164 DIRECTION OF DEVELOPMENT. 



successions rather more readily. The only objection to be urged against them 

 is that their use tends to suggest that some process other than normal succession 

 is concerned. Used as synonyms of succession, they are unobjectionable, 

 though as a consequence, they are also of little value. 



Correlation of progressive developments. — While all successional develop- 

 ment is progressive, the concrete seres of every cUmax formation may bear a 

 direct relation to the whole course of development. This is fundamentally true 

 of the seres which arise in primary and secondary bare areas and hence are 

 distinguished as primary and secondary seres. The one recapitulates the entire 

 succession, the other repeats only more or less of its later sequence. Seres, 

 moreover, show an essential difference with respect to the direction of reaction, 

 depending upon the nature of the extreme conditions in which they arise. 

 Primary seres may arise on rock or in water, or they may develop on new soil, 

 such as that of dunes or bad lands. While secondary areas do not depart so 

 widely from the climatic mean, they may also be xerophytic or hydrophytic. 

 Though often mesophytic, they axe always drier or wetter than the climax area. 



The basic developmental relation of every sere is indicated by the terms 

 prisere and svbsere. The one is a concrete example of primary succession, 

 the other of secondary succession. Since they mark a fundamental distinction 

 in the development of a climax formation, their further treatment is deferred 

 to the chapter upon classification. 



As water-content is the controlling factor in all succession, either directly or 

 indirectly, it furnishes the best basis for indicating the direction of movement. 

 This arises from the fact that it represents the primary interaction of habitat 

 and community in the course of development. In the origin of every sere, 

 the amount of water is the critical factor, and the rate and direction of develop- 

 ment will be recorded more or less clearly in its increase or decrease. There 

 are in consequence three possible bases for distinguishing direction in terms of 

 water-content. These are (1) the actual direction of movement itself, (2) the 

 initial condition, (3) the final condition. It is of interest to note that all of 

 these have been used. Clements (1904:124; 1905:267) made use of the 

 actual successional change in water-content, as well as the final term: 



"The direction of the movement of a succession is the immediate result of 

 its reaction. From the fundamental nature of vegetation, it must be expressed 

 in terms of water-content. The reaction is often so great that the habitat 

 undergoes a profound change in the course of succession, changing from hydro- 

 phytic to mesophytic or xerophytic, or the reverse. This is characteristic of 

 newly formed or exposed soils. Such successions are xerotropic, meaotropic, 

 or hydrotropic, according to the ultimate condition of the habitat. When the 

 reaction is less marked, the type of habitat does not change materially, and 

 the successions are xerostatic, mesostatic, or hydrostatic, depending upon the 

 water-content. Such conditions obtain for the most part only in denuded 

 habitats." 



Cooper (1912 : 198) has made the initial conditions the basis of classification: 



"The plant successions leading up to the establishment of the climax forest 

 are conveniently classified in two groups: the xerarch successions, having 

 their origin in xerophytic habitats; and the hydrarch successions, originating 

 in hydrophytic habitats." 



