Horneophytopsida
Encyclopedia
Horneophytopsida is a class
of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue
, found from the Late Silurian to the Early Devonian (around ). They are the simplest known polysporangiophyte
s, i.e. plants with sporophytes bearing many spore-forming organs (sporangia
) on branched stems. They were formerly classified among the rhyniophytes, but it was later found that some of the original members of the group had simple vascular tissue and others did not.
In 2004, Crane et al. published a cladogram
for the polysporangiophytes in which the Horneophytopsida are shown as the sister group of all other polysporangiophytes. One other former rhyniophyte, Aglaophyton, is also placed outside the tracheophyte clade, as it did not possess true vascular tissue (in particular did not have tracheid
s), although its conducting tissue is more complex than that of the Horneophytopsida.
(See the Polysporangiophyte
article for the expanded cladogram.)
The class currently includes the genera Caia
, Horneophyton
, and Tortilicaulis
. Horneophyton is the best known genus; see that article for a description.
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of extinct plants which consisted of branched stems without leaves, true roots or vascular tissue
Vascular tissue
Vascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
, found from the Late Silurian to the Early Devonian (around ). They are the simplest known polysporangiophyte
Polysporangiophyte
Polysporangiophytes, also called polysporangiates or more formally Polysporangiophyta, are plants in which the spore-bearing generation has a structure of branching stems terminating in sporangia...
s, i.e. plants with sporophytes bearing many spore-forming organs (sporangia
Sporangium
A sporangium is an enclosure in which spores are formed. It can be composed of a single cell or can be multicellular. All plants, fungi, and many other lineages form sporangia at some point in their life cycle...
) on branched stems. They were formerly classified among the rhyniophytes, but it was later found that some of the original members of the group had simple vascular tissue and others did not.
In 2004, Crane et al. published a cladogram
Cladogram
A cladogram is a diagram used in cladistics which shows ancestral relations between organisms, to represent the evolutionary tree of life. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational...
for the polysporangiophytes in which the Horneophytopsida are shown as the sister group of all other polysporangiophytes. One other former rhyniophyte, Aglaophyton, is also placed outside the tracheophyte clade, as it did not possess true vascular tissue (in particular did not have tracheid
Tracheid
Tracheids are elongated cells in the xylem of vascular plants that serve in the transport of water and mineral salts. Tracheids are one of two types of tracheary elements, vessel elements being the other. All tracheary elements develop a thick lignified cell wall, and at maturity the protoplast...
s), although its conducting tissue is more complex than that of the Horneophytopsida.
(See the Polysporangiophyte
Polysporangiophyte
Polysporangiophytes, also called polysporangiates or more formally Polysporangiophyta, are plants in which the spore-bearing generation has a structure of branching stems terminating in sporangia...
article for the expanded cladogram.)
The class currently includes the genera Caia
Caia (plant)
Caia is a genus of small fossil plants of Late Silurian age . The diagnostic characters are naked parallel-sided axes branching isotomously, terminating in vertically elongate sporangia which bear spinous emergences particularly at the distal ends. Spores are trilete and retusoid...
, Horneophyton
Horneophyton
Horneophyton, a member of the Horneophytopsida, was an early plant which may form a "missing link" between the hornworts and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert.-Description:...
, and Tortilicaulis
Tortilicaulis
Tortilicaulis is a moss-like plant known from fossils recovered from southern Britain, spanning the Silurian-Devonian boundary . Originally recovered from the Downtonian of the Welsh borderlands, Tortilicaulis has since been recovered in the famous Ludlow Lane locality.Whilst it is generally...
. Horneophyton is the best known genus; see that article for a description.
