DISTRIBUTION OF VEGETATION IN UNITED STATES. 31 



for several States. We have not only drawn upon ecological literature 

 and the publications of the United States Geological Survey, but have 

 been greatly aided by consultation of the photographic illustrations in 

 numerous works of a non-botanical character, and we are particularly 

 indebted to many of our colleagues, who have generously given us the 

 information at their disposal. 



In the preparation of the maps showing the cumulative distribution 

 of growth-forms and the distribution of individual species we have used 

 all of the manuals, floras, and local lists that it was possible to secure. 

 We have not consulted the specimens that are to be found in any of 

 the large herbaria, but have depended solely upon published statements 

 of occurrence. We have done this because it is so frequently possible 

 to secure, in ecological literature or in the publications of the United 

 States Biological Survey, statements regarding the ranges of plants 

 which would in all probability be represented in the herbaria by only 

 a few collections, on the labels of which the information regarding the 

 ecological occurrence would be extremely meager. 



Some of the ranges of individual species are naturally outlined much 

 more accurately than others. The distribution of trees is in general 

 much better known that that of herbaceous plants, and the distribu- 

 tion of grasses is better known than that of plants whose economic 

 importance is not so great. Some of the ranges exhibited in our maps 

 have been based on extremely few stations, as is true particularly of 

 such plants as Floerkea occidentalis and Trautvetteria grandis. It may 

 be taken for granted that all of our distributional areas which are 

 represented by smooth and wide-sweeping lines are in general based 

 upon less precise information than are the areas limited by very sinu- 

 ous lines. The limitations of our method of mapping have required 

 that the range of numerous mountain plants occurring in the Western 

 States be exhibited by passing a bounding-line around the entire region 

 in which they are found locally at their appropriate elevations. In 

 similar manner several of the aquatic and palustrine plants have been 

 plotted as if growing continuously throughout extensive stretches of 

 country in which they are really of very local occurrence. This is 

 notably true for Sium cicutoefolium and for Cephalanthus occidentalis, 

 which is not actually known in New Mexico and is very uncommon in 

 Arizona, although it reappears in great abundance in interior Cali- 

 fornia. The scale of our maps has made it necessary to include in the 

 ranges of very many of our plants certain extreme coastal locations 

 in w^hich they are actually absent. This matter is mentioned only 

 because several important climatological stations are located on the 

 coast in areas which differ very markedly from the adjacent mainland 

 in the character of their vegetation. This is true of Key West, Cape 

 Hatteras, and Point Flattery. Further details regarding the vege- 

 tational maps will be given in the succeeding pages. 



