Psilotaceae

Psilotum nudum

The fork -leaved plants ( Psilotaceae ), also called Urfarne, are a family of class Psilotopsida, which is among the ferns. Due to the lack of roots, their reduced leaves and the bifurcated branch they have a very earthy.

Features

The fork -leaved plants ( Psilotaceae ) are the least complex structure of vascular plants under the extant land plants. The sporophyte is persistent and has no root. It forms rhizomes, on which sit rhizoids. Also, the embryo has no root system. The shoot axis is low and dichotomous ( forked ) branched. It is no real dichotomy, but next to the old apical cell a neighboring cell is determined to the second apical cell. The axis bears reduced leaves with or without a vascular bundles. The above-ground axes have a aktinostele or approximately a siphonostele, said Mark is lignified.

The sporangia are great, its wall is two cell layers thick ( eusporangiat ). An annulus as an opening mechanism is missing. Two to three sporangia are fused into a Synangium and are seemingly on the adaxial ( facing the shoot axis ) side of a fork blade. A tapetum is missing. The spores are fed by sterile archesporial.

The spores are all the same size ( homospor ), kidney-shaped, monolet ( have a scar ). They are made to many ( over 1,000 ) per sporangium.

In Psilotum the gametophyte grows underground, is not photosynthetically active and feeds through the mycorrhiza. The gametophytes are a few inches tall and are cylindrical and branched. The antheridia sit on the surface and are vielkammerig, the spermatozoids are vielgeißelig. The archegonia are small and sunk. Strong developed gametophytes possess vascular bundles with lignified Ringtracheiden and a endodermis.

The basic chromosome number is x = 52

Dissemination

The fork -leaved plants are mainly found in the tropics and live terrestrial or epiphytic.

System

The systematic position of the fork -leaved plants has long been controversial. Some of them were even considered with the Urfarnen than the basal group of vascular plants. However, molecular biological studies have clearly provided to the ferns, where they form the sister group of the adder's tongue plants.

The family contains two genera with about 12 species:

• Psilotum Sw. two types. They are preferably used in symbiosis with fungi before ( mycorrhiza ). The plants have no leaves; photosynthesis happens in the sprouts.
• Tmesipteris Bernh. is often an epiphyte on fern Dicksonia. The scion is surrounded by up to 2 cm long leaf-like scales. The genus is no longer managed as a separate family Tmesipteridaceae. The following species belong to it (selection): Tmesipteris tannensis ( Spreng. ) Bernh, occurrence. Australia, Tasmania, New Zealand.
• Tmesipteris vieillardii P. A. Dang, occurrence. Caledonia.

From the fork blade plants no fossils are known. Because of molecular clocks has been calculated that they have separated from their sister group, the adder's tongue plants, already in the Carboniferous.

Documents

• Peter Sitte, Elmar Weiler, Joachim W. Kadereit, Andreas Bresinsky, Christian Körner: textbook of botany for colleges. Founded by Eduard Strasburger. 35th edition. Spektrum Akademischer Verlag, Heidelberg 2002, ISBN 3-8274-1010- X, pp. 734
• Alan R. Smith, Kathleen M. Pryer, Eric Schuettpelz, Petra coral, Harald Schneider, Paul G. Wolf: A classification for extant ferns. In: taxon. Volume 55, No. 3, 2006, ISSN 0040-0262, pp. 705-731, Abstract, PDF file.
• John W. Thieret: Psilotaceae. In Flora of North America Editorial Committee ( eds.): Flora of North America North of Mexico. Volume 2: pteridophytes and Gymnosperms, Oxford University Press, New York / Oxford et al 1993, ISBN 0-19-508242-7, pp. 16-17 ( limited preview on Google Book Search ). online. (English )