Rhacophytales

Rhacophyton sp.

Features

The representatives are characterized by a unique branching system, occur in the units of bilateral and quadriseriate that are in the armpit of special structures, the Aphlebiae. These are abnormal leaflet at the rachis of ferns. The appendage of the last branch level may be branched dichotomously, in some forms they are in one plane. The vascular system consists of uhrglasförmigem in cross section ( clepsydroidem ) primary xylem, which is surrounded by secondary xylem. The sporangia are in groups and have no annulus. All representatives must have been homospor.

Rhacophyton

The most completely known species is Rhacophyton. She is known worldwide by a number of sites, all of which are dated to the Upper Devonian.

Rhacophyton ceratangium is an upright, about one meter tall plant. The side branches, which were also referred to as fronds consist of quadriseriaten page axes. This arrangement was also referred to as tetra- stitch, so standing in four rows. The side axes are in two vertical rows on the main axis. The Astsegmente are about 50 cm long. The Astmorphologie is highly variable, there are all transitional forms between two - and three-dimensional shapes. Aphlebia - like structures are paired to the main axis. At the distal ends of the last branch axes are pinnules - like structures.

The strains of Rhacophyton ceratangium have a diameter of up to 2 cm. They have a central, hour glass strand primary xylem. This is surrounded by secondary xylem, which consists of radially arranged rows of stairs - tracheids. There are also uniseriate wood rays to seven cells are a high. At both ends of the narrow primary xylem sit called peripheral grinding, which is empty, in some places parenchyma may contain.

Rhachophyton zygopteroides from the Upper Devonian of Belgium had an upright main axis with spirally arranged fronds. The leaflets are in two rows and carry small leaflets of up to 1 cm in length. The strain has a star-shaped in cross-section xylem, the outer zone consists of tracheids and the inner zone of poorly preserved cells. The leaf traces 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 found in organic connection with the other institutions. A frond is three-dimensional with two long, sterile pinnae and two branched, fertile pinnae. Each fertile pinna consists of a basal stalk, which several times in quick succession branched dichotomously. Thus producing a dense, three-dimensional structure. The penultimate branches are curved slightly inward and carry small branches on their inward-facing side. These in turn branch dichotomously, at each of the branches is a terminal sporangium.

The sporangia of Rhacophyton ceratangium are spindle-shaped, with long, curved tip. They are up to 2.4 mm long and 0.4 mm wide. Open longitudinally. The spores are ovoid with a diameter of about 50 microns. They have a trilete ( three-rayed ) scar. The spores are similar to sporae dispersae type Perotriletes.

Other genera

Protocephalopteris is known from the Middle Devonian of Spitsbergen and Siberia. There are large, double- pinnate fronds with alternate pinnae arranged. The fertile unit consists of pairs of terminal fertile segments, where are the sporangia in pairs. These pairs are in the axils of Aphlebiae. Unlike Rhacophyton there is no quadriseriate branching.

Chlidanophyton ranges from the Late Devonian to the early Tournaisian. There are plants with terminally standing recurved sporangia on a branched system.

Eocladoxylon from the Middle Devonian ( Givetian ) of China has sterile and fertile branches that resemble Rhacophyton. Secondary xylem is missing. The terminal segments in a plane. The fertile segments are three-dimensional. The sporangia of Eocladoxylon minutum resemble the pollen sacs of some seed ferns such as Telangiopsis.

Protopteridophyton from the Devonian ( Givetian to Frasnian ) of southern China combines features of Trimerophyten, the ferns and the Progymnospermen.

Ellesmeris sphenopteroides from the Frasnian of Ellesmere Iceland ( Canadian Arctic) combines features of Rhacophytales and Zygopteridales. The stele of the main axis is bipolar and hourglass. The Aphlebiae are at the base of each primary pinna. The leaf segments are flat. Ellesmeris is significantly older than the Zygopteridales which completely flat leaflets are older than the known until then. But there is no clear differentiation between the axle and leaf, which is why they must be made more likely to be Rhacophytales as to the Zygopteridales.

Systematic position

The exact systematic position of the Rhacophytales is still unclear. Rhacophyton was provided in the past to the Aneurophytales when they were still viewed as precursors of seed ferns and true ferns. They were considered to Protopteridales, the Zygopteridales or as a precursor group of Coenopteridales.

The representatives of the order share characteristics with some representatives of the Iridopteridales and Progymnospermen. Some authors they are made as a family to the Zygopteridales. With the Zygopteridales they share the hour-glass shaped xylem and the quadriseriate branching. With the Progymnospermen they share the presence of secondary xylem.

Taylor, Taylor and Krings (2009) see them as more sophisticated than the Trimerophytophyta from which they originated; but they lack the organ differentiation, as they occur in the slightly younger fern-like plants.

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.