CHAPTEE XXXV 

 PLANT SUCCESSIONS* 



431. Nature of plant successions. Whenever a portion of 

 the earth's surface is stripped of its vegetation, or undergoes 

 any decided change in its physical condition, the way is usually 

 opened for invasion of plants from the surrounding territory 

 (Sec. 427). In most cases the immigrants are not all of them 

 thoroughly adapted to their new home, and cannot become so ; 

 or the condition of the territory may continue to change, so 

 that a series of new populations appears, each in turn wholly 

 or partly giving way to that which follows it. Such a set of 

 colonizings is called a plant succession. 



432. Causes of successions. It would require too much 

 space to state more than a very few of the causes which 

 originate plant successions. 



First. They may be brought about by the introduction into 

 a region of new species which are able, without change of soil 

 or climate, to drive out some or all of the original occupants 

 (Sec. 428). 



Second. They may be brought about by changing the supply 

 of light, heat, water, or other important factors in the surround- 

 ings of the plant. Such changes are sometimes natural, some- 

 times produced by man. 



Such a river as the Mississippi, with over 12,000 square 

 miles in its delta, affords a good instance of the power of natural 

 agencies to alter the conditions of plant life. Perhaps one third 

 of the delta is a sea marsh, with the vegetation characteristic of 



* To THE INSTRUCTOR : As this chapter is somewhat more technical than 

 many of the others of Part III, it may be omitted if limitations of time 

 demand a briefer course. 



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