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PHYSICAL FACTORS IN PLANT DISTRIBUTION 61 



tion in this connection some of the features of the Encinal, or 

 evergreen oak region, of the mountains of southern Arizona. 

 This assemblage is made up characteristically and uniformly 

 by such dissimilar forms as the evergreen broad-leaved oaks, 

 the evergreen needle-leaved piiion, the evergreen scale-leaved 

 juniper, several evergreen broad-leaved shrubs {Arctostaphylos, 

 and Garrya), the stem-succulent grass-like Nolina and Dasylirion, 

 the leaf-succulent Agave, the stem-succulent Opuntia, the small 

 deciduous and fine-leaved Acacia and Mimosa, the large-leaved 

 and deciduous Ingenhousia and Erythrina the small and hoary- 

 leaved species of Artemisia and Eriogonum, and a multitude of 

 equally dissimilar shrublets and root-perennials. These plants 

 live together under the conditions which prevail between 4500 

 and 5500 feet of elevation. The dissimilarity of general physio- 

 logical behavior and collective life requirements which is indi- 

 cated by the unlike features of form in these plants is confii'med 

 by their unlike general distributional ranges, by the fact that 

 some of them range lower toward the desert than others, while 

 some ascent to the forested elevations. Their dissimilarity is 

 further attested by what we know of the root habits, growth 

 phenomena and seasonal behavior of the members of the com- 

 munity. Unpublished work which I have recently carried out 

 with relation to the transpirational behavior of this assemblage 

 of plants still further and still more concretely emphasises the 

 unlikeness of these plants which live in dissimilar relations to 

 the environment, and even present habits of root distribution 

 and of foliar periodicity that place adjacent plants under as 

 unlike conditions as if they lived many hundreds of miles apart. 

 A very similar and almost equally striking condition exists 

 in the desert valleys of southern Arizona, where a highl}^ dis- 

 similar set of species are found, almost any two of which may 

 sometimes be found in close association, no two of which are 

 invariable associates. These facts which are true of the south- 

 western desert region are also true, although less conspicuously 

 so, in the moister portions of the United States. In the rain- 

 forests of Jamaica, both in the Blue Mountains and in the lower 

 limestone hills, the diversity of life forms is almost as striking ^ 



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