Lepidodendrales

Lepidodendron lycopodioides

The Lepidodendrales are an extinct group of Bärlapppflanzen ( Lycopodiopsida ). They came to Perm from Devon. Your tree-shaped representatives were highly up to 40 m and formed the coal - swamp forests of the northern hemisphere. Due to the large amount of finds the Lepidodendrales are the best known group of fossil Bärlapppflanzen.

• 4.1 Notes and references

Features

The representatives of the order Lepidodendrales are characterized by the presence of a ligule, of secondary xylem, a thick periderm and a tripartite bark. They have spirally arranged, root-like attachments with a monarch vascular bundles. They form a single functional megaspore, which germinates in the sporangium per megasporangium.

The following are the features of the two families Lepidodendraceae and Diaphorodendraceae are discussed. These representatives of the two families were summarized earlier due to their scale-like leaf scars on the trunk predominantly in organ - genus Lepidodendron.

Vegetative characteristics

The trees were up to 40 m high with a trunk diameter of at least 2 m ( Lepidodendron ). Many members, however, remained much smaller. Diaphorodendron vasculare, Diaphorodendron scleroticum and Diaphorodendron phillipsii include trees 8-20 m in height, representatives of Synchysidendron were high 10 to 15 m. The shoot axes consist for the most part from the bark, not, as in extant trees from the wood. Even in the case of 20 mm diameter, the branches bark occupies about 98 % of the volume, at recent trees only about 25%.

The most common fossils of the group are imprints of the trunk surface. This is characterized by permanent, something asymmetrical, more or less rhombic leaf pads. The leaf pads is the remnant of the enlarged leaf base, the above remains, since the sheet does not completely fall when leaf fall. The upper and lower end of the cushion sheet are scharfwinkelig, while the sides are rounded. The actual leaf scar sits slightly above the middle of the leaf cushion and is usually elliptical to rhombic. In the area of the leaf scar, there are three small pits. The mean corresponds to the individual vascular bundles, which supplies the sheet. The two lateral mark the two Parichnos strands, loose parenchyma channels. The Parichnosgewebe rises in the bark and runs through two pits on the abaxial side of the leaves. In Lepidodendron there are two additional Parichnoskanäle that something emerge at the root surface below the leaf scar. These two are absent in Diaphorodendraceae. Directly above the leaf scar is a mark indicating the position of the ligule.

The epidermis has a waxy cuticle and easy without special cells such as trichomes or glands. Stomata are frequent and sunk in shallow pits.

The tribes of Lepidodendrales are proto stelae ( Diaphorodendron ) or Siphonostelen ( Diaphorodendron and Lepidodendron ), or have mixed marks. Strains with Proto stelae have a central strand of primary xylem, which is surrounded by a narrow ring of protoxylem, this. Again of secondary xylem In mixed marrow, the center has parenchymal cells intermingled with tracheids, the tracheids of the form rather short parenchyma cells are similar as long slender cells. This is seen as an indication that the evolution of Marks led a not consummated differentiation of the cells to tracheids in the center at the Lepidodendrales. In metaxylem Leitertracheiden occur. The maturation of the xylem exarch done.

To the primary xylem, a narrow ring of secondary xylem are, which includes more than a few centimeters. The tracheids in the secondary xylem are Leitertracheiden. Tracheids in the primary and secondary xylem have fine longitudinal bars between the ladder steps, so-called Williamson Strip. Through the secondary xylem pull some wood beams that are one cell wide and several cells high.

Just outside the secondary xylem are thin-walled cells, the cambium. This is a unifaziales cambium, it is divided from only inward secondary xylem, secondary phloem is not formed.

The phloem is called the secondary xylem by a band of thin-walled cells, parenchyma, separated. At this sheath close to the outside screen elements with large, elliptical sieving panels on. Between these are strands of phloem parenchyma. Secondary phloem was the current state of knowledge is not formed after.

The bark of the Lepidodendrales in three zones - an inner, middle and outer - divided, which are characterized by particular cell types. The inner bark is the smallest zone and consists of small isodiametric parenchymal cells. Here also cell clusters come with dark contents, possibly secretory cells before and lacunae and various sclerotic cells. The middle crust is extensive and consists of larger parenchyma cells. In young stems, this area is characterized by radially elongated lacunae in older strains of this area is not preserved with the exception of some parenchymal cells. The outer cortex is composed of cells with slightly thickened cell walls, similar to the collenchyma cells.

