Earliest record of transfer cells in Lower Devonian plants.

Key sources of information on the nature of early terrestrial ecosystems are the fossilized remains of plants and associated organic encrustations, which are interpreted as either biofilms, biological soil crusts or lichens. The hypothesis that some of these encrustations might be the remains of the thalloid gametophytes of embryophytes provided the stimulus for this investigation. Fossils preserved in charcoal were extracted from Devonian Period (Lochkovian Stage, c. 410-419 Myr old) sediments at a geological site in Shropshire (UK). Scanning electron micrographs (SEMs) of the fossils were compared with new and published SEMs of extant bryophytes and tracheophytes, respectively. One specimen was further prepared and imaged by transmission electron microscopy. Fossils of thalloid morphology were composed almost entirely of cells with labyrinthine ingrowths; these also were present in fossils of axial morphology where they were associated with putative food-conducting cells. Comparison with modern embryophytes demonstrates that these distinctive cells are transfer cells (TCs). Our fossils provide by far the earliest geological evidence of TCs. They also show that some organic encrustations are the remains of thalloid land plants and that these are possibly part of the life cycle of a newly recognized group of plants called the eophytes.

Plant evolution: A tapetum is now effectively present in all land plant lineages.

The tapetum, a tissue that elsewhere ensures correct spore development, is missing in some bryophytes. A new study shows that, in the liverwort, Marchantia polymorpha, a gene controlling spore wall deposition is expressed in the capsule lining, so these cells essentially function as a tapetum.

Aeroterrestrial Coleochaete (Streptophyta, Coleochaetales) models early plant adaptation to land.

PREMISE OF THE STUDY: The streptophyte water-to-land transition was a pivotal, but poorly understood event in Earth history. While some early-diverging modern streptophyte algae are aeroterrestrial (living in subaerial habitats), aeroterrestrial survival had not been tested for Coleochaete, widely regarded as obligately aquatic and one of the extant green algal genera most closely related to embryophytes. This relationship motivated a comparison of aeroterrestrial Coleochaete to lower Paleozoic microfossils whose relationships have been uncertain. METHODS: We tested the ability of two species of the experimentally tractable, complex streptophyte algal genus Coleochaete Bréb. to (1) grow and reproduce when cultivated under conditions that mimic humid subaerial habitats, (2) survive desiccation for some period of time, and (3) produce degradation-resistant remains comparable to enigmatic Cambrian microfossils. KEY RESULTS: When grown on mineral agar media or on quartz sand, both species displayed bodies structurally distinct from those expressed in aquatic habitats. Aeroterrestrial Coleochaete occurred as hairless, multistratose, hemispherical bodies having unistratose lobes or irregular clusters of cells with thick, layered, and chemically resistant walls that resemble certain enigmatic lower Paleozoic microfossils. Whether grown under humid conditions or air-dried for a week, then exposed to liquid water, aeroterrestrial Coleochaete produced typical asexual zoospores and germlings. Cells that had been air-dried for periods up to several months maintained their integrity and green pigmentation. CONCLUSIONS: Features of modern aeroterrestrial Coleochaete suggest that ancient complex streptophyte algae could grow and reproduce in moist subaerial habitats, persist through periods of desiccation, and leave behind distinctive microfossil remains.

Spore wall ultrastructure in the early lycopsid Leclercqia (Protolepidodendrales) from the Lower Devonian of North America: Evidence for a fundamental division in the lycopsids.

Documenting the morphology and ultrastructure of spores from known Silurian-Devonian plants clarifies organization and probable affinities of dispersed spores and contributes to analyses of evolutionary changes and phylogenetic relationships in early plants. In this study of fossil in situ spores from the early protolepidodendralean lycopsid Leclercqia, we identified new characters including an additional synapomorphy of the ligulate lycopsid clade. A detailed light (LM), scanning electron (SEM), and transmission electron microscope (TEM) analysis of spores from two species of Leclercqia from the Lower Devonian (Emsian) of New Brunswick, eastern Canada, L. andrewsii and L. complexa, shows both are homosporous, yielding spores belonging to the dispersed spore form taxon Acinosporites lindlarensis. Important features of wall ultrastructure include the presence of a paraexospore, peculiar exospore-derived, peg-like structures located in the gap between the outer exospore/inner paraexospore, and multilamellate regions in the interradial areas of the proximal surface. Similar interradial multilamellate regions occur in other ligulate lycopsids (fossil and extant). This character is probably a further synapomorphy for the ligulate lycopsid clade, within which heterosporous lycopods form a monophyletic group. These data suggest the ligule and interradial multilamellate region appeared prior to heterospory.