VEGETATION ERAS. 



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the name of the last era, in order to have the names of the geologic and botanic 

 eras exactly parallel. The subdivision of vegetation eras into periods differs 

 but little from the current division, and in the present state of our knowledge 

 at least, the retention of the latter is desirable. 



Relation of vegetation era and eosere. — In terms of development, a vegeta- 

 tion era is that portion of geological time marked by an eosere . A vegetation era 

 is characterized by the dominance of a distinct population, i. e., pteridophytes, 

 gymnosperms, or angiosperms. An eosere is the total development of a par- 

 ticular vegetation throughout this period of dominance. In the case of each 

 of the three eras of great land vegetations, namely, Paleophytic, Mesophytic, 

 and Cenophytic, it is true that the characteristic flora appeared before the 

 beginning of the era. The relative unimportance of the evolution or presence 

 of a flora in the constitution of eras, as compared with its dominance, is well 

 shown by the fact that Paleophytic and Mesophytic floras have continued to 

 the present time, but with a negligible effect in the dominance of existing 

 vegetation. 



CoMPABisoN OP Geologic and Vegetation Eras. 



Ceneosere or angeosere. 



Meseosere or gymneosere. 



Paleosere or ptereosere. 



For each of the three vegetation eras there is a major successional develop- 

 ment represented by the eosere. The geosere. which comprises the sum total 

 of vegetational development, falls into three eoseres, viz, Paleophytic, Meso- 

 phytic, and Cenophytic. Since each eosere is a definite organic process termi- 

 nating in the characteristic vegetation then dominant, it is often desirable to 

 distinguish them by means of this dominance. As the dominant vegetation 

 of the Paleophytic era was pteridophytic in the large sense, the corresponding 

 eosere may well be termed the ptereosere. Similarly, for the Mesophytic and 

 Cenophytic eras with their Gymnosperms and Angiosperms, the terms are 



