CORRELATION OF DISTRIBUTIONAL FEATURES. 503 



both edges of a region with such great north-and-south extent as the 

 Grassland there is abundant opportunity for the interaction of condi- 

 tions of critical importance, or for the influencing of the effects of a 

 critical condition by one that is nowhere of primary critical importance 

 itself. This is well exemplified in the changing values of the moisture 

 ratio which are found to coincide with the western edge of the region. 

 If this composite condition is as critical as its general distributional 

 relations seem to indicate, we have here a case in which the vegetation 

 of the Grassland is able to endure lower and more exacting values of 

 this condition at the north than it is at the south, owing to the differ- 

 ence between the temperature conditions at north and south. On the 

 eastern edge of the Grassland there is a similar change in the apparent 

 controlling condition, but in this case we have the Grassland extending 

 into the region possessing the same values for the moisture ratio that 

 are found in the forested regions. It is possible that this is due to the 

 inter-operation of temperature conditions and those expressed by the 

 moisture ratio. Such a possibihty is indicated by the fact that the 

 values of the moisture-temperature index which are found in the 

 Grassland also extend eastward in the Northern States. 



Grassland Deciduous-Forest Transition (fig. 30). — A comparison of 

 the temperature blocks in figures 29 and 30 will show that the Transi- 

 tion region is very similar to the Grassland in the limits and ampli- 

 tudes of its temperature conditions. The principal divergence lies in 

 the fact that the frostless season is shghtly narrower in its amphtude 

 in the former area, and does not reach such low values. A comparison 

 of the precipitation conditions shows a difference in each case in the 

 direction of more moist conditions for the Transition region, although 



TcmmiATuftC 

 D*ra IN Normal rnoaTica* SUMN <F. SJ 

 Hot Oat*, F. S. 

 Colo Oats. F. S. 

 PHvsroLOGicAi. Summation. F. S. 

 Normal Dailt Mcan, coldcst XA o«ts or Vc«n 

 Normal Oailv Mcan, Vcau 



Prcciritation 

 Normal Oailv Mcan. F. S. 

 Days in lonccst Normal Rainy Period. F. S. 

 Oats in longc«t Normal Dry PcRioe. F. S. 

 Mcan Total. Ycar 



Evaporation 

 Daily Mcan. ie87>8, F. S. 



Moisture Ratios 

 Normal p/E, F. S. 

 Normal fr/E, F. S. 

 Normal P/E. rcAii 



Humidity 

 Normal Mean, F. S. 

 ; Sunshine 

 Normal Oaily Ouration. f. S. 



Moisturc.Tcmrcraturc tNDICCV 

 Normal P/E « T, F. 8., Phy*iolo«ic«i Metmoo 



Fig. 30. Climatic extremes for the Grassland Deciduous-Forest-Transition. 



