supplying power 

 of the soil is determined by its water-content and its physical make-up. 

 Charts can not yet be made, however, to represent the mean water- 

 content of the soil throughout any considerable area, and the charts 

 now in existence,^ of physical soil properties, can be of no quantitative 

 value in such discussions as the present, until the corresponding mois- 

 ture-contents (with their seasonal fluctuations) may be sinailarly 

 represented. Our inquiry is thus forced back once more to a considera- 

 tion of the factors determining the soil-moisture content. These factors 

 are (1) precipitation, (2) superficial supply by overflow, superficial 

 drainage, and subterranean supply and run-off, and (3) removal of 

 w^ater from the soil by plant-absorption and by direct evaporation. 



In the first of these tertiary conditions influencing the supply of 

 moisture to vegetation we have, finally, a well-recognized climatic 

 factor that has been measured and recorded, in a way, for many years 

 throughout the area of the United States. It is impossible at the 

 present time, however, to make any quantitatively comparative use of 

 what Httle information is at hand regarding the second set of factors 

 just mentioned ;2 this information is still far too general and qualitative 

 to be of service in an inquiry such as the present. With the third set of 

 conditions above mentioned (plant absorption and direct evaporation) 

 we shall have to deal in the following subsection, for the same environ- 

 mental conditions that control the removal of water from the plant 

 are effective to determine plant absorption— in a great measure, at 

 least— and loss of soil-moisture by evaporation into the air. 



While precipitation is thus clearly seen to be in no sense a direct or 

 immediate condition influencing water-supply to plants, it is very 

 frequently a condition that may be roughly related to plant activity, 

 as is well recognized by everyone; the mean annual rainfall of a given 

 area has long been regarded as of great value in estimating the possi- 

 bility of plant growth in such an area.^ 



As in the case of temperature, precipitation and evaporation should 

 be considered as they affect plants, rather than as they affect any given 



1 See, in tliis connection, the numerous soil surveys of the Bureau of Soils of the U. S. Depart- 

 ment of Agriculture. , , .,i i x j • r 



2 The reader interested in underground waters will find numerous bits of still unrelated informa- 

 tion in the series of Water Supply and Irrigation Papers published by the U. S. Geological Survey. 

 Especially interesting is also the following paper: McGee, W J, Wells and subsoil water, U. b. 

 Dept. Agric, Bur. Soils Bull. 92, 1913. 



3 Of course it is obvious enough that this proposition hold? only with certain restrictions, as, 

 for example, where the subterranean water-table is considerably below the soil surface. Thus, 

 the cat-tail (Typha) or tule swamps in the vicinity of springs in the Sal ton Basin of California 

 have the same ecological aspect as have similar marshes near the Atlantic seaboard, though a 

 comparison of the precipitation data for these two regions utterly fails to show any reasons for 

 expecting such similarity. The sand-dunes of the Salton Basin and those of the Lake Michigan 

 shores, on the other hand, show differences in vegetational aspect which may clearly be related 

 to differences in rainfall between these portions of the continent. 



