286 PLANT PHYSIOLOGY AND ECOLOGY 



attains a maximum in the intermediate ones, and again 

 decreases in the ultimate formation, on account of the 

 dominance of a few species. 



4. The normal sequence of vegetation forms in succession 



is: (1) algffi, fungi, and mosses, (2) annuals and bi- 

 ennials, (3) perennial herbs, (4) bushes and shrubs, 

 (5) trees. 



5. The number of individuals of a species increases con- 



stantly to a maximum for each stage, and then gradu- 

 ally decreases as the next stage develops. 



6. A secondary succession does not begin with the initial 



stage of the primary one that it replaces, but usually 

 at a much later stage. 



7. At present successions generally tend to end in meso- 

 phytic formations, grassland, or forest, though many 

 remain xerophytic or hydrophytic. 



8. The operation of succession must have been essentially 

 the same during the geological past that it is to-day. 



307. The study of succession is carried on by the use of 

 instruments for determining the physical factors of the habitat 

 and the reaction of the stages upon them, and by the use of quad- 

 rat, transect, and migration circle to discover and record the 

 changes in structure during the various stages. It is usually 

 impossible to apply these methods to the initial stage of a suc- 

 cession and then to the succeeding stages, owing to the length 

 of time taken by the development. A few secondary successions 

 run their course within a few years, but ordinarily the period is 

 much longer, especially in primary successions, where it may 

 reach several hundred years. Since it is rarely possible to 

 follow a succession from beginning to end, it is necessary to em- 

 ploy indirect methods of determining its course. The three 

 methods used for the purpose are: (1) the method of alternating 

 stages, (2) the relict method, (3) the experimental method. 

 By the combined use of these, it is possible to reconstruct a suc- 

 cession with something of the same detail and accuracy that 

 could be oljtained by following it from beginning to end. 



308. Method of alternating stages. The same succession often 

 starts at the same time or at different times upon several areas 

 of similar nature. When the start is made at different times, 

 the various areas show different stages in development. If one 



