64 THE VEGETATION OF THE UNITED STATES. 



IV. DISTRIBUTIONAL AREAS OF SELECTED INDIVIDUAL SPECIES. 



A relatively large number of individual species have been so selected 

 as to include a few of the dominant plants in each of the leading vege- 

 tations. Many of the minor plants have distributional areas which 

 coincide roughly with vegetational areas, as has already been shown 

 for some of the northeastern and southeastern evergreen needle-leaved 

 trees. Each of the dominant species that we have used for correlation 

 with the climatic conditions is accompanied by numerous minor or 

 subordinate species for which the same cUmatic controls must often 



be of importance. 



The trees are predominant in the list of species which we have used, 

 partly because they are the dominant element in so many of our types 

 of vegetation and partly because it is easier to secure full and accurate 

 distributional data for them than for plants of any other type. The 

 distribution of most of the flowering plants of the United States would 

 seem to be fairly well known if a list of the known occurrences of these 

 species were examined. When, however, the attempt is made to plot 

 the distribution on a map, the very great gaps which exist in our 

 knowledge become very evident. We have depended largely on the 

 invaluable data of the United States Forest Service, as given in various 

 pubUcations, for our maps of the distribution of trees. 



The species which we have used fall into some 22 groups, which will 

 now be enumerated. The distributional areas of these plants have 

 been arranged on the maps so that their limits will intersect as little 

 as possible and so as to economize space. The plates on which these 

 distributions are represented are given in each case. 



1. Northwestern Evergreen Needle-Leaved Trees. (Plate 13.) 



Tsuga heterophylla (Raf.) Sarg. 



Picea sitchensis (Bong.) Trautv. and Mayer. 



These two trees are taken as typical of the numerous evergreen 

 needle-leaved forms found in the hygrophytic forest of the North- 

 western States. Tsuga ranges eastward to the Rocky Mountains of 

 northern Idaho and northwestern Montana, but Picea is closely 

 restricted in its range to the hygrophytic forest region itself. 



2. Western Evergreen Needle-Leaved Trees. (Plates 14 and 15.) 

 Pseudotsuga mucronata (Raf.) Sudw. ( = P. taxifolia (Lam.) Britton = P. doug- 



lasii Carr). 

 Pinus ponderosa Laws, (including P. scopulorum (Engelm.) Lemmon). 

 Pinus contorta Loud, (including P. murrayana Oreg. Com.) Pinus edulis Engelm. 



In this group are comprised four of the leading trees of the western 

 portion of the Northern Mesophytic Evergreen Forest. It has been 

 possible to map the occurrences of Pseudotsuga with considerable 

 accuracy, in fact, in far more detail than our series of climatological 

 figures would warrant. Pinus ponderosa is likewise widely distributed 



