CHAPTEK XXXVII 

 PLANT ASSOCIATIONS; ZONATION* 



451. Plant associations. One of the first things which the 

 young field botanist learns is the fact that the distribution of 

 plants depends largely on the character of the ground they occupy. 

 There is in any small territory, such as a county, for example, 

 one assemblage of plants for the waters of ponds and another 

 for their shores, one for swamps, one for moderately diy uplands, 

 one for very dry hilltops, and so on. The aquatic plants of the 

 sea are very different from those of fresh water. Sandstone and 

 limestone soils have vegetations peculiar to themselves ; J the 

 long-leaved pine, the scrub pine, and the chestnut are character- 

 istic trees of sandy soils, while most of the oaks, the hackberries, 

 and the black walnut are generally found in limestone regions. 



The collection of plants as found in any given kind of station 

 or habitat, especially when prominent and well defined, is called 

 an association. Thus we have marine aquatic associations, sea- 

 beach associations, pond associations, bog associations, sand- 

 hill associations, meadow associations, heath associations, forest 

 associations, and many others. 



452. Subdivisions of plant associations. Usually the plant 

 association is divisible into assemblages, which are much more 

 alike in their vegetation than is the association as a whole. Thus 

 a woodland association may consist of pine patches, oak patches, 

 and birch patches. A grass-land association may consist of 



* To THE INSTRUCTOR : If it is necessary to cut down the discussion of 

 these topics to little more than definitions, only the first three sections of the 

 chapter need be read. 



1 Perhaps this is sometimes due to physical rather than to chemical causes. 

 In other words, the chemical differences in soils are usually accompanied by 

 differences in their porosity, their capacity for retaining water, for absorbing 

 heat from the sun's rays, and so on, which greatly modify their effect on plants. 



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