DEPARTMENT OF BOTANICAL RESEARCH. 71 



trees, how this abundance shades off in all directions, and what are 

 the outermost limits of this group of trees. In another case the range 

 of buffalo grass is shown so as to indicate the region in which it is a 

 dominant element in the vegetation, the region in which it is frequent, 

 the region in which it is rare, and the lunit beyond which it is not 

 known to occur. Under the third heading have been secured the 

 geographical ranges of a number of trees, shrubs, grasses, and other 

 plants, selected to represent the different types of distribution common 

 in the United States — or, in the case of certain aquatics, selected because 

 of their independence of the moisture factors in distribution. 



A total of 115 distributional areas were worked out under these 

 three headings and drawn on large-scale maps. Overlays were made 

 from these maps, and used in connection with climatological maps 

 drawn to the same scale and bearing the readings of all the stations. 

 Of the 38 climatological maps, 31 were used in this manner, and the 

 maximum and minimimi values of each factor were secured for each 

 of the vegetational areas, making a total of 3,565 sets of values. This 

 operation has yielded, in brief, a set of 31 climatic values for each of 

 the 115 vegetational areas. For each type of vegetation or for each 

 individual species it is possible to state the maximum and minimum 

 values of each temperature, moisture, humidity, or other factor 

 characterizing that area. 



The data secured in this manner serve to show, for example, the 

 highest and the lowest minimimti temperatures recorded at any of the 

 stations in the grassland, or Great Plains, area; or to show the highest 

 and lowest rates of evaporation that have been found to occur in the 

 range of the sage-brush. These maximum and minimum values are, 

 in short, the particular intensities which seem to Hmit the occurrence 

 of the particular vegetation or plant under consideration, in so far as 

 such a geographical correlation may be taken in lieu of experimental 

 evidence. The maximum and minimum values will be presented in 

 tabular form, and the total range of conditions within the distribu- 

 tional areas will be shown in graphic form, so that it will be possible 

 to use either of our sets of basic data in extending this somewhat pre- 

 liminary investigation with respect to other climatic features and other 

 plants or vegetational areas. 



Our results have also served to indicate which of the climatic features 

 are the most critical in determining particular cases of distribution. 

 This phase of the investigation has been prosecuted by a comparison 

 of the climatic and vegetational maps themselves, and by a search 

 for the coincidence of isoclimatic lines with distributional limits. In 

 this manner the moisture ratios have been found to show a close rela- 

 tion to the distribution of the leading plant formations of the United 

 States. While certain of the temperature features appear to be critical 

 in limiting the ranges of some of the individual species, the controls 



