362 



PALAEOZOIC SYSTEM OF ROCKS. 



Cycads, on the one hand, and the vascular Cryptogams on the other. 

 Many writers ally the Cordaites and Noggerathia with the Cycads 

 instead of the Conifers. They probably connect these two families of 

 G-ymnosperms. 



2. Ferns. — Ferns are the most abundant plants of the Coal period, 

 both as to individuals and as to variety of species. About one third to 

 one half of all the known species of coal-plants, both in this country 

 and in Europe, belong to this order. They represent both ordinary 

 forms, i. e., those with creeping stems, and Tree-ferns, like those now 

 growing only in warm latitudes (Fig. 470). They are known to be 

 ferns by their large complex fronds (Fig. 471), sometimes six to eight 

 feet long ; by the dichotomous venation of their leaves (Fig. 475) ; 

 and by the position of their organs of fructification (spore-cases) on the 

 under-surfaces of the leaves (Figs. 476 and 477). In some localities 

 these spore-cases are so abundant that the coal seems to be almost 

 wholly made up of them. The trunks of Tree-ferns are known by the 

 large, ragged, -ovoid marks left by the falling of the fronds (leaf-scars 

 — Figs. 485-487), and by the peculiar arrangement of the vascular 

 tissue in the cellular in the cross-section. Some coal Tree-ferns had 

 their large fronds in two vertical ranks (Megaphyton — Fig. 471). 



The Ferns of the Coal are, 

 therefore, unmistakably Ferns, 

 yet botanists recognize some 

 features which connect them 

 with other classes. Caruthers 



Fig. 471.— Megaphyton, a Coal-Fern 

 Fig. 470.— Living Tree-Fern. restored (after Dawson). 



thinks that he finds in the internal structure of the stems of Tree-ferns 

 of the Coal two types which are the foreshado wings of the Monocotyls 

 on the one hand, and the Dicotyls on the other ; and that therefore 

 they are probably the progenitors, not only of the Tree-ferns of the 

 present day, but also of the Palms and the foliferous Exogens.* 



* Nature, vol. vi, p. 480, and Scott, American Journal, vol. ix, p. 45. 



