Aglaophyton †
Pre-vascular, branching plants
Aglaophyton is an extinct, spore-bearing, non-vascular plant that was small, leafless, and rootless, but produced forked upright stems with terminal spore cases. It has a well-known sporophyte phase from the Rhynie chert, and has been shown to have a gametophyte phase called Lyonophyton. This plant lacked true vascular tissue, but had simple water conducting cells like mosses. Today, all living non-vascular plants lack the ability to branch in the spore-bearing phase, and Aglaophyton represents an intermediate stage of plant evolution.
Ecology & Form
Sporophyte (Spore-bearing phase)
Form genus: e.g. Aglaophyton majus
Stem
Isotomous aerial axes/stems and rhizomes
15 cm in height, and maximum width of 6 mm
No true vascular tissue
Distinct, hydroid-like conducting cells (Edwards 1986)
Stomata found on aerial and rhizomatous axes (Edwards et al. 1998)
Presence of stomata and preservation in the Rhynie chert suggests that these rhizomes crept on top of the soil.
It is assumed that these stomata had passive control, in which the loss of turgor pressure from drought resulted in stomatal closure
Evidence of mycorrhizal fungi found associated with the rhizomes
Leaves
Leafless aerial axes
Roots
Rootless, but exhibits rhizoids
Reproduction
Spore-bearing, homosporous plants
Sporangia are relatively large, fusiform in shape with spiral slits for dehiscence of spores
Aerial axes are terminated by these sporangia
No columella present in sporangium
Columella is present in other non-vascular plants such as mosses, hornworts, and Horneophyton †
Stomata are found on sporangium, but in lower frequency than axes
Stomata in these ancestral plants may have promoted dehiscence of sporangia (Chater et al. 2016)
Above: lifecycle of Aglaophyton-Lyonophyton
Gametophyte (gamete-forming phase)
Lyonophyton rhyniensis †
Remy & Remy 1980
Form genus of the male gametophyte; female gametophytes have been discovered but never described
Aerial axis that widens and terminates in a conspicuous cup-like structure which bears the antheridia
Smaller than sporophyte, but the axis of the gametophyte is very similar in anatomy
Possibly independent from sporophyte, but may be connected and reliant on the female gametophyte for nutrition
This gametophyte phase possesses stomata, which is unique among land plants
No bryophytes possess stomata in their gametophyte stage, only in the sporophyte on the sporangium.
Hornworts possess a 2-cell pore complex similar to stomata, but these pores lack the complexity of sporophytic stomata
Above: Lyonophyton gametophyte with an antheridium
Above: Model of an early land plant, Aglaophyton, employing apical growth to grow aerial axes. In this hypothesis, axes are created as apical meristems grow upwards, leaving cells to expand and mature. This mode of growth is seen in the Lycopodiaceae.
Above: Model of an early land plant, Aglaophyton, employing intercalary growth to grow aerial axes. In this hypothesis, several orders of axes are pre-formed before lengthening and then expand through intercalary growth between the branching points. This form of growth is seen in the Equisetaceae.
Diversity
Few species, with Aglaophyton as main representative
Aglaophyton majus †
Early Devonian (Lochkovian–Pragian) from Rhynie Chert in Aberdeen, Scotland
See descriptions above
Originally named Rhynia major, but was separated from that genus since this plant does not have vascular tissue
Teruelia diezii †
Early Devonian (Lochkovian–Pragian) from the Nogueras Formation of the Iberian Peninsula
Compression fossil with isotomous branching
Robust stems terminated in large, fusiform, twisted sporangia
This taxon is probably the compression fossil representative of Aglaophyton, which is known only from permineralizations
Above: Reconstruction of Teruelia diezii (from Fig 4, Cascales-Manana & Gerrienne 2017)