128 THE STORY OF THE EARTH AND MAN. 



Stigmaria in general appearance, though differing in 

 details. In the coal period there were several generic 

 forms of these plants, all attaining to the dimensions 

 of trees. Like the Sigillariae, they contributed to 

 the materials of the coal; and one mode of this has 

 recently attracted some attention. It is the accumu- 

 lation of their spores and spore-cases already referred 

 to in speaking of the Devonian, and which was in the 

 Carboniferous so considerable as to constitute an im- 

 portant feature locally in some beds of coal. A similar 

 modern accumulation of spore-cases of tree-ferns 

 occurs in Tasmania ; but both in the Modern and the 

 Carboniferous, such beds are exceptional; though 

 wherever spore-cases exist as a considerable consti- 

 tuent of coal, from their composition they give to it 

 a highly bituminous character, an effect, however, 

 which is equally produced by the hard scales support- 

 ing the spores, and by the outer epidermal tissues 

 of plants when these predominate in the coal, more 

 especially by the thick corky outer bark of Sigillaria. 

 In short, the corky substance of bark and similar 

 vegetable tissues, from its highly carbonaceous cha- 

 racter, its indestructibility, and its difficult permea- 

 bility by water carrying mineral matter in solution, 

 is the best of all materials for the production of coal ; 

 and the microscope shows that of this the principal 

 part of the coal is actually composed. 



In the wide, open forest glades, tree-ferns almost 

 precisely similar to those of the modern tropics reared 

 their leafy crowns. But among them was one peculiar 



