kidston: the flora of the carboniferous period. 197 
to these stems in two opposite rows, the frond on one side of the 
stem alternating with that on the other side. 
The stem, except at the part to which the fronds are attached, 
was densely covered with aerial rootlets. 
Caulopteris in its general aspect would much resemble one of 
the recent Tree Ferns, but Megaphyton, with its two opposite 
rows of fronds would have a very different aspect from any of 
the Tree Ferns at present existing. 
Before passing from this brief consideration of the more im- 
portant groups of pal^eozoic ferns, a few remarks must be made on 
their internal organisation, though this subject can only be 
touched on very slightly here. 
The stems or rhizomes of recent ferns have no exogenous 
growth, that is, when the vascular bundle is once fully formed 
no new elements are subsequently added to it. Hence Tree 
Fern stems when once fully developed retain the same diameter 
of trunk for years. 
Among fossil ferns whose structure is known, a few, 
generally of small size, possess the same structural peculiarities, 
but there is another type of palaeozoic fern structure where, 
among other characters, an exogenous increase to the vascular 
system takes place. In these, after the formation of the primary 
vascular bundles, whose size is limited as in the first type, a 
cambium layer appears from which an outer circle of exogenously 
developed vascular tissue arises. This ring of secondary xylem 
or wood may increase indefinitely in size by additions from the 
cambium zone, the ultimate size of the stem being limited only 
by the life of the plant. 
These Fern Stems with exogenous growth present certain 
anatomical characters intermediate between ferns and Cycads, 
and are now placed in a group to which Potonie has given 
the name of Cycadofilices. There is reason to believe, though 
little is known of their fructification, that they may be 
ferns, though in their anatomy they possess certain characters 
not found in existing members of this group. This discovery 
is one of the most interesting and important advances recently 
