18 MISC. PUBLICATION 303, U. S. DEPT. OF AGRICULTURE 



Climate, which is an expression of temperature and moisture con- 

 ditions, is without question of first importance. It is, in fact, a 

 summary of factors pertaining to water and to heat. Such factors 

 may be further subdivided into amount and distribution of water 

 or heat; or where the two interact the terms of the analysis are 

 evaporation, kind of precipitation, and humidity. 



Soil, which involves both chemical and physical structure, is un- 

 doubtedly of second importance. Plant distribution within areas of 

 uniform climate may be modified within each area by soil factors, 

 but it seems obvious that the climate plays the major role in pre- 

 venting vegetation from spreading. The interaction of soil and 

 climate (the latter is often a contributing factor in the formation 

 or modification of the former) may be of great importance. With 

 sufficient rainfall, for instance, the leaching out of certain bases may 

 produce an acid or neutral soil ; with insufficient rainfall the soil may 

 be alkaline. 



Both the preceding factors may operate provided sufficient time has 

 elapsed. A newly-formed species must have time to spread before 

 its range can be delimited by changes in climate or soil. If a species 

 is young, its climatic and edaphic boundaries may not yet be operat- 

 ing. In its distribution, time is still a very important factor. If a 

 species is old enough so that its further distribution is limited by 

 factors of climate or soil, time has ceased to be an important factor 

 in its present distribution. 



It is true that climatic and soil factors may be uniform or may 

 change very slightly over a given area of country, as in any of Mul- 

 ford's plant-growth regions. Knowing empirically that a number of 

 cultivated plants succeeded about equally well throughout such a 

 growth region, we are reasonably safe in assuming that in general 

 the range of a native species can coincide with boundaries of such 

 growth regions provided it has had the necessary time to spread. 

 It is significant that where there is a large number of endemics and 

 where great variation in characters is evident the growth-region 

 boundaries do not coincide at all well with limits of distribution, but 

 where there are few endemics and variation is much less, the bounda- 

 ries coincide much better. 



The correlation of Mulford's plant-growth regions with natural 

 vegetation areas has as yet not appeared in print. The present 

 author has been interested in the; idea for the last 5 or 6 years, has 

 checked a few species with care, and a/ great many in a general man- 

 ner. The subject cannot be dealt with adequately here, but it can 

 be said that in a general way correspondence of the lines bounding 

 the range of a given woody species with the lines bounding Mulford's 

 plant-growth regions! is very close. Correspondence of the growth- 

 region lines with climatic and soil boundary lines is also close, where 

 either the temperature, precipitation, or soil condition, or any com- 

 bination of them, acts as a limiting factor in plant distribution. 



A line bounding a growth region may be considered to consist of 

 series of attacheof shorter lines, each representing a change in soil 

 condition, in average precipitation, in average temperature, or some 

 other change displayed by plant distribution. On the map (fig. 1), 

 the lines must be considered not as sharply delimiting one region, 

 but as being drawn to separate two regions differing in one or more 



