Horneophyton
Encyclopedia
Horneophyton, a member of the Horneophytopsida
, was an early plant which may form a "missing link" between the hornwort
s and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert
.
had bare stems (axes) up to 20 cm high and about 2 mm in diameter with an undivided cortex; stomata were present but rare. There was a thin central strand of conducting tissue, but this was not reinforced with spiral and reticulate thickenings (and thus does not constitute true vascular tissue
). Early stages of development of the sporophytes of Horneophyton (as of hornworts) may have been dependent on their parent gametophytes for nutrition, but mature specimens have expanded, corm-like bases to their stems, up to 6 mm in diameter, that bore rhizoids and appear to be anchored in soil, suggesting a capacity for independent existence after the gametophyte had degenerated.
The sporangium
(spore-forming organ) is unique among both living and fossil plants, consisting as it does of branched lobes at the apex of some of the branches of the stem. Each lobe contains a central collumella, analogous to the sporangia of hornworts; however, the sporangia of hornworts do not dichotomise. The number of lobes possessed by a sporangium varied; at least three orders of dichotomous branching have been found, resulting in more than four lobes. The sporangia were much less regular than shown in most reconstructions (including that opposite), and they had 'bumps' or emergences on them. Spores were released through a slit at the top of each lobe. The sporangia of Horneophyton contained trilete meiospores, the surfaces of which were decorated with short conical protuberances.
The female gametophyte
of the plant has been recognised and described as the form taxon Langiophyton mackiei. It grew to a height of around 6 cm, and was free living. It may have been monoicous
, i.e. produced male and female gamete
s on separate plants.
Horneophyton grew on sandy, organic-rich soil in damp to wet locations. They usually grew as isolated individuals.
led Kenrick and Crane in 1997 to create a new class, Horneophytopsida
, for this and apparently similar genera.
With vascular tissue but "bryophyte"-like alternation of phases and sporangia, the organism has been considered a missing link between the hornworts and the vascular plants or tracheophytes (which molecular data suggest are sister groups). Features suggesting a relationship with the hornworts include the general form of its sporangia; its corm also resembles the foot of some hornworts. The free living nature of its sporophytes, and the fact that they display branching, are marked differences which force it into the stem group of tracheophytes (along with Aglaophyton).
Horneophytopsida
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 . They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs on...
, was an early plant which may form a "missing link" between the hornwort
Hornwort
Hornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...
s and the Rhyniopsida. It is among the most abundant organisms found in the Rhynie chert
Rhynie chert
The Rhynie chert is an Early Devonian sedimentary deposit exhibiting extraordinary fossil detail or completeness . It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield chert, is located some 700 m away...
.
Description
The sporophyteSporophyte
All land plants, and some algae, have life cycles in which a haploid gametophyte generation alternates with a diploid sporophyte, the generation of a plant or algae that has a double set of chromosomes. A multicellular sporophyte generation or phase is present in the life cycle of all land plants...
had bare stems (axes) up to 20 cm high and about 2 mm in diameter with an undivided cortex; stomata were present but rare. There was a thin central strand of conducting tissue, but this was not reinforced with spiral and reticulate thickenings (and thus does not constitute true 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:...
). Early stages of development of the sporophytes of Horneophyton (as of hornworts) may have been dependent on their parent gametophytes for nutrition, but mature specimens have expanded, corm-like bases to their stems, up to 6 mm in diameter, that bore rhizoids and appear to be anchored in soil, suggesting a capacity for independent existence after the gametophyte had degenerated.
The sporangium
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...
(spore-forming organ) is unique among both living and fossil plants, consisting as it does of branched lobes at the apex of some of the branches of the stem. Each lobe contains a central collumella, analogous to the sporangia of hornworts; however, the sporangia of hornworts do not dichotomise. The number of lobes possessed by a sporangium varied; at least three orders of dichotomous branching have been found, resulting in more than four lobes. The sporangia were much less regular than shown in most reconstructions (including that opposite), and they had 'bumps' or emergences on them. Spores were released through a slit at the top of each lobe. The sporangia of Horneophyton contained trilete meiospores, the surfaces of which were decorated with short conical protuberances.
The female gametophyte
Gametophyte
A gametophyte is the haploid, multicellular phase of plants and algae that undergo alternation of generations, with each of its cells containing only a single set of chromosomes....
of the plant has been recognised and described as the form taxon Langiophyton mackiei. It grew to a height of around 6 cm, and was free living. It may have been monoicous
Monoicous
Monoicous is a botanical term used to describe plants which bear both sperm and eggs on the same gametophyte. Dioicous is the complementary term describing species in which gametophytes produce only sperm or eggs but never both. The terms are used largely but not exclusively in the context of...
, i.e. produced male and female gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
s on separate plants.
Horneophyton grew on sandy, organic-rich soil in damp to wet locations. They usually grew as isolated individuals.
Taxonomy
First named by Kidston & Lang in 1920 from Early Devonian fossils forming the "Rhynie flora", the original generic name Hornea transpired to be a synonym, leading to Barghoorn and Darrah renaming the genus to Horneophyton in 1938. It was classified as a rhyniophyte (subdivision Rhyniophytina) by Banks; the absence of true vascular tissueVascular 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:...
led Kenrick and Crane in 1997 to create a new class, Horneophytopsida
Horneophytopsida
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 . They are the simplest known polysporangiophytes, i.e. plants with sporophytes bearing many spore-forming organs on...
, for this and apparently similar genera.
Phylogeny
A possible phylogeny for Horneophyton is shown below (based on Crane et al. for the polysporangiophytes and Qiu et al. for the bryophytes).With vascular tissue but "bryophyte"-like alternation of phases and sporangia, the organism has been considered a missing link between the hornworts and the vascular plants or tracheophytes (which molecular data suggest are sister groups). Features suggesting a relationship with the hornworts include the general form of its sporangia; its corm also resembles the foot of some hornworts. The free living nature of its sporophytes, and the fact that they display branching, are marked differences which force it into the stem group of tracheophytes (along with Aglaophyton).
External links
- Horneophyton at the University of Aberdeen. Includes images. (The apparent size of the corms in the reconstruction is inconsistent with the cross-sections shown earlier. See .)
- Cladogram from
