Horneophytes †
Earliest branching land plants
The horneophytes are an extinct group of plants that are similar to bryophytes, such that they lacked true vascular cells, but differed from the bryophytes because they have the ability to branch in the spore-producing phase (sporophyte). They are a basal group of plants with branching sporophytes (polysporangiophytes) on the land plant tree of life, which are now extinct.
Geologic Age
Late Silurian (Pridoli) - Early Devonian (Pragian)
Ecology & Form
Probably lived in wet, shore environments; grew on sandy, organic-rich soil in damp to wet locations
Fossils of chytrids, ancient fungi, have been found in the tissue of Horneophyton
Sporophyte (spore-bearing) phase
Horneophyton lignieri †
Kidston & Lang 1920; Barghoorn & Darrah 1938
Stems
Upright axes 20 cm tall (2 mm wide) growing from corm-like base
These plants exhibit equal-branching (isotomous) in distal branches; slightly anisotomous in proximal axes
Axis tips terminated by sporangia
Stomata are relatively rare and are enclosed by distinctly modified cells (Hass 1991)
Lack true vascular tissue, but possessed water-conducting cells similar to bryophyte hydroids, and sugar-conducting cells similar to leptoids
Roots
Lack roots, but possess rhizoids to anchor to the soil
Leaves
Leafless
It is assumed that the upright stems were photosynthetic, although they may have not been wide enough to support chlorophyllous tissues (see Langiophyton below)
Reproduction
Spore-bearing plants with sporangia that are stem-like (cylindrical) and the sporangia were forked
Bumps or emergences are on the sporangia
Central column of sterile tissue (=columella) found in "horn"
Similar (homologous?) to Aglaophyton †, hornworts, and mosses
Spores were released through a slit at the top of each lobe
Spores were trilete meiospores with short conical protuberances
Above: Horneophyton corm (c) with rhizoids (r) and hydroids (v)
Above: Horneophyton stem (in longitudinal section) with forked sporangia (s)
Above: Model reconstruction of Horneophyton
Above: Reconstruction of Horneophyton lignieri
Gametophyte (gamete-forming) phase
Langiophyton mackiei †
Remy & Hass 1991
Free-living, female gametophyte growing to 6 cm tall
The aerial shoots with cup-like structures with numerous archegonia inside
Possibly independent from the sporophyte
After fertilization, as the sporophyte grew, Horneophyton began to create the characteristic corm-like base.
The presence of this “rooting structure” may indicate that the sporophyte (Horneophyton) could eventually become independent from Langiophyton, with the ability to obtain soil nutrients after Langiophyton died
If Horneophyton was independent, then this means that this generation was probably green with the ability to conduct photosynthesis
Diversity
Caia langii †
Late Silurian from Hereford, England
Naked, parallel-sided sporophyte axes branching isotomously
Axes terminating in vertically elongate sporangia which bear spinous emergences particularly at the distal ends
Caia means "cudgel" referring to the shape of the sporangia
Sporangia are homosporous with spores that are trilete, laevigate, and retusoid.
Above: Reconstruction of Caia langii (from Fig 5, Fanning et al. 1990)
Tortilicaulis transwalliensis †
Late Silurian (Pridoli) - Early Devonian (Lochkovian) of Wales
Branched sporophyte axes exhibiting twisting (10 mm long, 0.1-0.4 mm wide) with fusiform to oval sporangia (Kenrick & Crane 1997)
Originally thought to be a bryophyte, but later found to possess branched sporophyte axes
Above: Sporangia of Tortilicaulis transwalliensis † (from Plate 5 of Edwards 1979)