Cordaitales

Cordaites lungatus, leaf imprints

The Cordaitales are an extinct, Paleozoic order of tree-shaped seed plants. They are related relatively closely with the conifers.

Features

Several members of the order have now been fully reconstructed. These are usually named after the Morphotaxon of the tribe. Tree -shaped Cordaitales have a monopodial trunk and a distal crown of large, strip-shaped leaves. Representatives from the tropical zone of the Pennsylvaniums reached up to 45 m height. Cordaites dumusum however, was a long-lived shrub, Shanxioxylon sinense a small tree. Some species were stilt roots.

Strain

The Cordaitales possess the characteristic root of the gymnosperms: a Eustele, in which the secondary xylem is formed by a bifaziales cambium and periderm by a cork cambium. Often, the strain has a significantly distinct marks. This consists of thin partitions between which are hollow chambers. Some species also have a solid, parenchymatous Mark. Are drains the hollow marrow obtained fossil, they are placed in the form genus Artisia. The wood of the secondary xylem is pronounced. There is in some species of narrow tracheids with uni- or biseriaten spotting and uniseriaten rays (similar to conifers). In other species the tracheids are larger with multiseriaten spotting. Rays are then numerous and biseriat. Wood parenchyma or resin canals do not occur in Cordaiten. Isolated preserved secondary xylem is called Dadoxylon.

The tips of the shoot axes are always inside parenchymatous. The formation of the vaginal walls due to shrinkage of the cells takes place only in the course of the root growth.

The branch is usually axillary, where from the armpit of the liner sheet emanate from one or two side branches.

The bark of young stems is constructed according to one of two patterns: one with alternating strands of sclerenchyma and parenchyma, on the other hand it is thin with significantly above leaf bases.

Twigs and branches with spirally arranged leaf scars are summarized in the form genus Cordaicladus.

Single parent genera are:

• Cordaixylon has a Eustele with big mark, which is surrounded by a small number of primary xylem strands. They resemble in this point so the recent conifers. The Mark has partly a diameter of about 10 cm.
• Pennsylvanioxylon Cordaixylon similar and can not always be distinguished from the latter.
• Mesoxylon has a parenchymatous marrow and no septa. Stem and leaf traces are mesarch.
• In Piracicaboxylon the marrow is divided into a central and a peripheral zone, which are separated by a parenchymatous ring.
• Shanxioxylon mostly missing trace strands of side branches, just missing the Sklerenchymbänder in the bark.

Root

Isolated roots found, which are attributed to the Cordaiten are placed in the form genus Amyelon. Amyelon species are, however, also found in areas and layers of which are not known Cordaiten.

The protostele has a diarches to pentarches vascular bundles. The tracheids of primary xylem have spiraled to multiseriate pits. The secondary xylem has uniseriate rays and radially arranged tracheids with one to five rows of pits on the radial cell walls. The secondary phloem consists of sieve elements, parenchyma, fibers and rays. On the outside of the secondary phloem followed by a layer of phelloderm and a dense layer of phellem with lenticels.

Lateral roots arise in large bundles in phellem -covered growths on the larger roots. From the main roots they differ by a distinctly developed endodermis.

The leaves of the Cordaiten are the organs most frequently obtained. They are mainly associated with the genus Cordaites, after the order has received its name. Cordaites includes euramerikanische species. They are usually spatulate and are available in a spiral arrangement on the branches. The vascular bundles rarely and dichotomous branching out; they therefore appear parallel. A midrib is missing. Grand'Eury 1877 has divided the genus by morphological criteria into three subgenera: Eu Cordaites has large spatulate leaves with rounded tip; Dory - Cordaites somewhat smaller leaves with sharpened; and Poa Cordaites with narrow, grass-like leaves.

Most leaves are 10-20 cm long, but they can reach 100 cm in length.

The Cordaitales also Heterophyllie occurred. Flake or needle- shaped leaves were at rnd to buds and at the base of branches of some species, dumusum at Cordaixylon even along the entire length of some branches.

The stomata are in longitudinal rows between the vascular bundles ( intercostal ). The gap opening apparatus consists of two bean-shaped guard cells, two lateral side cells and at each end each have a terminal cell. The mesophyll is often differentiated into palisade parenchyma and spongy. Along the vascular bundles are often located fiber strands that result in a double T- beam structure in the leaf cross section. The cuticle surface is very diverse, an indication that the number of species of Cordaiten is underestimated until now.

