518 CORRELATION OF DISTRIBUTIONAL FEATURES. 



of 0.80 in the North. The area lying inside the line for values of 0.20 

 is entirely occupied by Desert, which exceeds the line only to short 

 distances in Texas and Washington. 



The importance of the moisture ratio in controlling the leading 

 vegetations was. shown by Transeau for the eastern United States/ 

 and our investigation has served to confirm his deductions, as well as 

 to extend their application to the entire country. The comparisons 

 which we have made between the vegetational areas and the various 

 other climatic conditions have served to emphasize the importance of 

 the moisture ratio even more than was done by Transeau, since no 

 other single datum has been found in our work to approach it as an 

 expression of the controlling conditions for forest, grassland, and 

 desert. 



The importance of the moisture ratio is due partly to the fact that 

 it is, a combined expression of several other conditions, and still more 

 to the fact that these conditions are ones which have a combined 

 effect upon the physiological processes of the plant. The moisture 

 ratio is, in brief, an expression of the relation existing between the 

 water available for plants and the amount of water lost as a result of 

 atmospheric conditions. The moisture ratio gives us a single numerical 

 expression for the group of conditions which control a single important 

 physiological condition of the plant, namely, its maintenance of a 

 balance between intake and outgo of water. When we secure the 

 product of the moisture ratio and the temperature summation we have 

 a still more comprehensive expression of conditions, the moisture- 

 temperature index. In this index, however, we have no such succinct 

 expression of a set of conditions that are closely coordinated in their 

 relation to the physiology of the plant, in spite of the individual impor- 

 tance of each factor in the product. The moisture-temperature index 

 is correspondingly of less value in interpreting distributional etiology 

 than is the moisture ratio itself. 



A much more ideal derivation of the moisture ratio is one employing 

 the soil-moisture rather than the precipitation, since it is the former 

 rather than the latter condition that is of immediate relation to the 

 activities of the plant. Shreve has used the ratio of soil-moisture to 

 evaporation in a discussion of the annual march of moisture conditions 

 at Tucson, Arizona,^ and also in describing the gradient of conditions 

 from the base to the summit of the Santa Catalina Mountains, in 

 southern Arizona, a range surrounded by desert and capped by heavy 

 forest. For detailed work, and particularly in .arid regions or regions 

 with pronounced periodicity of rainfall, the ratio of soil-moisture to 

 evaporation will be found to express the prevailing conditions for 



'Transeau, E. N., Forest centres of eastern North America. Am. Nat., 39: 875-889, 1905. 

 'Shreve, Forrest, Rainfall as a determinant of soil moisture, The Plant World, 17: 9-26, 1914. — 

 Idem, 1915. 



