The genus Phaeoceros was segregated from Anthoceros by Proskauer (1951). However, as pointed out by Renzaglia and Vaughn (2000), interrelationships among genera and delineation of generic boundaries are still problematic in hornworts. The type species of the genus is Phaeoceros laevis (L.) Prosk., but the genus also includes several other species.

The gametophyte is a flattened thallus. It lacks internal differentiation and its margins are slightly lobed. The gametophyte cells contain single lens-shaped chloroplasts similar to the chloroplasts of charophytes, with abundant starch, central pyrenoids and channel thylakoids. The shape of cells varies from more isodiametric cells along the thallus margins to elongated cells in the median region of the thallus. Inside the thallus mucilage-filled cells are scattered amongst the photosynthetic parenchyma.

Growth regions each containing a wedge-shaped apical cell and immediate derivatives are located in the notches along the thallus edges. The growth pattern leads to an orbicular form of the thallus, i.e. its thickness gradually decreases from the center to margins.

Rhizoids develop as an elongation of a ventral cell and are not separated by any septum. They are unicellular and smooth. Sometimes they may have branched tips.

Also ventrally on the gametophyte thallus mucilage clefts are formed. Their opening is surrounded by two cells that resemble the guard cells of stomata, but lack the ability to open and close. Nostoc cyanobacteria enter the thallus through them and establish as colonial endosymbionts, whereas thallus outgrowths penetrate the cyanobacterial colonies. The internal chambers containing the cyanobacteria increase in size and fill with mucilage secreted by the hornwort as the cyanobacterial colonies multiply.

Vesicular-arbuscular endomycorrhizae are common in thallus cells of most of the Anthocerote taxa.

The gametangia are produced along the dorsal thallus midline.

Archegonia develop from epidermal cells, but are ultimately sunken in thallus tissue and only the neck cells slightly protrude from the thallus surface. The venter is a 1-2 cell-layered jacket wrapping the egg, whereas the archegonial neck is comprised of six rows of cells.

Antheridia develop from cells located in the coating of schizogenous antheridial chambers in the thallus. Antheridia are wrapped in a jacket composed of four layers of cells and are supported by a short stalk. Each antheridial chamber incorporates several antheridia, all of which are developed from one antheridial initial.

The sporophyte is protected in its early stages by gametophytic tissue forming an involucre. As the sporophyte develops the involucre is broken and remains as a cylinder that surrounds the base of the sporophyte.

The sporophyte is comprised of a bulbous foot embedded in the gametophyte and an elongated cylindrical spore-bearing region. A basal meristem located between the two assures the continuous production of sporogenous tissue throughout the growing season. The sporophytic foot cells emit haustoria that penetrate between the cells of the gametophyte.

The spore-bearing region of the sporophyte has an epidermal layer, an assimilative layer immediately below it, sporogenous tissue and a central columella.

The sporophytic epidermis shows stomata with 2 guard cells and is made of elongated cells. Stomatal guard cells have the ventral (inner) walls thickened.

The columella is persistent throughout the sporophytic life stage.

The sporogenous tissue produces spores and sterile cells.

Multicellular pseudoelaters made of sterile cells are interspersed among the spore mother cells. They form a girder system in the meshes of which the spore mother cells lie. Pseudoelater cells lack helical wall thickenings in Phaeoceros, although they may show some irregular cell wall thickenings. They are dead at maturity.

Spore maturation occurs progressively from the base to the apex of the sporophyte. Spores are formed in tetrahedral tetrads and bear the characteristic trilete mark. They have sculptured exine and are a translucent yellow in color. Spores remain in tetrads (click here to see spore tetrads in two stages of development), surrounded by the wall of the spore mother cell until nearly mature, but are dispersed individually.

Dehiscence of the spore-bearing region (sometimes called capsule, although it is not morphologically differentiated as such) occurs typically along two longitudinal lines that originate near the sporophyte tip and separate two valves. Dehiscence is coupled with a twisting of the valves. Occasionally, dehiscence may occur by one longitudinal split only. Pseudoelaters and columella facilitate spore separation and assist in dispersal.

Spore germination is exosporous. The sporeling development pattern includes a globose massive protonema developed outside the spore.

Live material for the images was kindly provided by Harold Blazier.

The images were taken at the Scientific Imaging Facility, Ohio University.

Bibliography

Written by Alexandru Mihail Florian Tomescu
May 2001