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.

Classification

└Embryophytes

└Polysporangiophytes

└Horneophytes †

Geologic Age

Ecology and 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"

  • 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 †

  • Fanning et al. 1990

  • 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.

Caia langii

Above: Reconstruction of Caia langii (from Fig 5, Fanning et al. 1990)

Tortilicaulis transwalliensis †

  • Edwards 1979

  • 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

Tortilicaulis transwalliensis

Above: Sporangia of Tortilicaulis transwalliensis † (from Plate 5 of Edwards 1979)