64 THE STORY OF PLANT LIFE 



favourable to the extension of range of a species, 

 plants tending to travel along such barriers and to 

 penetrate new regions. 



When a flora is isolated it may retain primitive 

 characters and form a characteristic type of vegeta- 

 tion. 



When w^idely diffused a flora may exhibit great 

 range of variation, in so much that the original 

 centre of dispersal may become obscured, and 

 different centres of diffusion may in the same cir- 

 cumstances tend to intermingle and to become 

 fused. 



A main cause of the demarcation of vegetation 

 into special regions is the influence of climate. 

 Owing to the division of the earth into zones of 

 temperature, cold, temperate, and hot regions, we 

 have different types of flora in each. 



The Arctic flora is largely made up of lowly types 

 of cryptogams, such as mosses and lichens, and 

 dwarf hardier trees, and shrubs, sedges, willows, 

 saxifrages, Primulas, etc. When the flora occupies 

 the heights of mountains and is akin to the Arctic 

 flora it is called an Alpine flora, and many Alpine 

 plants are Arctic types, which, brought south in a 

 glacial phase, have become stranded on hills and 

 have retreated as the temperature was raised with 

 the retreat of the ice. 



Other types of plants may have been forced to 

 retreat to lofty hills owing to the intense competition 

 in the lowlands. 



