kidston: the flora of the carboniferous period. 197 



to these stems in two opposite rows, tlie frond on one side of the 

 stem alternating with tliat on the other side. 



The stem, excej^t at the part to which the fronds are attached, 

 was densely covered with aerial rootlets. 



Caulopleris 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 paheozoic 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 C ycadojilices. 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 



