LEPIDODENDRON. 



69 



to substitute scalariforra tissue for the gymnospermatous woody tissue, and scalariforni 

 utricles for true medullary parenchyma, and finally exclude the medullary rays, the 

 description of the Cycadean stem would apply to that of Lepidodendron. And it deserves 

 special notice, that the surface of the Cycadean trunk is composed of the bases of the old 

 leaves, together with the scales which in some species are interspersed among them or 

 alternate with them. The leaves do not disarticulate at the circumference of the stem, 

 but at some distance from it, leaving a small portion of the base persistent. The scars of 

 the outer surface of the stem give a different impression from those presented when the 

 persistent bases of the leaf-stalks are removed. Whoever is even a little familiar with 

 coal fossils is aware that there are two sets of scars on the stems of Lepidodendron — one 

 superficial and the other internal. The fossils that present the first set are generally said 

 to be ' corticated' stems, and those exhibiting the others ' decorticated.' The ' bark' is 

 generally converted into a compact structureless coal, the outer surface of which has the 

 one set of scars, and the inner surface the other. I believe this coal is produced by the 

 external of the two epidermal series, and that the outer scars were truly superficial, while 

 the inner were produced by the vessels which passed to the bases of the leaves. The two 

 sets of scars in Cycadean stems are analogous structures ; but in Lepidodendron the layer 

 which bears the scars on its two surfaces is a compact cylinder ; while in CycadecB there 

 is no connecting tissue uniting the bases of the leaves ; they are closely packed together, 

 but quite free from each other. It is evident that in many respects the fossil stem had a 

 striking analogy in the arrangement of its parts to that of the recent Cycads, while it was 

 a true Cryptogam ; and if we now examine the slender stem of Li/copodlim, we shall find, 

 I believe, that Lepidodendron, though more highly developed, does not differ essentially 

 from it. 



" Spring, in his ' Monographic des Lycopodiacces' (p. 293), describes the stem of this 

 Order as composed of five parts: — 1st, the woody axis; 2nd, a layer of delicate cells; 

 3rd, the liber ; 4th, the herbaceous envelope ; and, 5th, the epidermis. 



" The axis is composed of bundles of scalariform vessels, scattered through a very 

 delicate cellular tissue, in a regular figure, which varies in the different species. This 

 axis is surrounded by a layer of lax, delicate, cellular tissue, which Spring considers to be 

 the channel through which the sap circulates, and the seat of growth in the stem, the 

 inner portion being developed into wood vessels, and the outer into ' liber/ The ' liber' 

 is composed of elongated cells, with thickened walls. Spring gives to it this name because 

 of its analogy to the liber in dicotyledons. This layer is often so thin that it is difficult 

 to detect. It is surrounded by a thick greenish layer, composed of large elongated cells 

 with thin walls ; and this is covered with an epidermis, consisting of small cells with thick 

 walls. The vascular bundles pass through the various layers of cellular tissue from the 

 axis to the circumference. 



" The great difference between the stem of Lepidodendron and Lycopodiim is the 

 existence of a pseudo-medulla and the arrangement of the vascular tissue as a solid 



