CORRELATION OF DISTRIBUTIONAL FEATURES. 489 



long blocks in the figures, would therefore give a suggestion as to the 

 more important of these two climatic conditions in the control of the 

 distribution of such a hypothetical plant. 



Although the comparative amplitudes of two climatic conditions 

 may serve as indicators of their relative importance in controlhng a 

 stated case of distribution in an area as large as the United States, it 

 needs always to be borne in mind that we are deahng with only a part 

 of the total possible amplitude of the conditions for the entire globe. 

 Our method would be much more useful if we were studying the vege- 

 tation of an area sufficiently large to comprise the greatest known 

 extremes under which plants occur, and it would be of little use in 

 investigating the controlling conditions of a small area. 



It must also be kept in mind that the minimum and maximum values 

 of a condition which shows wide amplitude in a particular distribu- 

 tional area are just as truly the limiting intensities as are those of a 

 condition exhibiting narrow amplitude in the same area. The desert 

 grass Hilaria jamesii, for example, encounters a range of length of 

 frostless season from 72 to 296 days, which is a rather wide amplitude, 

 being nearly two-thirds that for the United States. Although this 

 perennial grass is able to accommodate its vegetative activities to a 

 frostless season which is twice as long in some parts of its range as it 

 is in others, there is still every reason to believe that Hilaria is unable 

 to carry through its development with a frostless season of less than 

 72 days, at least under the conditions which accompany a season of 

 that length in the region in which Hilaria encounters them. It is 

 unable, likewise, to spread into regions with a frostless season of more 

 than 296 days, due undoubtedly to some accompanying adverse con- 

 ditions rather than to too long a season favorable for growth. Although 

 there is a wide range of differences in length of growing-season which 

 have no apparent restricting influence on the distributional movements 

 of Hilaria, we must not allow this fact to obscure the possibility that 

 the ultimate extremes which it encounters are indeed of importance to 

 its limitation. While Hilaria is able to accommodate itself to frost- 

 less seasons of widely differing length, it is able to grow only within 

 narrowly restricted limits with respect to the values of the moisture 

 ratios; it encounters extremely narrow amplitudes of all of these ratios. 

 Its limitation, at different parts of its distributional edge, by frostless 

 seasons of very different lengths, indicates that associa,ted conditions 

 have much to do with whatever influence the length of season may be 

 able to exert. Its limitation by moisture ratios of so nearly the same 

 value means that the influence of this compound condition is but little 

 affected by the various values of associated factors that are to be 

 found on different parts of the distributional limit. This is as much 

 as it is possible to infer from the statement that one environmental 

 condition is more important that another in limiting plant distribution. 



