UPPER PALEOZOIC FLORAS 141 



intervals of exposure to the air. The stomatiferous surfaces may have 

 been very small for a time, and the stomata of periodic function only 

 while the greater part of the carbonic-acid gas to serve as plant food 

 was still drawn in the old way from the richly charged waters. The 

 expansion of a proper leaf and the production of an aerial system of 

 transpiration were presumably gradually evolved as the plant became 

 weaned from its subaqueous habitat and accustomed to gain its food 

 from an atmosphere which, it may be, was then better adapted to the 

 nourishment of the emergent amphibian. However this may be it is 

 fairly clear that the early representatives of the dominant Devonian 

 types were of limited foliar expanse. Cuticular transmission of gases 

 is still observed in the living ferns and Lycopods, the latter being far 

 less susceptible to carbon-dioxide poisoning than are the higher plants. 



It also appears that to support their weight in air a reinforced 

 cuticle, later developed as a very thick and complicated cortex, was 

 made to serve until a woody axis and, eventually, secondary wood 

 should be fully produced by their descendants. From the characters 

 of some of the fossils it seems probable that, unable to stand alone, 

 they sprawled or clambered about on the ground or on other plants. 



The mode of occurrence of their fossil remains usually in fresh or 

 brackish water lagoonal or estuarine deposits, which are frequently 

 ripple-marked, or even sun-cracked, may be regarded, though not 

 without caution, as pointing out the conditions of their earliest 

 habitats. 



MIDDLE DEVONIAN 



Characters. — The first Paleozoic land flora sufficiently known to 

 make it eligible to the series of correlation discussions is that of the 

 Middle Devonian. 



This flora, whose apparent meagerness is perhaps due mostly to 

 meagerness of information, is of strange and forbidding aspect. Its 

 most characteristic types are Psilophyton, Arthrostigma, and Rhachi- 

 opteris of Dawson. It is also marked by the presence of Proto- 

 lepidodendron, a primitive forerunner of the great lycopod group 

 and, before reaching the Portage we find added Archaeopteris, 

 together with the curious Pseudosporochnus, which may supply 

 the missing fronds to the defoliated Caulopteris-like stems from Ohio 

 and New York. 