The secondary cortical tissue or periderm is formed in the outer bark. It is very extensive and represents the largest part of the trunk diameter. In Diaphorodendron the periderm consists of two zones, an inner and an outer phelloderm phellem. The inner zone consists of alternating thick - and thin-walled cells with dark contents, which is interpreted as resin. Lepidodendron has a massive periderm, which is uniform or two layers.

The cork cambium replaces the epidermis of Lepidodendrales made ​​little to the outside, but a large amount of phelloderm inwards - in contrast to the current seed plants.

For the periderm were within the Lepidodendrales different types of education. Some species show several tangential bands of meristematic tissue. This outer cortical layers are meristematic and form for a certain period of radially arranged rows of cells. After a time, they represent the division activity and a new layer is meristematic and forms new periderm.

The leaves are Mikrophylle as with all Bärlapppflanzen. They are linear, some up to 1 m in length, but usually much shorter. The length correlated with the diameter of the scion portion where sit the leaves. This correlation is due to the growth of the deterministic Lepidodendrales. For isolated leaf fossils found different species have been established, it should, however, usually involve different sized sheets of different areas of a plant. The old name Lepidophyllum had since been awarded for a flowering plant, to be replaced by Lepidophylloides. Through the entire leaf blade is a single vascular bundle, which is accompanied on the abaxial surface of two shallow grooves pulls. Stomata placed abaxial and are also arranged in rows parallel to the pits. They are placed in shallow pits.

The leaves have a distinct hypodermis of fibers surrounding the mesophyll parenchyma and vascular bundles lying in the middle. Palisade parenchyma is not known.

The underground organs of Lepidodendrales be designated by the generic name Stigmaria or as Stigmariae. You branch dichotomously, among the most common club moss fossils and are mainly used in the clay layers directly below the coal layers before. The anatomy is primarily known for studies on Stigmaria ficoides.

From the base of the trunk arise from four main axes which are horizontal, the root system is shallow rooted. Lateral appendages are arranged spirally. These are shed in the course of growth and leave characteristic circular scars on the main axis. The lateral appendages are sometimes referred to as Stigmariae fine roots; However, their schraubige arrangement and rejection are more typical of leaves as for fine roots.

The principal axes branch out repeatedly dichotomously and form a vast underground system that can reach a diameter of 15 m. The principal axes have a parenchymatous Mark. The primary xylem endarch matures and is arranged in a series of belts and is in turn surrounded by a cambium. The secondary xylem is characterized by wide beams that give the wood a segmented appearance. The tracheids of the secondary xylem are arranged in radial rows and carry ladder-shaped thickenings with the characteristic longitudinal beams. Secondary phloem is absent in Stigmaria, the cambium was one-sided ( unifazial ), the mass transfer took place only on the primary phloem.

As primary secondary bark of Stigmaria ficoides have a complex structure of different cell and tissue types. The formation of secondary cortical tissue resulted similar to that. In surface axes to a narrow zone of periderm

The lateral appendages are up to 40 cm long and 0.5 to 1 cm in diameter. They are usually unbranched, branched dichotomously at most once. To tip gradually tapering. Each tag has a single, small monarches vascular bundles, which is surrounded by a compact inner bark. It close to the outside of a hollow middle and a thin outer cortical zone. The vascular bundle is bilaterally symmetrical and collateral. These and the above-mentioned characteristics have led to the interpretation that this tag is not the roots, but the sheets are homologous, and were converted to the function of anchoring and nutrient uptake. Thus, the main axis of the stem axis would be homologous.

Protostigmaria from the early Mississippian has a bulbous underground axis schraubig the lateral appendage sitting at the. It is divided by furrows into several lobes, in the course of growth more furrows were added. Protostigmaria was probably the underground part of Lepidodendropsis, a tree-shaped representatives of Lepidodendraceae.

Root hairs are so far not known, possibly formed the Lepidodendrales with mushrooms a mycorrhiza. In some fossils fungi were found in cells of the cortical parenchyma.

Reproductive organs

The reproductive organs of Lepidodendrales are cones that are formed at the distal branches in the tree crown. This may be late formed branches as Synchysidendron, or later dropped lateral branches as in Diaphorodendron. The cones were up more than 50 cm long ( Lepidostrobus goldenbergii ). A pin consists of a central axis, are at the sporophylls in schraubiger arrangement. The sporangia are of the adaxial (upper) side of the Sporophylle whose end portions are bent upwards, and the upper Sporophylle overlap. The ligule is sitting in a small pit in front of the sporangium. Most fossils were placed in the genus Lepidostrobus. An attempt was made to divide the genus was, with brackish - Hanes and Thomas 1983 only monosporangiaten pin ( with a variety spores) beließen in this genus, the bisporangiaten pin ( with two kinds of spores) in the genus presented Flemingites.

