CHAPTER THIRTY-TWO 



THE DISTRIBUTION OF PLANTS IN NATURE 



The distribution of plants in nature is determined by the 

 hereditary qualities of the plant on the one hand and by the 

 characteristics of the environment on the other. Some plants, 

 like the dandelion and sheep sorrel, have such indefinite require- 

 ments that they can thrive in, or at least endure, the conditions 

 in many different habitats. Most plants, however, have a far 

 more definite set of requirements, and if any one of these require- 

 ments is not met by a habitat, the plant is excluded from the 

 habitat. Orange trees, for example, cannot withstand freezing 

 temperatures; the low creeping Arctic willows will not grow 

 where the summer season is hot; cacti are excluded from soil 

 that is wet and poorly aerated for even a few weeks each year ; 

 cat-tails die on land that is not submerged at least a part of each 

 year; sycamore, poplar, and willow seedlings will not thrive in 

 dense shade ; alfalfa and certain species of clover thrive only in 

 well-drained soils that are neutral or slightly alkaline (many 

 limestone soils furnish both these conditions) ; rhododendrons, 

 azaleas, blueberries, and cranberries grow well only on soils that 

 are acid — they soon die on limestone soils. 



In general, it may be said that plants requiring similar environ- 

 mental conditions are restricted to certain regions of a continent 

 and to certain habitats within these regions, because there only 

 are environmental conditions suited to their hereditary struc- 

 tures and qualities and favorable to their complete development. 



Vegetation. By vegetation is meant the plant covering of the 

 earth or of its subdivisions. The plant covering of any region 

 is a great organization of hundreds, perhaps millions, of individ- 

 uals. Some of these are dependent only upon the conditions 

 determined by climate and soil. Some are also dependent upon 

 nitrate supply, water supply, or the presence of certain other 



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