The Problems of Plant Distribution 267 



the presence of these imported plants in nearly every 

 part of the world. Few persons would imagine that 

 the daisies, dandelions, and buttercups, which span- 

 gle the meadows and lawns of our Eastern States 

 are probably all of them European immigrants, and 

 that the thistles and burdocks along the roadsides 

 are likewise aliens. Railways and ships spread the 

 seeds everywhere, and when the conditions are fa- 

 vorable, the newcomers quickly adjust themselves to 

 their new home. 



Barriers to Plant Distribution. The great nat- 

 ural barriers to plant migration are deserts and high 

 mountains, and large bodies of water, like the 

 oceans. The presence of over a thousand miles of 

 desert and mountains between the Mississippi and 

 the Pacific Coast largely explains the striking dif- 

 ferences in the vegetation of California and that of 

 the Atlantic States. 



While mountains are the barriers which prevent 

 the passage of many plants, they may also be high- 

 ways along which plants travel. Thus the great 

 ranges of mountains running north and south, per- 

 mit the southward migration of northern plants, and 

 the northern migration of antarctic ones. Ascend- 

 ing higher and higher as they go southward, many 

 arctic or north temperate plants have established 

 themselves on the mountains far southward of their 

 original habitat. 



