322 DAVID WHITE 
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 aérial 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. 
