182 CLASSIFICATION OF SERES. 



prove desirable to shorten these by using merely the initial of the specific 

 name, e. g., Picea a. hylion, Picea c. hylion, etc. 



Priseres and subseres. — ^Within the same climax, seres are classified as pri- 

 mary and secondary, i.e., as priseres and subseres. The fundamental value of 

 this developmental distinction has been sufficiently dwelt upon. In the actual 

 study and classification of the seres of any climax, the significance of the dis- 

 tinction will be obvious in the vast majority of cases. Subseres will be found 

 chiefly confined to bare areas due to superficial and usually artificial disturb- 

 ance, especially as a consequence of man's activities. They are much more 

 numerous than priseres, and are much more readily investigated, since the 

 persistence of the preceding reactions causes succession to go forward rapidly. 

 In the grassland and forest stages the associes are often normal, and thus 

 throw much light upon the slower but corresponding stages of the primary suc- 

 cession. Priseres are typical of the three extreme areas, water, rock, and sand, 

 in which no effective reaction has occurred. In the case of sand, this is 

 perhaps true only of primary dunes, in which the extreme condition due to 

 complete lack of humus and to low surface water-content is reinforced by the 

 great instability. Priseres are relatively infrequent in great midland regions 

 of forest and grassland, but they are sufficiently common to furnish a reliable 

 comparative basis for the study of succession. In lowland and montane 

 regions examples of priseres are often more numerous than those of subseres, 

 and such regions are of the first importance for serai investigations (plate 

 51, A, b). 



Hydroseres and xeroseres. — It has already been suggested that the water- 

 condition of the initial area furnishes a better basis for the subdivision of 

 priseres and subseres than does the water-content of the climax. A complete 

 classification upon the basis of water relations would require a primary division 

 into hydrotropic, xerotropic, and mesotropic seres, but the latter are so over- 

 whelmingly predominant in the present state of our knowledge that they alone 

 demand consideration. With increasing study of desert and tropical succes- 

 sion it is probable that the direction of the water-reaction will assume its 

 basic importance. At present, however, it is most convenient to regard seres 

 as primarily mesotropic, and to distinguish them as hydrarch and xerarch in 

 accordance as they arise in wet or in dry areas (Cooper, 1912 : 198). For the 

 sake of uniformity in classification, the corresponding terms hydrosere and 

 xerosere are here proposed. 



In the case of subseres, extreme conditions of water-content are rare or 

 they persist for a brief period only. Hence it is sufficient to recognize but 

 the two subdivisions, hydrosere and xerosere. With priseres the extremes are 

 marked, and the quality of the water-content often becomes controlling also. 

 As a consequence, it seems desirable to distinguish hydroseres, as haloseres 

 (Gr. aXs, AXos, salt) and oxy seres (6?us, acid),, with the corresponding terms, 

 halarch and oxarch. It must be recognized, however, that haloseres and 

 oxyseres are properly adseres, since they depart from the normal development 

 only for a portion of the sere. Moreover, while the surfaces of rock and of 

 dune-sand may be almost equally dry, the differences of hardness and stability 

 result in very dissimilar adseres. These may be distinguished as lithoseres 

 (Gr. \Wos, rock) and psammoseres (Gr. ^omaws, sand), or as Utharch and 

 psammarch. Finally, hydroseres and xeroseres may be also distinguished 



