PHYSICAL FACTORS IN PLANT DISTRIBUTION 57 



This great class of physically controlled plants can not be 

 specifically 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 constellations of conditions which are them- 

 selves rare. We may find areas of vegetation in which a relict 

 plant is dominant, as is true of the groves of Cupressus macro- 

 carpa 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 cases of this sort are fairly com- 

 mon they are usually readily detected by a more thorough study 

 of the adjacent regions. Such trees as Liriodendron, Taxodium, 

 and Liquidamhar 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 look- 

 ing for among factors at present operative. Whether the physi- 

 cally 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 trees, 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 opera- 

 tive or w^ere operative in very recent time. Such factors may be 

 acting directly through the conditions of climate and soil, or may 

 be acting indirectly through geographic or physiographic changes, 

 through the influence of associated plants, through animal, fungal 

 or bacterial 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 

 bv the direct factors of the climate and soil thev are at bottom 



