LEPIDODENDRON. 73 



direction, and nearly at right angles to their previous course, to reach the rootlets. But 

 only a small number reach their destination, the great majority of the deflected vessels 

 terminating in the woody zone. A very thick bark surrounds the woody zone. Imme- 

 diately in contact with the latter it consists of a thin layer of delicate, vertically elongated, 

 cellular tissue, in which the mural tissues of the outer extremities of the medullary rays 

 become merged. Externally to this structure is a thick parenchyma, which quickly 

 assumes a more or less prosenchymatous form, and becomes arranged in thin radiating 

 laminae as it extends outwards. The epidermal layer consists of cellular parenchyma, with 

 vertically elongated cells at its inner surface, which feebly represents the bast-layer of the 

 other forms of Lepidodendroid plants. The rootlets consist of an outer layer of parenchyma, 

 derived from the epidermal parenchyma. Within this is a cylindrical space, the tissue of 

 which has always disappeared. In the centre is a bundle of vessels surrounded by a 

 cylinder of very delicate cellular tissue, prolonged either from one of the medullary rays, 

 or from the delicate innermost layer of the bark, because it always accompanies the vessels 

 in their progress through the middle and outer barks. 



" The facts of which the preceding is a summary lead to the conclusion that all the 

 forms of plants described are but modifications of the Lepidodendroid type. The leaf- 

 scars of the specimens so common in the coal-shales represent tangential sections of the 

 petioles of leaves when such sections are made close to the epidermal layer. The thin 

 film of coal of which these leaf-scars consist, in specimens found both in sandstone and in 

 shale, does not represent the entire bark as generally thought, and as is implied in the 

 term ' decorticated' usually applied to them, but it is derived from the epidermal layer. 

 In such specimens all the more central axial structures, viz., the medulla, the wood, and 

 the thick layer of true bark, have disappeared through decay, having been either 

 destroyed, or in some instances detached and floated out ; the bast-layer of the epiderm has 

 arrested the destruction of the entire cylinder, and formed the mould into which inorganic 

 materials have been introduced. On the other hand, the woody cylinder is the part 

 most frequently preserved in Stigmaria ; doubtless because, being subterranean, it was 

 protected against the atmospheric action which destroyed so much of the stem. 



" It is evident that all these Lepidodendroid and Sigillarian plants must be included 

 in one common family, and that the separation of the latter from the former as a group 

 of Gymnosperms, as suggested by M. Brongniart, must be abandoned. The remark- 

 able development of exogenous woody structures in most members of the entire family 

 indicates the necessity of ceasing to apply either to them or to their living representatives 

 the term Acrogenous. Hence the author proposes a division of the vascular cryptogams 

 into an Exogenous group, containing Lycopodiacea, Equiseticece, and the fossil Calamitacecs, 

 and an Endogenous group, containing the Perns, — the former uniting the Cryptogams 

 with the Exogens through the Cycadea and the other Gymnosperms, and the latter 

 linking them with the Endogens through the Pahiacea." 



