Cordaitales
| Cordaitales | ||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Cordaites lungatus , leaf prints |
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| Temporal occurrence | ||||||||||||
| Mississippian to Permian | ||||||||||||
| 360 to 250 million years | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Cordaitales | ||||||||||||
| Scott , 1909 | ||||||||||||
The Cordaitales are an extinct, Paleozoic order of tree-shaped seed plants . They are relatively closely related to the conifers .
features
Several representatives of the order have now been completely reconstructed. These are usually after Morphotaxon named 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 Pennsylvania reached a height of up to 45 m. Cordaites dumusum, on the other hand, was a long-lived shrub , Shanxioxylon sinense a small tree. Some species formed stilt roots .
tribe
The Cordaitales have the characteristic trunk of the gymnosperms: a Eustele , in which the secondary conductive tissue is formed by a bifacial cambium and the periderm by a phellogen . Often the trunk has a clearly pronounced pith . This consists of thin partitions between which there are hollow chambers. Some species also have a firm, parenchymal pith . If spouts of the hollow marrow have been preserved in fossil form, they are placed in the Artisia genus . The wood of the secondary xylem is pronounced. In some species it consists of narrow tracheids with uni or biseriate pits and uniseriate rays (similar to conifers). In other species the tracheids are larger with multiseriate pits. Their rays are then numerous and biseriat. Wood parenchyma or resin canals are not found in Cordaites. Secondary xylem obtained in isolation is known as dadoxylon .
The tips of the shoot axes are always parenchymatic inside. The formation of the partition walls through shrinking of the cells only takes place in the course of the trunk growth.
The branching is usually axillary, with one or two side branches emerging from the axilla of the support sheet .
The bark of young trunks is structured according to one of two patterns: on the one hand, with alternating strands of sclerenchyma and parenchyma, on the other hand, it is thin with clearly protruding leaf bases.
Branches and twigs with spirally arranged leaf scars are summarized in the form genus Cordaicladus .
Individual trunk genera are:
- Cordaixylon has a large marrow eustele surrounded by a small number of primary xylem strands. In this respect they are similar to the recent conifers. The pith is sometimes over 10 cm in diameter.
- Pennsylvanioxylon is similar to and cannot always be distinguished from Cordaixylon .
- Mesoxylon has a parenchymal marrow and no septa. Stem and leaf marks are mesarchic.
- In Piracicaboxylon , the marrow is divided into a central and a peripheral zone, which are separated by a parenchymal ring.
- Shanxioxylon usually lacks traces of side branches, as well as the sclerenchyma bands in the bark.
root
Isolated found roots that are attributed to the Cordaiten be in the form of generic Amyelon asked. Amyelon species are, however, also found in areas and layers from which no cordaites are known.
The protostele has a diarchic to pentarchic vascular bundle . The tracheids of the primary xylem have spiral 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 follows a layer of phelloderm and a dense layer of phellema with lenticels .
Side roots arise in large bundles in phellem-covered growths on the larger roots. They differ from the main roots by a clearly developed endodermis .
leaves
The leaves of the cordaites are the most commonly preserved organs. They are predominantly assigned to the genus Cordaites , after which the order also got its name. Cordaites include Euramerican species. They are usually spatulate and are arranged in a spiral on the branches. The vascular bundles rarely branch and then dichotomously; therefore they appear in parallel. A midrib is missing. In 1877 Grand'Eury divided the genus into three sub-genera according to morphological criteria: Eu-Cordaites has large spatulate leaves with a rounded tip; Dory-Cordaites slightly smaller leaves with a pointed tip; and Poa-Cordaites with narrow, grass-like leaves.
Most leaves are 10 to 20 cm long, but they can also reach 100 cm in length.
The Cordaitales also had heterophyllia . Scale- or needle-shaped leaves stood around buds and at the base of branches of some species, with Cordaixylon dumusum even along the entire length of some branches.
The stomata are in longitudinal rows between the vascular bundles (intercostal). The stomata consists of two bean-shaped guard cells , two side cells and one terminal cell at each end. The mesophyll is often differentiated into palisade and sponge parenchyma . Along the vascular bundles there are often fiber strands that result in a double T-beam structure in the blade cross-section. The cuticle surface is very diverse, an indication that the number of Cordaite species has been underestimated to date.
