46 THE VEGETATION OF THE UNITED STATES. 



desirable to give them separate study, in view of the fact that they are 

 so widely separated, at least within the geographical limits of the 

 United States. 



The generalized map (plate 2)^ is, therefore, a simplification of the 

 detailed vegetation map, in addition to being a generalization from 

 it in the sense that the lines between the plant formations have been 

 smoothed, although their location has in no case been changed in such 

 manner as to throw any of our leading climatological stations into 

 vegetations other than those in which they actually belong. 



The areas represented on the generalized map of the vegetation 

 have been designated as follows : 



1. Desert. 



2. Semidesert. 



3. Grassland. 



4. Grassland-Deciduous Forest Transition. 



5. Deciduous Forest. 



6. Northwestern Hygrophytic Evergreen Forest. 



7. Western Section of the Northern Mesophytic Evergreen Forest. 



8. Eastern Section of the Northern Mesophytic Evergreen Forest. 



9. Southeastern Mesophytic Evergreen Forest. 



III. DISTRIBUTIONAL AREAS OF CONFORMIC GROUPS OF PLANTS. 



Under this heading we desire to discuss briefly the groups of con- 

 formic plants (plants of the same growth-form) which we have used in 

 the correlations dealt with on the following pages. Four groups of 

 such plants have been charted and are exhibited in plates 3, 4, and 5. 

 It seems desirable to give here some of the detailed data upon which 

 these maps have been based. 



CUMULATIVE DISTRIBUTION OF EVERGREEN BROAD-LEAVED TREES. 



The evergreen habit in broad-leaved trees is commonly regarded 

 as one that has developed in moist, warm climates, and this view is 

 confirmed by the predominance of trees of this type in the tropical 

 rain-forests of both hemispheres. We have endeavored to define both 

 the term "tree" and the term "broad-leaf" as definitely as possible for 

 securing the list that we have used. We have regarded as trees only 

 those woody plants which have a well-defined trunk and a height of 

 20 feet or more, and have regarded as evergreen all of those trees which 

 retain some of their leaves throughout the year, at least holding the 

 old ones until the time of appearance of the new leaves. The needle- 

 leaved evergreen trees have not been included in this class. The trees 

 of this group merge into shrubs and in such a manner that it is 

 extremely difficult to draw a hard-and-fast line between them, and 

 indeed some of the species which are arborescent in one portion of their 



^On Plate 2, and also on plates 6, 7, 11, 34-37, 39, 42-72, the description "Southeastern meso- 

 phytic forest" should read "Southeastern mesophytic evergreen forest." 



