406 MESEOSERE AND PALEOSEEE. 



Baiera, Ginkgo, Nilsonia, Pinus, Sequoia, Taxodium, etc., among the conifers, 

 and Cycadites, Cycadoidea, Dioonites, Podozamites, Pterophyllum, etc., among 

 the cycads must have constituted the forests, especially after the early Triassic. 



The climax dominants of the Mesophytic era were conifers and cycads 

 especially after the Permian, when Lepidodendron, Sigillaria, and Calamitei 

 had disappeared. There are good if not convincing climatic and botanic 

 evidences that Permian glaciation had produced at least three climaxes, rep- 

 resented by the araucarians and pines, the cycadeans, and the Walchia- 

 Voltzia type. It is probable that the first two were often much mixed, and 

 indeed conifers and cycadeans may well have existed in the same climax. 

 In western North America in particular the cycadeans seem to have occupied 

 the more arid uplands, while the lowlands were dominated by araucarians 

 especially. The Glossopteris flora of the southern hemisphere in particular, 

 with its shrubby gymnosperms, Walchia, VoUzia, Brachyphyllum, etc., seems 

 to represent a cold or arid climate resulting from glaciation. 



Of the serai dominants we have very little direct evidence. In the hydro- 

 sere, Chara suppHed the submerged form, and bryophytes such as Riccia and 

 Fontinalis are not improbable. Floatiag forms are unknown, but amphibious 

 dominants such as Equisetum, Dryopteris, and Onocka must have been uni- 

 versal. The lichens, mosses, and herbs of the xerosere must be taken for 

 granted, as we have no record of them. It is not improbable, however, that 

 this r61e may have been taken, in part at least, by Glossopteris and its herba- 

 ceous associates. The presence of a scrub stage is attested by the existence 

 of scrub climaxes, and it is probable that the same genera took a part in both. 



Structure of the vegetation. — From the above it appears that vegetation 

 must have been differentiated during the Permian, and must have retained 

 this character throughout the era, though perhaps with decreasing emphasis 

 in the Jurassic and Comanchean. There is no evidence of polar zones, though 

 it seems that they must have developed to some extent at least. The occur- 

 rence of glaciation in widely separated regions, such as Australia, India, 

 South Africa, South America, and North America (Schuchert, 1914:267) 

 doubtless produced as many centers, about which were differentiated two or 

 three climax zones at least. In a vegetation as uniform as that of the Carboni- 

 ferous is supposed to have been, glaciation should have produced similar if 

 not identical zones in the various regions. The occurrence of the Glossopteris 

 flora, which is regarded as an index of glacial conditions, in Australia, Asia, 

 Africa, and South America, lends support to this view. Its absence in North 

 America might well indicate a pre-existing differentiation which prevented the 

 evolution of the new flora. This is opposed, however, to the accepted view 

 of the uniformity of Carboniferous vegetation. 



In any event, there must have been two distinct climaxes in the Permo- 

 Triassic, marked by Lepidodendrea and by the Glossopteris flora respectively. 

 The appearance of Walchia and VoUzia, and the association of the last with 

 Noeggerathiopsis in the Glossopteris flora, suggests that the latter may have 

 shown at least two climaxes, a pteroid one of herbs or scrub and a gymno- 

 spermous one of small trees. By the Jurassic these climaxes had disappeared, 

 and their places had been taken by araucarians and pines and by cycadeans. 



Deformation and climatic cycles. — In its general aspect, the Mesophytic 

 is a single great deformational cycle, beginning with the Permian and ending 

 with the lesser deformation which marked the close of the Comanchean. 