Cordaites - like leaves from the Carboniferous and Permian of the southern hemisphere are called Noeggerathiopsis. More sheet genera of the southern hemisphere are Pantophyllum, Kawziophyllum and Euryphyllum.

Reproductive organs

The seed- and pollen-producing organs are composite cones and are usually at the ends of the leaf -bearing branches. The cones are always monosporangiat, so either contain ovules or pollen sacs. Since cones are rarely found on the plants, it is unknown whether the plants were monoecious or dioecious. The pins are generally 10 cm long, but may reach 25 cm. On the main axis are bracts in cross against permanent arrangement. In the axil of each leaf springs supporting a lateral axis, are at the scale-like leaves in schraubiger arrangement. Most are vegetatively -sharpened, some wear at the tip ovules or pollen sacs. In Cordaitanthus the pollen sacs are in a ring at the top of the shed and mature in sequence. In Gothania they stand in a row on the scale and matured at the same time.

The ovules are often flattened. The base is usually heart-shaped, pointed the end. The micropyle is narrow. The nucellus is fused at the base with the integument, then free up. Is located at the base of the nucellus a slice of tracheids, two vascular bundles run in the integument. The representatives of Cardiocarpus and Nucellangium are oval in cross section, Mitrospermum bears on the edge of two small wings. The pollen chamber consists of a conical region at the tip of the nucellus. The state of the micropyle before and after fertilization, similar to the in recent conifers. Therefore, it is assumed that this takes place as in the fertilization of a Polli Nation drop that draws in and collects the drops pollen chamber.

The ovules enlarged and the integument formed a sarcotesta. The Megagametophyt enlarged and took almost the entire seed cavity, while the nucellus was reduced to a paper-thin layer. When the seeds had reached its final size, the fiber cells of Sklerotesta differentiated from. The Megagametophyt was cellularly and formed mikropylenseitig two archegonia.

From Nucellangium seeds with young embryos are known, a rarity in Paleozoic gymnosperms. The embryo is 0.2 mm long, ellipsoidal, and cellular. This means that the embryo development in contrast to extant gymnosperms from the beginning was cellular or was very early cellular. From the rarity of mature seeds is concluded that there must have been at the Cordaiten no seed dormancy.

The pollen is monosaccat. Germination took place either proximally with a trileten or monoleten germ opening and are considered Präpollen, or there was no germ opening and it is probably true to pollen. Präpollen type Felixipollenites is known from pollen sacs of Gothania lesliana, type Sullicaccites from Gothania -type pin of Mesoxylon priapi. Florinites pollen is known from the microsporangia of Cardaitanthus concinnus and Cordaitanthus Journal of Cordaixylon dumusum. Little is known about the structure of the Mikrogametophyten.

Dissemination

Especially numerous are findings from Europe, North America and China. Here the Cordaitales formed a substantial part of spätpaläozoischen flora. Many species were growing in the lowlands in peat swamps. Here they formed either monotypic or stocks they grew together with Calamiten, tree ferns and lycophytes different. Other representatives were growing on well-drained mineral soils. On dry sites, they were also part of fire- tolerant societies. At higher altitudes, particularly high Cordaiten grew.

Cordaiten are often regarded as inhabitants of sea -influenced marshes and would thus represent the oldest forms of mangroves. The lack of significant physiological adaptations to brackish water and the absence of signs of permanent flooding leave this hypothesis in recent years appear to be unlikely.

System

About the systematic position of the Cordaitales there are different views. Since Rudolf Florin's work is generally accepted a close relationship of the Cordaitales with the conifers. The view that conifers evolved from the Cordaitales, was later in favor of the view, both groups evolved from a common ancestor, abandoned.

As ancestors sees Beck (1981 ) to the Archaeopteridales ( Progymnospermen ), this view is supported by similarities in vegetative structure and in the wood anatomy. Rothwell (1982 ) called seed ferns from the Pennsylvanian, especially the Callistophytales as possible ancestors. Here the similarities lie with the reproductive organs. Taylor et al. (2009) speak the latter theory, however, too little persuasion.

A cladistic analysis showed that the studied genera of Cordaitales ( Shanxioxylon, Cordaitanthus - Pennsylvanioxylon and Mesoxylon ) form a clade that has as a sister group, the clade of Paleozoic, Mesozoic and extant conifers.

Documents

• 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. 787-804.
• Gar W. Rothwell: Cordaitales. In: Charles B. Beck (eds.): Origin and Evolution of Gymnosperms. Columbia University Press, New York, 1988, ISBN 0 - 231-06358 -X, pp. 273-297.