Lepidodendron

Lepidodendron Sternb., 1820, literally tree with scales, is a genus of arboreal Pteridophyta known from its fossil remains from the late Devonian to early Permian period between 382 and 286 million years ago, when it was replaced by gymnosperms. This plant, which probably developed in swampy areas or even in water, was one of the largest of its time and formed extensive forests, judging by the extensive paleontological record that is preserved, especially during the Carboniferous period.

Morphologically, its representatives were formed by a large main trunk with a wide crown at its apex formed by dichotomous ramifications, microphylls and strobili, and an extensive root system at its base. The main characteristic of the Lepidodendron fossils are the rhomboid impressions of the scars that it had on the surface of its trunk, branches and roots formed by the fall of the leaves.

Morphology

Stolen scars of the trunk Lepidodendron.

Printing of leaves Lepidodendron.

Raice of Lepidodendron showing the scars.

The fossil remains of Lepidodendron show a tree-like plant that must have reached more than 35 meters in height and a diameter of 2 meters, which are the dimensions of the largest specimen known to date. This plant has an erect main axis or stem that branches from a certain height depending on the species. The dichotomous ramifications form a wide cup or crown with increasingly thin axes and carriers of lanceolate leaves and strobili. The base of the trunk also dichotomized several times to form a horizontal root system bearing leaf-like structures.

The leaves or microphylls of Lepidodendron are always acicular to lanceolate and highly variable in size depending on the species, reaching the largest of them up to 1 m in length. These leaves are helically inserted in the youngest ramifications, have a single longitudinal vein and stomata organized in two bands only on the underside. The leaf base is decurrent and ligulate, and has a basal sheath formed by a continuation of the bark. In this way, the leaves of all the species, which were deciduous, fell, leaving a patent and conspicuous rhomboid scar on the bark of both the main trunk and the ramifications, a determining factor in the description of the different species of Lepidodendrales.

Leaf scars expose the vascular tissues of the axis from the leaf trace and in the oldest areas of the stems they are always very blurred by the passage of time and by the deformation suffered after the thickening of the stems. The scars on the main axis are always larger than those on the ramifications. Although it has been assumed that this difference in size is also due to a deformation of the bark after the abscission of the leaves, the finding of juvenile individuals of Lepidodendron with large leaves suggests that the scars on the stem they maintain their dimensions throughout the life of the plant.

The root system is made up of several roots with horizontal growth and a large diameter that emerge from the base of the stem in a helical fashion. These roots dichotomized several times. The two roots formed first divided rapidly, forming a characteristic base of four. Leaf-like structures are found on the surface of the smaller roots, which increase the capacity for binding to the substrate and uptake of nutrients. The root appendages are also deciduous, scarring the older root portions and the basal zone of the stem.

The growth of these plants is determined and their reproduction is monocarpic, so that at approximately 15 years of age the plant reaches its maximum size from which it begins to form reproductive structures and then die. The vascular cylinder of Lepidodendron has a unifacial vascular cambium, which only forms secondary xylem towards the interior of the stele. The thickness of this secondary xylem, formed by scale-like tracheids, decreases from the base to the apex of the plant, a fact that indicates its determined development. Most of the vascular cylinder is formed by a thick cortex, with fibrous and anastomosed cells, in its outer area. Surrounding the cortex is the periderm, a tissue morphologically similar to cork and with an important supporting function.

At the end of the youngest and thinnest ramifications Lepidodendron produces sporophylls, the structures that carry the reproductive organs, and which, due to their dimensions, make the branches hang down. These sporophylls are grouped in strobila between 3 and 30 cm long and about 6 cm in diameter. Each of the sporophylls is made up of a pedicel, which emerges perpendicularly from the axis of the strobila helically or in whorls, and a lamina. terminal or perpendicular bract more or less wide. In each one of the pedicels there is an oval megasporangium and wider than the sporophyll with a short stipe and ligule, the latter character characteristic of current and fossil heterosporeal lycopodia. Inside the sporangium there is a single functional megaspore of the Cystosporites type and three aborted megaspores.

Taxonomy

Most of the known species, more than a hundred, have been described from bark impressions based on the morphology and dimensions of the leaf scars. Some species, however, were described from deformed or degraded impressions of leaf scars (with the names Ulodendron, Halonia or Aspidiaria)., strobili (with the names of Lepidostrobus, Lepidocarpon and Achlamydocarpon), bases of stems with root system (with the name of Sigillaria or Stigmaria) or trunks without bark (with the name of Knorria), structures all of which are very different from each other and are rarely associated.

Some of its most representative species are these:

Lepidodendron aculeatum – Lepidodendron acuminatum – Lepidodendron acutangulum – Lepidodendron acutum – Lepidodendron africanum – Lepidodendron anomalum – Lepidodendron aolungpylukense – Lepidodendron arberi – Lepidodendron brevifolium – Lepidodendron canobianum – Lepidodendron carinum – Lepidodendronistondrotomumum –Lepidodendronistonepidodendrotumum subdichotomum – Lepidodendron subrhombicum – Lepidodendron szeianum – Lepidodendron taichinganense – Lepidodendron tripunctatum – Lepidodendron vasculare – Lepidodendron vasiuchitschevii – Lepidodendron veltheiii – Lepidodendron volkmanianum – Lepidodendron worthenii.