568 CORRELATION OF DISTRIBUTIONAL FEATURES. 



Sapindus, but the values of all three of the moisture ratios are nearly 

 the same. The amplitudes of sunshine duration do not overlap at all, 

 and those of the moisture-temperature index are very dissimilar. 



The southern limit of Populus balsamifera and the northern limit of 

 Sapindus marginatus have such a direction as to indicate that both 

 trees are controlled by temperature. The edge of the area of the 

 former corresponds closely to the line for 30 hot days in the normal 

 year, although the tree extends far enough south to encounter 88 days 

 at the edge of its range, at Toledo, Ohio. The area of Sapindus is 

 limited in central Kansas by an average frostless season of slightly 

 less than 180 days, by slightly more than 60 cold days, and by a physio- 

 logical summation of 10,000. The position of the northern lunit of 

 Sapindus is so placed, however, as to indicate very clearly that its 

 range is determined by the interaction of temperature and moisture 

 conditions in such a manner as to require a detailed investigation based 

 on a more accurate knowledge of the distribution of the tree than is 

 yet available. The arid conditions of the Grassland and Desert regions 

 apparently limit the western extension of both Populus and Sapindus. 



TtM*CR*TUtlC 

 P*VS IN NOKMOI. FROSTLtS* SEASON (F. S.> 



• Hot D»»». F. S 

 Cold D*ys, F. S. 

 Phvsioloqical Summation. F. S. 



Normal Oailv Mean, colocst 14 oats or Year F^-"-"^' 

 Normal Daily Mean, Vear 

 Precipitation 



Normal Dailv Mean. F. S. I , .„„.„.. n„n..,:„..,„„^,„„„,,,.zrr^7^ 



Oats in longest Normal Rainv Period, F. S. ^^T^^^ggggggjogg^ 

 Days in longest Normal Dry Perioo. F. S. 



Mean Total. Year H -^V""""""" \m'MM U mW' m i 



Evaporation 

 Daily Mean. 1687-8, F. S. I „ . ■„ ,,■.,■,,..,,■, . ,i^^^,,|g,„j„^^,„^^^„^^-.^a: 



Moisture Ratios 

 .Normal P/E, F. S. 

 Normal it/E. F. S. 

 Normal P/E, Year 



Humidity 

 Normal Mean. F. S. 



Sunshine 

 Normal Daily Duration. F. S. 



Moisture-Temperature Indices 



-111" 1 1 > n 1 1 n > ^^^^^^^^^^^^^^^^g^ 



vnnrrnnn ith ■ 



NORMAL P/E X T, F. S.. PHYSIOtOG.eAL METHOD I -^""•" •• n„ .^g^^ 



Fig. 73. Climatic extremes for Popiilus balsamifera (shaded) and Sapindus marginatus (black) 



Comus canadensis and Spermolepis echinatus (fig. 74). — ^These plants 

 are examples of species with northern and southern transcontinental 

 distributions, respectively, and then- climatic extremes have been 

 placed together in the same diagram for comparison. Comus ranges 

 from northern California and the Sierra Nevada through the Rocky 

 Mountains and the Black Hills to the extreme Northeastern States, 

 being nearly coextensive with the Northern Mesophytic Evergreen 

 Forest (plate 30). Spermolepis ranges from central California through 

 southern Arizona and western Texas to Arkansas, western Tennessee, 

 and western Florida (plate 31). 



