394 CORRELATION OF DISTRIBUTIONAL FEATURES. 



values for the center, and also for each of the zones, of diminishing 

 abundance. 



The discovery of close relationship between the position of plant 

 boundaries and isoclimatic lines has been made by a mere comparison 

 of the vegetational and climatic maps, checked by the use of the vege- 

 tational overlays on the climatic maps. 



It is true of both sets of the maps which embody our basic data that 

 some are constructed from much fuller figures or information than 

 others. Over 1,600 stations have been used for the map of absolute 

 minimum temperature and over 1,200 stations for the map of average 

 length of frostless season, whereas, on the other hand, the map showing 

 the annual total evaporation is based on only 139 stations, and that 

 showing the normal mean annual precipitation is based on Gannett's 

 chart, showing no readings for individual stations and smoothed to 

 exhibit the means by 10-inch increments. The most poorly determined 

 plant ranges are those of Floerkea occidentalis, based on four published 

 occurrences, and of Trautvetteria grandis, based on sixteen occurrences, 

 nine of which are in western Washington. The most satisfactory plant 

 ranges are those of the southeastern species of pines, worked out by 

 Mohr, and those of Pseudotsuga mucronata, Pinus ponderosa, P. con- 

 torta, and other western and eastern trees, worked out by the United 

 States Forest Service. 



A large percentage of our botanical areas extend beyond the geo- 

 graphical limits of the United States, and this circumstance has proved 

 particularly unfortunate in connection with our efforts to express the 

 cUnmtic extremes characterizing their hmits. Nearly all of the 

 chmatic factors also reach higher or lower values in Canada or Mexico 

 than are shown for the United States. In the table showing the 

 extreme values of the cUmatic data for the United States (table 152) 

 the bold-faced figures indicate the cases in which the highest or lowest 

 possible values are found in this country. These cases are only 11 out 

 of a total of 62 extremes, for the 31 climatic features. 



In a number of cases the extreme values of the climatic indices are 

 found to occur on capes or coastal islands. The smallest number of 

 dry days is found at Cape Hatteras, as well as the highest value of 

 the physiological moisture-temperature index; the highest normal 

 daily mean precipitation and the highest values of the moisture ratios 

 are found at Cape Flattery, and the highest temperature summations 

 at Key West. Nineteen out of the 62 extremes have been derived 

 from stations of this character. The fact that all of these coastal 

 stations are situated where vegetation is extremely sparse and very 

 different from that of the adjacent mainland, has led us to attribute to 

 some of our botanical areas extreme values which may be shghtly too 

 high or too low. We have considered, however, that the clinmtic 

 conditions of these coastal stations are very nearly like those that are 

 endured by the nearest bodies of vegetation growing on the mainland, 

 and that they probably represent the extremes for this vegetation 



