104 THE AMERICAN BOTANIST 



jjlants far beyond their known limits are now matters of fre- 

 quent occurrence. Classic illustrations of the extensive spread 

 of species in liistoric times may be found in the behaviour of 

 the prickly pear in Australia, of the water-weed {Elodea) in 

 Europe and of the prickly lettuce and Russian thistle in our 

 own country. 



Sooner or later, the most persistent species must reach 

 a barrier that limits its further progress. Such barriers are 

 naturally most diverse and their importance often depends 

 upon the species, for what constitutes an insuperable barrier 

 to one species may prove to be no hindrance to another. An 

 extensive forest may bar the spread of plants that thrive on 

 plains or prairies, and a desert or large body of water be 

 an efficient barrier to woodland forms. Indeed, the desert 

 might limit the forest while offering new regions for colon- 

 ization by plants of the plains. 



The effect of temperature on the distribution of species 

 is a familiar one, for we see a cold period annually put an 

 end to many kinds of tender vegetation and realize that the 

 northward extension of any species must ultimately be limit- 

 ed by the cold. That an increase in temperature may prove 

 harmful to other species is not so well known but a consider- 

 ation of the [)lants of the world show an immense number that 

 cannot endure any great increase in temperature and when 

 brought into milder regions die because the weather is too 

 warm. We may even account for the fact that the tree flora 

 of Europe is not as rich as our own by a matter of temper- 

 ature, for it is known that the last great ice-sheet which cover- 

 ed the northern hemisphere pushed the plants southward until 

 they were stopped by the mountain chains across their path 

 and thus obliged to perish, while in our own country the north 



