ON COAL. 



161 



If we make a section of the stem of a Sigillaria, (figs 24 and 25,) we 

 find externally a hai^k, or, more probably, a rind{a) of condensed cellu- 

 lar tissue, sometimes a half or an inch thick; within this an enormous 

 amount of loose cellular tissue, (c,) often 2 feet or more thick. Through 

 the centre of this runs a slender sheath(J) of vascular or woody tissue, 

 which in a Sigillaria 5 feet in diameter is not more than 3 inches in 

 diameter ; a mere thread of vascular in the midst of a mass of cellular 

 tissue. This again incloses a small pith (a) which occupies the very 

 centre of the trunk. These vascular cylinders, with their inclosed pith, 

 being the most indestructible portion of the trunk, are often found 

 alone, and described under the name of Endogenites. Figs. 24 and 25 

 represent cross and longitudinal ideal sections of this plant, (a) the 

 cellular tissue of the pith, (h) the vascular or woody sheath, (c) the 

 mass of cellular tissue between the vascular sheath and (d) the rind, 

 (e) slender vascular bundles connecting the leaves with the central 

 sheath. Upon closer examination of this woody or vascular cylinder 

 (b) it is found to consist of concentric layers, somewhat analogous to the 

 layers of growth of exogenous trees, and divided into wedges by medul- 

 lary rays, like the tree exogens. Upon still closer examination, how- 

 ever, of a good cross section under a microscope (fig. 26) no distinction 

 of vascular and woody tissue, such as is found in the wood of Dicotylo- 

 dons, is observed, but the whole is made up of one kind of tissue, open 

 and thin-walled, in comparison with woody tissue proper, but closely 

 resembling the wood of pines. But a longitudinal section shows no 

 disc-like markings such as characterize the wood of Conifers, but 



Figs. 26 and 27. Fig. 28. 



Fig. 28. A cross section and longitudinal section 

 of a Sigillaria. The letters a, b, e, d, e represent 

 same a.s in previous figs. 1, 2, 3 are the 3 layers 

 of the vascular cylinder 6, wi isa medullary ray'. 



reveals the fact that it consists entirely of spiral vessels, (figs. 27 and 

 27;) and that, therefore, the sheath of the Sigillaria consists of vascular 

 rather than of woody tissue. In consequence of the great predomi- 

 nance of cellular tissue, these stems, as well as those of the Lepido- 

 dendron and Calamites, are generally found very much flattened bv 

 pressure. '' 



11 s 



