1871.] Prof. Williamson on Lepidodendra and Sigillarise. 501 



Externally to this bast-layer is a more superficial epiderm of parenchyma, 

 supporting the bases of leaves, which consist of similar parenchymatous 

 tissue. Tangential sections of these outer cortical tissues show that the 

 so-called " decorticated " specimens of Lepidodendra and of other allied 

 plants are merely examples that have lost their epidermal layer or had it 

 converted into coal, this layer, strengthened by the bast-tissue of its inner 

 surface, having remained as a hollow cylinder when all the more internal 

 structures had been destroyed or removed. 



From this type the author proceeds upwards through a series of examples 

 in which the vessels of the medulla become separated from its central cel- 

 lular portions and retreat towards its periphery, forming an outer cylinder 

 of medullary vessels, which are arranged without order and enclose a defined 

 cellular axis ; at the same time the encircling ligneous zone of radiating 

 vessels becomes yet more developed, both in the number of its vessels and 

 in the diameter of the cylinder relatively to that of the entire stem. As 

 these changes are produced, the medullary rays separating the laminae of 

 the woody wedges become more definite, some of them assuming a more 

 composite structure, and the entire organization gradually assuming a more 

 exogenous type ; at the same time the cortical portions retain all the 

 essential features of the Lepidodendroid plants. Commencing with the 

 Lepidodendron selaginoides just described, we pass on. to L. Harcourtii, in 

 which there is a distinct cellular axis to the medulla, surrounded by a ring 

 of medullary vessels, externally to which is the second or radiating cylinder 

 of vessels, from which alone, as M. Brongniart has very correctly shown, 

 the bundles of vessels supplying the leaves are derived. Then we reach 

 the more highly organized of the forms which Mr. Binney has described 

 under the common name of Sigillaria vascularis, in which the woody 

 cylinder is more extensively developed. This conducts us to a series of 

 varieties from which the cells of the medulla have disappeared, but in 

 which there is a very distinct inner cylinder of large barred vessels not 

 arranged in radiating order, and an outer and much more ample cylinder 

 of smaller ones arranged on the exogenous type. In these examples the 

 line of demarcation between the vessels of the medulla and those of the 

 ligneous zone is sometimes straight and at others boldly crenulated. In 

 the latter examples the outside of the vascular medullary cylinder, detached 

 from its surroundings, exhibits the fluted appearance of a Calamite, for 

 which it might be mistaken, but it lacks the transverse nodal constrictions 

 of that genus. It is to some of these more highly organized Lepidodendra 

 just referred to that Corda has applied the name of Diploxylon, and Witham 

 that of Anabathra, both of which correspond in the closest manner 

 with the Siffillaria elegans of M. Brongniart. We are thus brought, by 

 the evidence of internal organization, to the conclusion that the plants 

 which Brongniart has divided into two distinct groups, the one of which 

 he has placed amongst the vascular Cryptogams, and the other amongst 

 the Gymnospermous Exogens, constitute one great natural family. 



