276 STUDIES FOR STUDENTS 



The above discussion is based on the present floral societies, com- 

 posed almost wholly of flowering plants. The conclusions, however, 

 in regard to the climatic relations of herbaceous and arboreal vegeta- 

 tion may with probability be extended backward in time to ages as 

 early as the Devonian, when all plants were either cryptogamic or 

 gymnospermous, since in the later Paleozoic, long before the advent 

 of the Mesozoic phanerogams, plant societies existed then as now 

 which included forms from arboreal to herbaceous and ranged in 

 adaptation from hygrophilous to xerophylous. The present usual 

 restriction of cryptogamic vegetation to small forms occupying habi- 

 tats moist, shady, or cold, habitats not strongly sought by the higher 

 vegetation, did not then necessarily hold; conditions to some extent 

 perpetuated in Australia, where tree ferns still abound in the coastal 

 districts of New South Wales and Victoria, and vascular cryptogams 

 with xerophytic adaptations are known to occur in other portions of the 

 island continent. 



In illustration of these conclusions a comparison may be made of 

 the fossil vegetation of the Mauch Chunk (Mississippian) shale of 

 eastern Pennsylvania, believed by the writer from other considerations 

 to be a continental deposit of a semiaricl climate,^ with the flora of the 

 overlying coal-measures, believed to be continental deposits of a cli- 

 mate cool and rainy. In the Mauch Chunk strata, as observed by 

 the writer, impressions of small plant fragments are not uncommon, 

 consisting of first, the fragments of slender grasslike reeds probably 

 belonging to the equisetae; second, impressions of flattened, straplike 

 coarser stems and leaves up to an inch in width and exhibiting sug- 

 gestions of parallel venation; third, impressions of stems with close-set 

 spiny leafage, the spines not being over half an inch in length ; fourth, 

 casts of roots showing branching rootlets, the latter clothed with fine 

 tendrils. The roots occur in massive argillaceous sandstones and in 

 favorable cases are exposed by the rock fractures for depths of a foot 

 with indications of being considerably more extensive. A striking 

 feature of those root casts found in place is that they branch downward 

 and not horizontally. Other observers have detected a leathery 

 character in certain of these plant impressions. No casts of logs have 



I J. Barrell, "Origin and Significance of the Mauch Chunk Shale," Bulletin oj the 

 Geological Society of America, Vol. XVIII (1907), pp: 449-76. 



