8 Contributions towards Establishing the General Character 



These vessels also vary in diameter ; they increase gradually from the 

 inner to the outer side of the cylinder. By means of longitudinal 

 sections, made both parallel and at right angles to the medullary rays, 

 the vessels are seen to be regularly marked with parallel transverse bars 

 on all their walls ; this is an important character, for, in the Conifers 

 and Cycases, the cells or vessels composing the ligneous system are in 

 general onlxj marked on their lateral walls, that is, on those which corre- 

 spond with the direction of the medullary rays. Another difference re- 

 mains to be mentioned ; in the latter plants the vessels are generally 

 marked with what are termed disks, but in Sigillaria elegans, the vessels, 

 as already stated, are marked with parallel transverse bars. 



The bundles or cords which Brongniart considers were connected 

 with the medullary sheath, and continued into the leaves or other ex- 

 ternal appendages, are, as previously mentioned, situated externall}^ to 

 the ligneous cylinder, and consequently they must have been imbedded 

 in the surrounding zone of cellular tissue. On a transverse section, these 

 cords are in some cases nearly cylindrical, and in others rather oblong, 

 and the vessels composing them have in general irregular angular ori- 

 fices ; on longitudinal sections, we discover that the vessels are formed 

 of a fine membrane, and that they have their walls transversely barred. 

 Like the vessels of the medullary sheath, they are irregularly grouped, 

 and not arranged in radiating series, in the manner of those forming the 

 ligneous cylinder. 



The external zone or bark possesses nothing very remarkable, except 

 its varying width, and this appears to be due to the ligneous and vas- 

 cular parts being eccentric, as in Lepidodendron and Stigmaria, and not 

 central, as in ordinary woody trees,* The inner, and by far the largest, 

 part of the bark, a, appears, from the few traces remaining, to have con- 

 sisted of a delicate parenchyma. The cuticle, or external part, which is 

 extremely well preserved in one place, is formed of a stronger tissue, 

 and consists of two different layers, a* a", in close contact. The cells of 



* Brongniart says, that the Lycopods agree with these fossils in the occasional 

 eccentricity of their vascular system. 



I have considerable doubt as to this eccentricity being a character of Lepido- 

 dendron, Stigmaria, and Sigillaria, when living. It is due, I strongly suspect, to 

 the destruction of the mass of delicate cellular tissue in which the axis was im- 

 bedded. Steinhauer was the first, I believe, to advance this opinion. He very 

 justly observes, that the destruction of the surrounding tissue would leave the 

 axis unsupported, and that the latter would necessarily sink to the under side of 

 the fossil. The creeping habit of the branches of Stigmaria is extremely favour- 

 able to such a position of their axis resulting from this contingency. With re- 

 ference to Lepidodendron, I have seen specimens in which the vascular cylinder 

 is nearly central. And as regards 5e^i7Zari«, Artis' figure of Rhytidolepis fibrosa 

 (PI. ix.), demonstrates that its ligneo-vascular axis was not eccentric. 



