Fossil Vegetables of the Coal Formation. 285 



nucleus of which these stems is almost entirely composed, is still 

 preserved, or has fallen off. It is in fact a character peculiar to 

 the fossil stems of the coal deposits, to be transformed or rather 

 entirely replaced, by an inorganic substance, deposited in the way 

 of sediment, often very coarse, and retaining no traces of the in- 

 ternal organisation of the stem ; while around this nucleus there 

 occurs a layer, more or less thick, of very friable lamellar char- 

 coal, which has exactly preserved the form of the surface of the 

 vegetable. According as this cortical layer has a thickness more 

 or less great, and more or less equal, the central nucleus, when 

 it is deprived of it, preserves more or less accurately the form 

 of the external surface of the vegetable. In the Stigmariae, the 

 Sagenariae, the Calamites, and some Sigillarice, this bark forms 

 an extremely thin layer, a sort of epidermis, which leaves to the 

 stony nucleus the same form which the surface of the vege- 

 table itself presented. In the greater number of the Sigillariae, 

 on the contrary, this bark, which has a thickness of from one to 

 two lines, does not preserve internally the form which it had on 

 the outside ; the disk produced by the entire base of the petiole 

 no longer exists. The vessels alone which traversed it still leave 

 a mark internally, and produce those narrow, and often puncti- 

 form, impressions which were observed on the Syringodendra. 

 This character still furnishes an additional reason for considering 



o 



their genus as allied to the tree ferns. In the small number of 

 stems of these plants which we have had an opportunity of ob- 

 serving, and particularly in those of the old continent, there is 

 observed a perfectly distinct bark, or rather external layer, of an 

 organisation very different from the bark of dicotyledonous ve- 

 getables. This bark appears to detach itself from the substance 

 which occupies the centre of the stem, and then forms a sort of 

 hollow cylinder, of a very dense substance, the external surface 

 of which presents, with much accuracy, the form of the bases of 

 the petioles, while the inner surface presents only the passage of 

 the vessels. Let us suppose this woody cylinder to be filled up 

 with an earthy substance, and the bark afterwards converted in- 

 to charcoal, stems will be obtained, having a nearly perfect re- 

 semblance to the Sigillariae ; if, again, the carbonaceous bark be 

 removed, the earthy nucleus will represent, with but slight dif- 

 ferences, the Syringodendra. 



