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10 THE VEGETATION OF THE UNITED STATES. 



time to extend themselves as far as the subtle factors of their environ- 

 ment, the crude power of great barriers, or the operation of suitable 

 agents of dispersal have permitted, and their course has not run so far 

 that climatic and orographic calamities have overtaken and restricted 

 them. 



This great class of physically controlled plants can not be specifi- 

 cally enumerated at the present time. There are many good reasons 

 for believing it to contain the bulk of the species which form the 

 dominant element in all vegetation, whether these are plants of 

 extended range and frequent occurrence, plants of more restricted 

 range, or plants whose occurrence is determined by complexes of con- 

 ditions which are themselves rare. We may find areas of vegetation in 

 which a relict plant is dominant, as is true of the groves of Cupressus 

 macrocarpa near Monterey, California, or others in which a novitiate 

 plant is very abundant, as is true of the Cababi Hills in southern 

 Arizona, where an undescribed Opuntia, known nowhere else, is 

 extremely abundant. Although eases of this sort are fairly common, 

 they are usually readily detected by a more thorough study of the 

 adjacent regions. Such trees as Liriodendron, Taxodium, and Liquid- 

 ambar are known to have undergone distributional recessions, but 

 they occupy such large areas at present that they can scarcely be 

 classed as relict species, and can surely be placed among the plants 

 whose distributional controls are worth looking for among factors at 

 present operative. Whether the physically controlled plants form a 

 large or a small percentage of our flora, we can at least state with some 

 assurance, based upon the correlation of distribution and climatic 

 conditions, that they form the predominant part of our vegetation. 

 The great bulk of the tiees, shrubs, grasses, root-perennials, and other 

 plants which make up the dominant natural vegetation of the world 

 may safely be held to have had their present distributional limits 

 imposed by physical factors which are either now operative or were 

 operative in very recent time. Such factors may be acting directly 

 through the conditions of cUmate or may be acting indirectly through 

 soil conditions, through geographic or physiographic changes, through 

 the influence of associated plants, through animal, fungal, or bac- 

 terial enemies, through fire or mechanical agencies, or through the 

 means of animate or inanimate agents of dispersal. Since these indirect 

 factors of environment can affect plants only through the same kinds 

 of physiological influences as are exerted by the direct factors of the 

 climate, they are at bottom of the same nature, whether we allude to 

 them as ''biotic" factors, "mechanical factors," or what not. 



In the isolated desert mountains of southern Arizona the great bulk 

 of the species are physically controlled in their distribution, as is 

 abundantly shown by the universality of each of these species through- 

 out a given altitudinal range or a given set of habitats, and by the 

 definiteness with which it is limited by rather sharply drawn lines. 



