No. 512] PRESENT PROBLEMS IN PLANT 



sumptions long- since proved untenable 

 experimental evidence. In other wore 

 of local plant formations which has cons 

 upon changes in the concomitant actn 

 climatic and historical factors, is made t 

 unit whose fundamental basis is a sing 

 Himatic factors. It is to be noted furth 

 Ion, most debated in connection with zo 

 is the boundary; that the term "zone" ir 

 of structure and homogeneity of compi 

 most striking fact about the geograph' 

 individual species is their dominance in 

 their decline in importance and frequei 

 thence in any direction. Plant format 



pears that the optimum areas of scores 

 species nearly coincide. In brief, actu^ 

 tion, through its lack of uniformity anc 1 

 tendency to concentric dispersal, and t 

 the optimum areas of many species, 

 larger units in harmony with the pr: 

 composition and origin of their compc 

 we choose to call them "centers of disp 

 moment, as compared with the recognith 

 zones and their sub-divisions are not 

 tions of plants or plant formations, 

 criticism does not refer to the use of th 

 expressions of certain spatial and temp 

 viz., torrid, temperate and polar zones. 



The unsatisfactory character of zon 

 classification is felt also by studentf 

 Especially is this true of the classificati 

 types prevailing on continents. The 

 (1892-3), Koppen (1900), Supan (1903 

 (1905) are especially interesting in 

 Further, the provinces pointed out by 



'Ward, R. DeC. "Classification of Climates," 

 38: 401^12, 465-477, 1906. 



