CHAPTER XV 

 ALTERNATION AND ZONATION 



310. The relation between alternation and zonation. The 



structure of all formations, as well as their arrangement into 

 vegetation, rests upon two principles, zonation and alternation. 

 Both of these are concerned in the structure of a formation. In 

 some cases zonation predominates to such an extent that alter- 

 nation is subordinated or becomes inconspicuous. More often 

 the reverse is true. The alternation of various groups is so marked 

 that zonation is completely obscured. Because of its striking 

 character, zonation is usually more conspicuous than alterna- 

 tion. The latter, however, is more fundamental and more uni- 

 versal. 



311. Alternation. This principle deals with the occurrence 

 of a formation at different places in a region or of a plant group 

 or species at separate points in a formation. It covers all re- 

 sponses of vegetation to the unlikeness of the many parts of the 

 earth's surface. Alternation stands in sharp contrast to zona- 

 tion, since it is caused by a lack of symmetry in habitats. It 

 is found in vegetation areas and plant groups of every rank. 

 The breaking up of vegetation into formations is a striking example 

 of alternation. It is also found within every formation, where 

 it is represented by consocies, societies, communities, and families, 

 and gives endless variety to its structure. In brief, alternation 

 deals with the structures and causes which give to vegetation 

 its typical and universal lack of uniformity. 



312. Causes of alternation. The primary cause of alternation 

 is unlikeness in the physical factors of various formations or 

 areas of the same formation. This condition is universally pres- 

 ent in vegetation; it occurs within zones as well as in formations 

 that lack them. Alternation is due also to competition and to 

 invasion, and naturally any two or all of these causes may act 



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