Rhacophytales
| Rhacophytales | ||||||||||||
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Rhacophyton sp. |
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| Temporal occurrence | ||||||||||||
| Middle Devon to Upper Devon | ||||||||||||
| Systematics | ||||||||||||
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| Scientific name | ||||||||||||
| Rhacophytales | ||||||||||||
The Rhacophytales are an extinct group of plants from the Devonian Mountains that stand near the base of the ferns .
features
The representatives are characterized by a unique branching system, in which bi- and quadriseriate units occur, which are in the axilla of special structures, the Aphlebiae. These are abnormal leaflets on the rhachis of the ferns. The appendages of the last branching stage can be dichotomously branched, in some forms they are in one plane. The vascular system consists of a clock-glass-shaped (clepsydroidem) primary xylem , which is surrounded by secondary xylem. The sporangia are in groups and have no annulus . All representatives are likely to have been homosexual .
Rhacophyton
The most fully known genus is Rhacophyton . It is known from numerous sites around the world, all of which are dated to the Upper Devonian.
Rhacophyton ceratangium is an upright plant around one meter high. The side branches, which were also referred to as fronds, consist of quadriseriaten side axes. This arrangement was also known as tetrastich, i.e. standing in four rows. The side axes are in two vertical rows on the main axis. The branch segments are over 50 cm long. The branch morphology is very variable, there are all transitional forms between two and three-dimensional forms. Aphlebia-like structures are paired on the main axis. At the distal ends of the last branching axes there are pinnate-like structures.
The stems of Rhacophyton ceratangium are up to 2 cm in diameter. They have a central, clock-glass-shaped strand of primary xylems. This is surrounded by the secondary xylem, which consists of radially arranged rows of stair tracheids . There are also uniseriate rays that are one to seven cells high. At both ends of the narrow primary xylem there are so-called peripheral loops, which are empty and can also contain parenchyma in some places .
Rhachophyton zygopteroides from the Upper Devonian Belgium had an upright main axis with spirally arranged fronds. The pinna are in two rows and have small leaflets up to 1 cm in length. The trunk has a xylem with a star-shaped cross-section, the outer zone of which consists of tracheids and the inner zone of poorly preserved cells. The traces of leaves branch off from the arms of the stele as rectangular bars, each of which has two peripheral loops.
The fertile fronds of Rhacophyton have not yet been found in organic connection with the other organs. A frond is three-dimensional with two long, sterile leaflets and two branched, fertile leaflets. Each fertile plumage consists of a basal stalk that branches dichotomously several times in quick succession. This creates a dense, three-dimensional structure. The penultimate branches are slightly bent inwards and have small twigs on their inward-facing side. These, in turn, branch dichotomously, with a terminal sporangium on each of the branches .
The sporangia of Rhacophyton ceratangium are spindle-shaped with a long, curved tip. They are up to 2.4 mm long and 0.4 mm wide. They open lengthways. The spores are egg-shaped with a diameter of around 50 micrometers. They have a trilete (three-pointed) scar. The spores resemble the sporae dispersae of the Perotriletes type .
Other genera
- Protocephalopteris
Protocephalopteris is from the Middle Devonian of Spitsbergen and Siberia known. They are large, double-pinnate fronds with alternate plumage. The fertile unit consists of pairs of terminal fertile segments on which the sporangia stand in pairs. These pairs stand in the armpits of Aphlebiae. In contrast to Rhacophyton, there is no quadriseriate branching.
- Chlidanophyton
Chlidanophyton ranges from the late Devonian to the early Tournaisian . They are plants with terminal, recurved sporangia on a branched system.
- Eocladoxylon
Eocladoxylon from the Middle Devonian ( Givetian ) of China has sterile and fertile branches which Rhacophyton similar. Secondary xylem is absent. The end segments are in one plane. The fertile segments are three-dimensional. The sporangia of Eocladoxylon minutum resemble the pollen sacs of some seed ferns , such as Telangiopsis .
- Protopteridophyton
Protopteridophyton from the Devonian ( Givetium to Frasnium ) of southern China combines features of trimerophytes , ferns and progymnosperms.
- Ellesmeris
Ellesmeris sphenopteroides from the frasnium of Ellesmere Island (Canadian Arctic) combines properties of the Rhacophytales and the Zygopteridales . The stele of the main axis is bipolar and shaped like a watch glass. The Aphlebiae are at the base of each primary leaflet. The leaf segments are flat. Ellesmeris is significantly older than the Zygopteridales, the completely flat leaflets are older than those previously known. But there is no clear differentiation between the axis and the leaf, which is why it is more likely to be related to the Rhacophytales than to the Zygopteridales.
Systematic position
The exact systematic position of the Rhacophytales is still unclear. Rhacophyton was placed among the Aneurophytales in the past , when these were still regarded as the forerunners of the seed ferns and real ferns. They were considered to be the Protopteridales , the Zygopteridales or as a precursor group of the Coenopteridales .
The representatives of the order share characteristics with some representatives of the Iridopteridales and the progymnosperms . Some authors place them as a family with the Zygopteridales. With the Zygopteridales they share the clock-glass-shaped xylem and the quadriseriate branching. With the progymnosperms, they share the presence of secondary xylem.
Taylor, Taylor and Krings (2009) see them as more developed than the Trimerophytophyta from which they arose; but they lack the organ differentiation that occurs in the somewhat younger fern-like plants.
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. 401-405.
Web links
- Rhacophyton on Devonian Times