Family Lepidodendreæ; Genus Lepidodendron, Sternberg.
These are arboreal Lycopods having linear one-nerved leaves, stems branching dichotomously, and with ovate or rhombic leaf-bases bearing rhombic leaf-scars, often very prominent. The fruit is in scaly strobiles, terminal or lateral, and there are usually, if not always, macrospores and microspores in each strobile. The young branches and stems have a central pith, a cylinder of scalariform tubes sending out ascending bundles to the leaves through a thick cellular and fibrous inner bark, and externally a dense cortex confluent with or consisting of the leaf-bases. Older stems have a second or outer layer of scalariform fibres in wedges with medullary rays, and strengthening the stem by a true exogenous growth, much as in the Diploxylon type of Sigiliaria. The development of this exogenous cylinder is different in amount and rate in different species.[CX] This different development of the exogenous axis is accompanied with appropriate external appearances in the stems, and the changes which take place in their markings. These are of three kinds. In some species the areoles, at first close together, become, in the process of the expansion of the stem, separated by intervening spaces of bark in a perfectly regular manner; so that in old stems, while widely separated, they still retain their arrangement, while in young stems they are quite close to one another. This is the case in L. corrugatum. In other species the leaf-scars or bases increase in size in the old stems, still retaining their forms and their contiguity to each other. This is the case in L. undulatum, and generally in those Lepidodendra which have large leaf-bases. In these species the continued vitality of the bark is shown by the occasional production of lateral strobiles on large branches, in the manner of the modern red pine of America. In other species the areoles neither increase in size nor become regularly separated by growth of the intervening bark; but in old stems the bark splits into deep furrows, between which may be seen portions of bark still retaining the areoles in their original dimensions and arrangement. This is the case with L. Pictoense. This cracking of the bark no doubt occurs in very old trunks of the first two types, but not at all to the same extent.
[CX] See “Memoirs of Dr. Williamson,” in “Philosophical Transactions,” for ample details.
As a type of Lepidodendron, I may describe one of the oldest Carboniferous species characteristic of the Lower Carboniferous in America, and corresponding to L. Veltheimianum of Europe.
Lepidodendron Corrugatum, Dawson.—(See [Fig. 43], supra.) “Quarterly Journal of Geological Society,” vol. xv.; “Acadian Geology,” page 451.
Habit of Growth.—Somewhat slender, with long branches and long, slender leaves having a tendency to become horizontal or drooping.
Markings of Stem.—Leaf-bases disposed in quincunx or spirally, elongate, ovate, acute at both ends, but more acute and slightly oblique at the lower end; most prominent in the upper third, and with a slight vertical ridge. Leaf-scars small, rounded, and showing only a single punctiform vascular scar. The leaf-scar on the outer surface is in the upper third of the base; but the obliquity of the vascular bundle causes it to be nearly central on the inside of the epidermis. In young succulent shoots the leaf-scars are contiguous and round as in Cyclostigma, without distinct leaf-bases. In this state it closely resembles L. Olivieri, Eichwald.[CY]
[CY] Lethæa Rossica, Plate Y, Figs. 12, 13.
In the ordinary young branches the leaf-scars are contiguous, and closely resemble those of L. elegans, Brongt. ([Fig. 43 C]). As the branches increase in diameter the leaf-scars slightly enlarge and sometimes assume a verticillate appearance ([Fig. 43 D]). As they still further enlarge they become separated by gradually increasing spaces of bark, marked with many waving striæ or wrinkles ([Fig. 43 I, N]). At the base of old stems the bark assumes a generally wrinkled appearance without distinct scars.
Knorria or Decorticated States.—Of these there is a great variety, depending on the state of preservation, and the particular longitudinal ridges. [Fig. 43 D] shows a form in which the vascular bundles appear as cylindrical truncate projections. Other forms show the leaf-bases prominent, or have an appearance of longitudinal ribbing produced by the expansion of the bark.