Aglaophyton

Aglaophyton major, reconstruction

Rhynie Chert ( Aberdeenshire )

Aglaophyton major is a fossil plant, which is due to its combination of features between the mosses and vascular plants. She was described in 1920 from the Rhynie Chert in the Scottish county of Aberdeenshire for the first time. You alone forms the genus Aglaophyton.

• 5.1 Notes and references

Features

Sporophyte

The rhizome grows horizontally. At certain points rhizoids, which are non-septate, arising from epidermal cells grow.

The shoots of the sporophyte are upright, naked and branch out evenly dichotomous. The plant is more than 18 inches high. The diameter of the rung is about six millimeters.

The xylem consists of an inner zone of elongate, thin-walled cells, and an outer zone of elongated, thick-walled cells. This water line tissues do not contain tracheids because the cells lack the appropriate thickening. This central part is surrounded by a thick layer of thin-walled cells around six elongated cells due to their position considered " phloem ". Because this vascular tissue, however, the typical guide elements ( such as tracheids ) are missing, they are also as in mosses called Hydroide and Leptoide. Externally, the bark closes, which has very thick-walled cells in the outer region. The epidermis consists of elongated cells. The stomata are formed by two companion cells, which are surrounded by six to eight slightly modified epidermal cells.

The sporangia are spindle-shaped in cross section and radial symmetry. They are terminal, ie at the tips of the axes, just above a dichotomous branching of the axis. The wall of the sporangium consists of three cell layers, and is therefore eusporangiat. The outer layer consists of thin epidermal cells. This also has stomata. The next layer is similar to the parenchymatous and inner bark of the axis. The third layer surrounding the tapetum layer the central spore mass. Pre-formed opening points of the sporangium are not available. The spores are 52-78 microns in size, trilet ( have a three-rayed scar ) and laevigat (smooth).

Gametophyte

From Renate Remy and Remy 1980 Winfried Lyonophyton rhyniensis has been described which is probably the gametophyte of Aglaophyton. It is known only to the distal end of antheridia carrier. It is a bare, at least 16 mm long axis, which terminates in a disk-shaped structure of 2.6 to 9 mm in diameter. On the surface, the sit roundish antheridia. Each antheridium has a central sterile tissue. In many cases, the spermatozoa are visible in the antheridia.

The gametophyte was self-employed, ie not connected to the sporophyte. The connection to the sporophyte is produced by similarities of the cuticle, the stomata, and the anatomy of the water-conducting cells.

Also, the female gametophyte has now been identified, but not yet published.

Ecology

Aglaophyton grew singly or in groups on dry substrates, which were covered with litter. For germination humid conditions were probably necessary.

Systematic position

Aglaophyton is not a vascular plant. It corresponds to the construction of vascular plants, with the exception of the construction of the leading tissue. Thus, it occupies a middle position between mosses and vascular plants. Traditionally Aglaophyton will be provided to Rhyniophyta, but these are a polyphyletic group and actually part of the vascular plants. In cladistic analyzes Aglaiophyton is the sister group of vascular plants

Botanical history

The fossils from the Rhynie Chert were first kept for Rhynia gwynne - vaughanii, and described in 1920 by Robert Kidston and WH Lang as Rhynia major. Long no sporangia were known by Aglaophyton what to suspect gave rise, Rhynia major is the gametophyte of Rhynia gwynne - vaughanii. This would be a confirmation of a theory of the origin of land plants have been built which provided equal to the origin ( isomorphic ) gametophyte and sporophyte. The discovery of Rhynia maior fossils with sporangia was unnecessary further discussion.

Dianne S. Edwards 1986 could show on the basis of a re-examination of the original material and based on new discoveries that Rhynia major clearly did not have vascular bundles. She put on the new, monotypic genus Aglaophyton.

Documents

• Paul Kenrick, Peter R. Crane: The Origin and Early Diversification of Land Plants. A Cladistic Study. Smithsonian Institution Press, Washington, D.C. 1997, pp. 320-322. ISBN 1-56098-729-4
• Thomas N. Taylor, Edith L. Taylor: The Biology and Evolution of Fossil Plants. Prentice Hall, Englewood Cliffs 1993, pp. 196-198, ISBN 0-13-651589-4 227