Rhyniophyta

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Rhyniophyta

(rī'nēŏf`ətə), division of plants known only from fossils, of which the genus Rhynia was perhaps the most important. These plants date from the Silurian and Devonian age. Relatively simple in structure, they resemble the PsilotophytaPsilotophyta
, division of vascular plants consisting of only two genera, Psilotum and Tmesipteris, with very few species. These plants are characterized by the lack of roots, and, in one species, leaves are lacking also.
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 in many features, such as the lack of clearly developed roots. Like modern higher plants the Rhyniophyta had the specialized conducting tissues xylem and phloem. The Rhyniophyta are the most primitive group of vascular plants so far known and appear to be ancestral to most of the major divisions of vascular plants.

Rhyniophyta

 

(also Psilophyta), the oldest and most primitive extinct group, or division, of higher plants. The plants were characterized by homospory, the terminal location of sporangia, the absence of roots and leaves, dichotomous or dichopodial (pseudomonopodial) branching, and primitive anatomical structure. The vascular system was a typical protostele. The protoxylem was located in the center of the xylem, and the metaxylem consisted of tracheids with annular or, less commonly, scalariform thickenings. There were no sustentacular tissues. The Rhyniophyta were not capable of secondary growth, since their meristems were only terminal. The sporangia were primitive and had thick walls; they ranged in shape from globose (approximately 1 mm in diameter) to elongate-cylindrical (to 12 mm in length). Gametophytes have not been identified conclusively. Some botanists consider the horizontal rhizomatous organs, or rhizomoids, to be gametophytes. The Rhyniophyta grew in damp and swampy places and in coastal shallows.

The Rhyniophyta embraced a single class, Rhyniopsida. There were two orders: Rhyniales (including the families Cooksoniaceae, Rhyniaceae, and Hedeiaceae) and Psilophytales (including the family Psilophytaceae). The order Rhyniales was characterized by dichotomous branching and a thin, poorly developed stele. The xylem consisted of tracheids with annular thickenings. The oldest representatives of the Rhyniophyta belonged to the genus Cooksonia, which was originally discovered in Wales in deposits from the late Silurian (approximately 400 million years ago).

The most fully studied genera have been from the Lower Devonian: the Rhynia and, to a lesser extent, the Horneophyton. In the Horneophyton the rhizomoid was separated into monili-formly arranged tuberous segments that lacked vascular tissues and consisted entirely of parenchymatous cells. Stems originated above the rhizomoid, and numerous rhizoids below. It is believed that in the process of evolution the rhizomoids gave rise to roots. Both the Rhynia and Horneophyton had a multilayer sporangial wall covered with a cuticle. Horneophyton was characterized by a unique sporiferous chamber that formed an archlike canopy over the central column of sterile tissue, which was an extension of the phloem of the stem. In this way the Horneophyton resembled modern-day sphagnum.

The family Rhyniaceae also included the genus Taeniocrada, many species of which formed underwater thickets in the Middle and Upper Devonian. The Lower Devonian genera Hedeia and Yarravia are sometimes classified in the separate family Hedeiaceae. The Lower Devonian genus Sciadophyton, which is usually placed in the separate family Sciadophytaceae, was a small plant consisting of a rosette of simple or weakly dichotomized thin stems with a stele. Dichopodial branching and a better developed stele were typical of the order Psilophytales. The best-known genus, Psilophyton (known from Lower Devonian deposits in Eastern Canada) had unevenly developed branches that formed a false main axis of dichopody with thinner lateral branches. The stem was surrounded by a cutinized epidermis with stomata, and its surface was bare or covered with thorns (2-2.5 mm long) whose ends were disklike, which probably indicated their secretory role. The sporangia were exposed by a lengthwise slit. The Lower Devonian genera Trimerophyton was similar to the Psilophyton.

The study of the structure and evolution of the Rhyniophyta is of great significance for evolutionary morphology and the phylogeny of higher plants. The dichotomously branching stem with terminal sporangia was apparently the ancestor of the sporophyte of higher plants; the roots and leaves appeared later than the sporangium and stem. There is every reason to consider the Rhyniophyta the original ancestral group from which the bryophytes, lycopsids, equisetums, and ferns descended. According to another point of view, only the bryophytes, lycopsids, and Rhyniophyta have a common ancestor.

REFERENCES

Osnovy paleontologii: Vodorosli, mokhoobraznye, psilofitovye, plaunovidnye, chlenistostebel’nye, paporotniki. Moscow, 1963.
Traité de paléobotanique, vol. 2: Bryophyta, Psilophyta, Lycophyta. Paris, 1967.

A. L. TAKHTADZHIAN

Rhyniophyta

[‚rī·nē′äf·əd·ə]
(paleobotany)
A subkingdom of the Embryobionta including the relatively simple, uppermost Silurian-Devonian vascular plants.
References in periodicals archive ?
The simplest version of this hypothesis requires a single origin of zygotic retention, a single origin of free-living sporophytes, and a loss of zygotic retention in the rhyniophyte lineage after this diverged from the ancestry of modern forms.
The simplest version of this hypothesis requires a single origin of zygotic retention, in an ancestor of modern embryophytes that did not include rhyniophytes among its descendants.