24 THE VEGETATION OF THE UNITED STATES. 



may contain individuals of a particular height, together with other 

 smaller individuals, as is true of the fresh-water marshes of the tribu- 

 taries of Chesapeake Bay. By far the most common condition is that 

 in which several successive stages of height are found together, so that 

 the vegetation is said to possess a stratification with respect to the 

 foliage of the plants concerned. This is found in every forest and 

 reaches a splendid climax in the tropical rain-forests. In the com- 

 munities which possess several strata of plants, the highest one has 

 been designated the "facies," but its members will here be alluded 

 to as the "major" plants of the community and the several lower 

 strata as the "minor" plants. 



A further salient feature of communities depends upon whether the 

 plants cover the ground closely or stand in an open formation. We 

 may discover open or closed communities among plants of every 

 stature, from the smallest grasses to trees of a height of 50 or 60 feet. 

 The greatest density of stand is reached by trees only under conditions 

 which favor the attainment of a greater size than this. In open com- 

 munities there may be plants of different heights, but in such cases the 

 low plants are not dependent on the large plants for the conditions 

 which render their existence possible. Even in open communities 

 there may be a certain degree of stratification under the largest in- 

 dividuals as is observable in southern Arizona, where many small 

 annuals occur abundantly beneath the trees as well as away from them. 



Last among the criteria of the community is the number of species 

 comprised in it. Even if there is a uniformity of growth-form through- 

 out the community, there is importance, from our standpoint, in know- 

 ing whether this uniformity is caused by the existence of a single 

 species or of many. It is customary to designate the species which is 

 most common in a community as the "dominant" one and the other 

 species as "subordinate." 



The salient features by which we distinguish types of vegetation 

 are, then, to sum up: the growth-forms involved, the presence of one 

 or many strata of plants, the open or closed condition, and the degree 

 of simplicity or complexity of the specific content. These are all 

 features which must be looked upon as products of the environmental 

 conditions just as truly as are any of the structures or physiological 

 reactions of the individual plants themselves. The presence of a single 

 growth-form or of many, the existence of a low carpet of plants or of a 

 lofty forest, the openness or density, and the dominance of a single 

 species or the successful association of many, are all features which are 

 determned by the environmental complex just as truly as is the rate of 

 growth or that of photosynthesis for an individual plant or species. 



The only other criteria that have been used in defining communities 

 are the physical nature of the habitat and the specific identity of the 