Lepidostrobus oldhamius from the Lower and Middle Pennsylvanian of North America and the UK are up to over 30 cm in length from two to six inches in diameter. The sporangia are massive and have an irregularly shaped cushion of sterile tissue that extends from the sporophyll in the lumen of the sporangium. All sporangia contain small spores from 20 to 30 microns, which carry a trilete ( three-rayed ) and fine scar on he side distal spines. Released spores of this type are placed in the genus Lycospora.

Flemingites schopfii bisporangiater a pin of about 8 cm long and 1.3 cm in diameter. The pins are similar to those of Lepidostrobus oldhamius, although there are two types of spores: distal sporangia form large quantities of small Lycospora -like spores which further below sporangia form 12 to 29 trilete megaspores 700-1250 microns in diameter.

Monosporangiate journals as the Journal of mikrosporangiaten straight spore- plants be interpreted in many ways. However, they can also be the cones of homo spore plants analogous to recent Lycopodium.

Some journals are monoporangiat, however, form only megaspores. The most common pins are Lepidocarpon belonging to the tribes of Lepidophloios. Lepidocarpon is the most highly developed reproductive structure of Bärlapppflanzen: the arrangement of the sporophylls is functionally equivalent to the integuments of the seed plants. The sporangia are sitting on the adaxial surface of the sporophylls. A sporophyll consists of two lateral Spreitenteilen and a distal shoulder. The lateral spreading cases the sporangium one such, which remains a slotted opening only at the top. In the sporangium matures only a large megaspore, while the other three wither. The wall of the megaspore, the latter, when found, Cystosporites is called, consists of loosely clustered strands of sporopollenin, a unique feature. The megagametophyte is that a few archegonia. Embryos are ellipsoidal, without vascular bundles and have a folded epidermis. During growth, the axis then provided with vascular bundles divides dichotomously: a branch develops into the shoot axis, the other to the underground Stigmariae system.

Other pin - genres such as Achlamydocarpon lacked this integumentartige envelope of the sporangia. Achlamydocarpon is considered cones of different species of the genus Diaphorodendron.

Knowing the gametophyte is based on a few finds. In Flemingites schopfii - pin micro-and megagametophyte are likely to receive. In the megaspores sometimes found near the trileten scar parenchymatous cellular megagametophyte. Some of the on the surface of cells bear rhizoids, which protrude from the scar and even pierce the sporangium. Archegonia have one to three layers of cervical cells and including a large stomach cell. Some microspores from the same kind of show stages in the development of Mikrogametophyten, about the division in antheridial initial and prothallium cell. Some cells contain material that is morphologically similar chromosomes. In general, the Mikrogametophyten more closely resemble extant Selaginella, while the megagametophytes extant Isoetes are similar.

In Lepidodendron Lepidodendron esnotense and rhodumnense are the megagametophytes multicellular structures that develop within the megaspore wall ( endospore ). When ripe they break through the trilete scar, where they form a mass of tissue in which several archegonia arise. Rhizoids missing here.

System

The karbon time Lycopsiden today are usually divided into three or four families, which the first three are monophyletic, while the fourth one rather badly -cut, paraphyletic group:

• Lepidodendraceae and
• Diaphorodendraceae: the first two families include species that were once largely found to be shed trees in the genus Lepidodendron form the basis of its common features.
• Sigillariaceae includes the seal trees that were recognized early on as an independent group.
• Ulodendraceae

The families of the Lepidodendrales provided by some authors to the Isoetales, then include all rhizomorphic - forming groups.

Prevalence and significance of

The tree-shaped Lepidodendrales built in large part on the Carboniferous forests from which the coal originated in Euramerica. In Westfalium ( Upper Carboniferous ), they formed up to 70 % of the biomass in the coal swamp forests. This proportion decreased in the subsequent Stefanium to around 5 %. At the end of the Carboniferous Lepidodendrales died out in Europe and America, in China, they survived into middle Permian. The reasons for their extinction a drier climate, tectonic activity, which resulted in lower marsh areas, or a combination of both are suspected.

Documents

• William A. DiMichele, Richard M. Bateman: The Rhizomorphic Lycopsids: A Case- Study in Paleobotanical Classification. Systematic Botany, 1996, Volume 21, pp. 535-552.
• Thomas N. Taylor, Edith L. Taylor, Michael Krings: Paleobotany. The Biology and Evolution of Fossil Plants. Second Edition, Academic Press 2009, ISBN 978-0-12-373972-8. Pp. 279-309.