112 VEGETATION OF A DESERT MOUNTAIN RANGE. 



other to a common geographical and habital Umit. The physical 

 requirements of plants are so varied and so elastic that the composition 

 of a series of communities occupying similar habitats in widely sepa- 

 rated places shows the constant overlapping of the ranges of individual 

 species which is due to the physiological inequivalence of these species. 



It is particularly true of the plant communities of arid and semi- 

 arid regions that the most closely associated individuals are not alike 

 in their life requirements, and this is true to a less pronounced extent 

 in all plant communities. The members of the many diverse biological 

 types, or growth forms, which are found together in Desert and Encinal 

 find their soil water at different levels, procure it at different seasons, 

 and lose it through dissimilar fohar organs, at the same time that they 

 react differently to the same temperature conditions. In brief, these 

 associated plants are not living in the same climate but are living in 

 different sections of the same climate, the demarcation of these sections 

 being either temporal or spatial. 



The use of the physical characteristics of the habitat as a criterion 

 in the definition of a plant community does something to give a greater 

 rigidity and a wider applicability to the definition. On the other hand 

 it confuses cause with effect and makes it impossible to investigate 

 the relation of physical conditions to a community defined in that 

 manner without reopening the whole question as to the nature and 

 identity of the community. There is much strong logic to support 

 the view that all necessary definitions and classifications of vegetation 

 should be made on the basis of the vegetation alone. When units of 

 vegetation are thus defined they lend themselves to the further study 

 of their life requirements, and it is such study — applied to individual 

 species as well as to vegetation — that affords the most promising and 

 important field for ecological activity. 



The distribution of vegetation in the Santa CataHna Mountains is 

 strongly controlled by a steep climatic gradient; the vegetation itself 

 is diversified in its display of growth forms; and the secular changes 

 of vegetation due to physiographic phenomena, and to the reaction 

 of the plant upon its habitat, are in almost complete abeyance. These 

 circumstances have made it possible to give a delineation of the vege- 

 tation upon purely vegetational characteristics, without regard to the 

 secular changes which are taking place in very restricted areas, and 

 with particular emphasis upon the individualism of behavior among the 

 characteristic species. The same circumstances have also made it 

 possible to lay side by side the facts respecting the vegetational gradient 

 and those respecting the climatic gradient in such manner as to reveal 

 the correlations between the two and to indicate some of the physical 

 controls which operate in the limitation of the activities and of the 

 ranges of species and of vegetations. 



