236 



PLANT PHYSIOLOGY AND ECOLOGY 



two successive stages of the same succession. The former usually 

 results in a zone of varying width between the formations concerned. 

 When one stage of a succession is gradually replaced by another, 

 the entire habitat is often occupied for many years by a more or 

 less equal mixture of the two. It is usually possible to determine 

 the formations that produce the mixed one, while the relative 

 abundance of their respective species indicates which formation 



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Fig. 95. A mixed formation of aspens and spruces, produced by the com- 

 ing in of the spruces, which will finally replace the aspens. 



plays the most important part. Since mixed formations often 

 persist for a long time, it is often necessary to consider them in 

 detail, very much as though they were distinct formations. In 

 some cases it is probable that new formations have arisen by the 

 permanent mixing of two contiguous ones. 



Experiment 62. Comparison of formations. Make a general study of 

 several formations of the neighborhood. Determine the facias and 

 principal species of each, and find out what species they have in common. 

 Classify them with respect to habitat, distinguishing open from closed 

 ones, and point out any that seem to be mixed formations. 



