3o6 General Botany 



The water lily, bulrush, and cat-tail associations of ponds are 

 small communities of plants dominated by these particular plants. 

 The hemlock forest, the redwood forest, the yellow-pine forest, 

 and the oak-hickory forest are all associations of large and 

 small plants of numerous species that thrive under certain 

 conditions. 



Climatic plant formations. In any particular part of the 

 country there are usually many different plant associations. 

 Local differences of elevation, topography, soil, drainage, and 

 slope exposure result in a diversity of local environments. 

 Some plants fit into each of these environments better than 

 others ; consequently in each there arises a local community, or 

 association, of plants. In general it has been found that through- 

 out a region having similar climatic conditions, the group of 

 plant associations is essentially similar. When we extend our 

 study into regions with very different climates, we find a very 

 different series of plant associations. 



The plant associations of Indiana are very similar to those of 

 Ohio because the climate in the two states is very similar. For 

 the same reason the plant associations of Kansas and Nebraska 

 are very similar. However, there is a vast difference between 

 the plant associations of the Ohio-Indiana region and those of 

 the Kansas-Nebraska region. This difference is primarily the 

 result of difference in climate. 



Different groups of plant associations, then, are character- 

 istic of different climates. For this reason it is customary to 

 group associations into larger units called climatic plant forma- 

 tions. The terms ^' evergreen forest," '' deciduous forest," 

 " prairie," "plains," and " desert " show the general recognition 

 of these larger groupings of vegetation that are primarily deter- 

 mined by the. light, moisture, and temperature conditions that 

 make up climate. 



Plant associations not permanent. Students of physiography 

 are familiar with the fact that land forms are constantly changing. 



