THE EVOLUTION OF HERBACEOUS PLANTS 



293 



because in number of species herbs are fully equal to woody 

 plants. 



From phy to geography. — That the differentiation and refrigera- 

 tion of climate in the temperate zones, particularly in the north 

 temperate land mass, have been the chief factors in the develop- 

 ment of herbs is indicated by the present distribution of these plants 

 over the earth. In temperate regions, which are subject to winter 

 temperatures considerably below freezing, from 75 to 85 per cent 

 of the dicotyledonous plants are herbaceous in habit. In arctic 

 and alpine regions, which are still colder, from 85 to 90 per cent or 

 more of the dicotyledons are herbs. The relative proportion of 

 the two types is precisely reversed in tropical countries and under 

 climates where freezing never occurs. In such regions only from 25 

 (or less) to 40 per cent of the dicotyledons are herbs. Table I 

 presents an analysis of the dicotyledonous vegetation in several 

 typical temperate, arctic, alpine, and tropical regions.' 



TABLE I 



Region 



Northeastern United States 



Rocky Mountains 



EUesmereland 



Great Britain 



Germany 



Switzerland 



Iceland 



Brazil 



British West Indies 



Tropical Africa 



Java 



Number of 

 Species 



Number of 

 Herbs 



Percentage of 

 Herbs 



2,280 



2,206 



76 



927 



1,117 



1,899 



221 



15,981 



2,249 



8,577 

 3,188 



1,748 

 1,910 



71 

 821 



947 

 1,726 



200 

 4,092 



675 



3,560 



867 



76 

 86 

 93 



90 

 90 



25 

 30 

 41 

 27 



This evident adaptation of herbs to life in regions subject to 

 low temperatures at certain seasons makes it very probable that 

 the advent of a cold winter has indeed been the chief factor respon- 

 sible for their origin, for there is a fairly constant relation between 

 the minimum winter temperature of a region and the proportion of 



^ In all of the tropical regions of which the floras are tabulated here, there are 

 more or less extensive upland or mountain areas which possess a relatively temperate 

 climate and hence a fairly high proportion of herbs. In the tropical lowlands in 

 each case, however, only from 10 to 15 per cent of the dicotyledons are typically 

 herbaceous. 