Cordaites -like leaves from the Carboniferous and Permian of the southern hemisphere are known as Noeggerathiopsis . Further leaf genera in the southern hemisphere are Pantophyllum , Kawziophyllum and Euryphyllum .
Reproductive organs
The organs that produce seeds and pollen are composed of cones and are usually located at the ends of the leaf-bearing branches. The cones are always monosporangiate, so contain either ovules or pollen sacs . Since cones are rarely found on the plants, it is unknown whether the plants were monoeconomic or diocese . The cones are usually around 10 cm long, but can also reach 25 cm. On the main axis there are bracts in a cross-opposed arrangement. In the axilla of each support sheet a side axis arises, on which there are flaky sheets in a helical arrangement. Most are vegetative with a pointed end, some have ovules or pollen sacs at their tips. In Cordaitanthus , the pollen sacs stand in a ring at the tip of the scales and ripened one after the other. At Gothania they stand in a row on the shed and ripened at the same time.
- Ovules and seeds
The ovules are often flattened. The base is usually heart-shaped, the end pointed. The micropyle is narrow. The nucellus is fused with the integument at the base, then free at the top. At the base of the nucellus there is a disc of tracheids, two vascular bundles lead into the integument. The representatives of Cardiocarpus and Nucellangium are oval in cross section, Mitrospermum has two narrow wings on the edge. The pollen chamber consists of a conical area at the tip of the nucellus. The condition of the micropyle before and after fertilization is similar to that of recent conifers. It is therefore assumed that fertilization takes place via a pollination drop, as with these, which catches the drop and sucks it into the pollen chamber.
The ovules enlarged and the integument formed a sarcotesta . The mega gametophyte enlarged and took up almost the entire seminal cavity, while the nucellus was reduced to a paper-thin layer. When the seed reached its final size, the fiber cells of the sclerotesta differentiated . The megagametophyte became cellular and formed two archegonia on the micropylene side .
From Nucellangium seeds are known with young embryo, a rarity in Paleozoic gymnosperms. The embryo is 0.2 mm long, ellipsoidal and cellular. This means that, in contrast to recent gymnosperms, embryo development was cellular from the beginning or became cellular very early. From the rarity of ripe seeds it is concluded that the Cordaites should not have had any dormancy .
- Pollen
The pollen is monosaccate. Germination took place either proximally with a triple or monolet germ opening and is considered prepollen, or there was no germ opening and it is possibly real pollen. Prepollen of the Felixipollenites type is known from pollen sacs from Gothania lesliana , the Sullicaccites type from Gothania -type cones from Mesoxylon priapi . Florinites pollen is known from the microsporangia of Cardaitanthus concinnus and the Cordaitanthus cones from Cordaixylon dumusum . Little is known about the structure of the micro gametophyte .
distribution
Finds from Europe, North America and China are particularly numerous. Here the Cordaitales formed a substantial part of the late Paleozoic flora. Many species grew in the lowlands in peat swamps. Here they either formed monotypical stands or they grew together with calamites , tree ferns and various lycophytes . Other representatives grew on well-drained mineral soils. In dry locations, they were also part of fire-tolerant societies. Particularly high cordaites grew at higher altitudes.
Cordaites are often seen as inhabitants of swamps influenced by sea water and would therefore represent the oldest forms of mangroves . The lack of essential physiological adaptations to brackish water as well as the absence of signs of permanent flooding make this hypothesis seem unlikely in recent years.
Systematics
There are different views on the systematic position of the Cordaitales. Since Rudolf Florin's work, a close relationship between the Cordaitales and the conifers has been generally accepted. The view that the conifers evolved from the Cordaitales was later abandoned in favor of the view that both groups evolved from a common ancestor.
Beck (1981) regards the archaeopteridales ( progymnosperms ) as ancestors ; this view is supported by similarities in the vegetative structure and in the wood anatomy. Rothwell (1982) names Pennsylvanian seed ferns , especially the Callistophytales, as possible ancestors. Here are the similarities in the reproductive organs. Taylor et al. (2009) believe that the latter theory has little persuasive power.
A cladistic analysis showed that the examined genera of the Cordaitales ( Shanxioxylon , Cordaitanthus-Pennsylvanioxylon and Mesoxylon ) form a clade that has the clade of Paleozoic, Mesozoic and recent conifers as a sister group .
supporting 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 (Ed.): Origin and Evolution of Gymnosperms . Columbia University Press, New York 1988, ISBN 0-231-06358-X , pp. 273-297.
