l82 HEREDITY AND EVOLUTION IN PLANTS 



dicotyledonous herbs preponderate in north temperate 

 regions, and woody plants in the tropics. The latter 

 climate probably approaches more nearly to that under 

 which Angiosperms first appeared. Herbs, having a 

 short life cycle (one to two or three seasons) are able to 

 survive periods of intense cold in the form of seeds, and 

 would, therefore, survive in larger numbers than woody 

 plants on the advance and retreat of the continental ice 

 sheet of the Glacial period. This would account for the 

 fact of a much smaller proportion of woody plants in the 

 flora of Europe, for these could not migrate southward, as 

 the ice encroached, since the mountain ranges there have 

 a general east-west trend (in contrast to the general north- 

 south trend of American ranges), and southern migration 

 would necessitate an ascent to high altitudes that would 

 be fatal to temperate or subtropical species. 



The above facts are not cited as established, but only 

 to illustrate a method. There is also evidence and argu- 

 ment suggesting the opposite conclusion, namely, that 

 herbaceous plants are older than woody. 



