XIV. MESEOSERE AND PALEOSERE. 

 THE MESOPHYTIC ERA. 



The flora.— The Mesophytic was as truly intermediate in the character of 

 its flora as it was in its position. Not only did Paleophytic types persist in 

 it to a considerable degree, but the dominant vegetation of the Cenophytic 

 also began in it and reached a marked development before its close. Moreover, 

 the flora was intenmediate from the phylogenetic standpoint as well, since the 

 dominant vegetation of the era was gymnospermous, in contrast to the fern- 

 worts and flowering plants of the Paleophytic and Cenophytic respectively. 

 The Mesophytic may well be regarded as a great transition era between the 

 characteristically dominant vegetations of the Paleophytic and Cenophytic. 

 It saw the loss of dominance of pteroid types, the rapid development and ex- 

 tension of gymnospermous types already present or suggested in the Paleo- 

 phytic, and the rise and triumph of the wholly new angiosperms. In floral 

 character, then, the Mesophytic was preeminently a time of great changes, 

 as a consequence of which there must have been a great mixing of types and a 

 corresponding confusion of successional dominants. 



It seems probable that the great reduction and practical disappearance of 

 Paleophytic pteroids during this era were connected with the evolution and 

 assimaption of dominance by the Cenophytic angiosperms. They both appear 

 to be the consequences of the same great event, namely, the Permian-Triassic 

 deformation cycle, with its widespread glaciation and aridity, and its many 

 glacial-interglacial cycles. Though less marked in some respects than Pleis- 

 tocene glaciation, the consequence of the Permian glacial period upon the flora 

 were much more far-reaching. This was due to the fact that the relatively 

 primitive flora of the Pennsylvanian possessed greater possibilities of evolution 

 than did that of the PHocene. Moreover, the pteroid flora must have found 

 effective migration and ecesis more difficult than the later angiosperms did, 

 and hence persistence by shifting was much less possible. Furthermore, 

 Permian glaciation appeared in the midst of an almost universally uniform 

 tropical flora, and its destructive effect must have been vastly greater than 

 that of the Pleistocene upon a flora which had already been strikingly differen- 

 tiated and adjusted to temperate climates. The very factors which are 

 thought to have led to the reduction and disappearance of the femwort types 

 and the evolution and final dominance of angiosperms would doubtless have 

 favored the rapid assumption of dominance by gymnospermous types, which 

 were already under way in the Paleophytic. In short, the gymnosperms may 

 be said to mark a transition from a cryptogamic to a phanerogamic flora, for 

 the same reason that the gymnospermous habit is intermediate between that 

 of the fernworts and the angiosperms. 



Composition of the flora. — ^Approximately 125 genera are known for the 

 Mesophytic era in North America. Of these, 50 are gymnosperms, while 

 fernworts and flowering plants are about equally represented in the remainder. 

 Of the gymnosperms, Abietites, Anomozamites, Araucaria, Araucarioxylon, 

 Araucarites, Arthrotaxopsis, Baiera, Brachyphyllum, Cedrus, Cephalotaxopsis, 

 Cycadella, Cycadoidea, Cycadeospermum, Cycadites, Dioonites, Encephalartopsis, 



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