ANTHOCERA: CHARACTERISTICS, LIFE CYCLE, REPRODUCTION AND EXAMPLES - BIOLOGY - 2025

The hornworts (Anthocerotophyta) are a group of non-vascular plants that constitute one of the most primitive members of the embryophytes. Initially cataloged as bryophytes, their genetic and structural relationship with higher plants has now been determined.

Recent research on the molecular phylogeny of anthocerans has made it possible to determine that they constitute an evolutionary step for terrestrial plants. However, the evolutionary disposition of the group is under discussion, despite the fact that they share a common ascendant with the tracheophytes.

Anthoceros sp. Source: Bramadi Arya

Most of the taxonomic groups that constitute hornworts are located around the world mainly in tropical climates. The ideal habitat for these plants are aquatic environments, as well as shady and humid environments.

The Anthocerotophyta group is made up of approximately 100-150 species of 5 recognized genera. They are productive plants that have the facility to reproduce and obtain moisture and nutrients from dew and precipitation.

They are effective colonizers of rocky surfaces and humid soils poor in nutrients, which favors their development in wild places. They exercise a significant ecological function as soil protectors, nutrient fixers, moisture maintenance and restoration of biodiversity.

General characteristics

Morphology

It presents a multilayered thallus with a flattened dorsiventral structure, generally forming rosettes with wavy or sinuous edges 3-10 cm in diameter. The cells of the thallus contain a large single, disk-shaped chloroplast and pyrenoids surrounding a discoidal chloroplast.

Phaeoceros carolinianus. Source: HermannSchachner

The thalli are made up of cells with a thin cell wall, they are attached to the substrate through unicellular rhizoids. The epidermis of the thallus has some pores or stomata formed by two reniform occlusive cells, in addition it does not have ventral scales.

The sexual organs -anteridia and archegonia- develop within the crypts or pores of the thallus. The sporophytes present stomata with occlusive cells and carry out the photosynthetic process due to the presence of chloroplasts.

The foot of the sporophyte has a tuberous appearance with a terminal capsule in the shape of a horn, also lacking a seta. The anthoceras maintain a symbiotic relationship with some cyanobacteria of the genus Nostoc, which subsist on the mucilaginous cavities of the thallus.

The sporophytes maintain a continuous growth and a sustained production of spores associated with pseudo-elaters or sterile hygroscopic cells. The capsules show progressive growth due to the presence of the intercalary meristem located between the capsule and the stipe.

Habitat and distribution

Anthocerotophyta are found in humid and shady environments in subtropical and tropical regions throughout the globe. They are common in mountainous areas, humid ravines, river banks, water sources, and swampy lands; they are cosmopolitan.

They adapt to conditions of hot climates with high relative humidity, they do not support intense cold or frost. Associated with water sources, they are resistant to short periods of drought, resuming their metabolic activity when wet.

Taxonomy

• Kingdom: Plantae
• Division: Anthocerotophyta or Anthocerophyta

Class: Leiosporocerotopsida Stotl. & Crand. -Stotl., 2005.

Photosynthetic organisms, with a broad green thallus and cells with chloroplasts and starch or pyrenoid storage organelles. They are characterized by the development of numerous sporangia with chloroplasts and stomata. It is made up of a single order and a single family.

• Order: Leiosporocerotales. Hässel, 1988.
  • Family: Leiosporocerotaceae. Hässel, 1986.

Class: Anthocerotopsida de Bary ex Jancz., 1957.

Individuals of this class are characterized by having numerous pores that they use to store reserve substances. Most cells have chloroplasts; anthoceras of this class are associated in symbiosis with cyanobacteria of the genus Nostoc.

This class is classified into three subclasses and four orders: Anthocerotidae (Anthocerotales), Notothyladidae (Notothyladales), Dendrocerotidae (Phymatocerotales and Dendrocerotales).

• Subclass: Anthocerotidae Rosenv., 1958.
  • Order: Anthocerotales Limpricht in Cohn, 1877.
    • Family: Anthocerotaceae (Gray) Dumort., 1829.
• Subclass: Notothyladidae RJ Duff, JC Villarreal, Cargill & R., 2007.
  • Order: Notothyladales Hyvönen & Piippo, 1993.
• Subclass Dendrocerotidae RJ Duff, JC Villarreal, Cargill & R., 2007.
  • Order Phymatocerotales RJ Duff, JC Villarreal, Cargill & R., 2007.
  • Order Dendrocerotales Hässel, 1988.

Dendroceros sp. growing on the bark of a tree. Source: J.Ziffer

Lifecycle

Anthocerus species -Anthocerotophyta- manifest two phases: a gametophytic and a sporophytic, which occur alternately in higher plants. Anteroceros have a haplo-diplobionic, heteromorphic life cycle, where a haploid gametophyte and a diploid sporophyte are distinguished.

In monoecious species, archegonia and antheridia develop in the same plant, but in dioecious species, archegonia and antheridia are formed in different plants.

On the lobed surface of the gametophyte the archegonia open and the antheridia are located in antheridial chambers under the surface of the thallus. In the Anthocerotophyta there are two forms of growth, a thaloid and a foliose.

The thaloid structures are flattened with a meristematic zone that can be divided by mitosis creating consecutive dichotomous branches. The sexual structures are located in specialized structures or on the ventral surface of the thallus.

Biflagellate anterozoids form within the antheridia and are transported through water. When the oosphere is fertilized by an anterozoid, it divides and forms the sporophyte, which by meiosis gives rise to the spores.

Phaeoceros life cycle. Source: derivative work: Smith609 (talk) Hornwort_life_cicle_svg_diagram.svg: Mariana Ruiz user: LadyofHats

After fertilization, the sporophytes containing the spores develop from the archegonium. In hornworts, the sporophyte produces numerous spores that are released when the structure opens as it grows.

In the sporophyte, specialized structures called pseudo-elaters are located, which facilitate the dispersal of the spores. Once the germ spores are dispersed, they settle in a nutritive substrate where they transform into new anthocera plants.

Reproduction

Hornbills are non-vascular plants that spread through sexual and asexual reproduction. In fact, hornworms alternate their life cycle between a gametophytic phase and a sporophytic phase.

Sexual reproduction

In sexual reproduction, spores are formed by meiosis from haploid cells that are released to settle on the substrate. The dominant and permanent thallus develops from the spores throughout the gametophytic generation.

The thallus is characterized by its irregular and wavy shape, lacking conductive tissues such as phloem and xylem. On the ventral face the rhizomes hold it to the substrate, on the back it presents pores where the sexual organs develop.

The antheridia remain exposed on the surface through the apex of the flagellate anterozoid. But the archegonium remains within the pore that contains the egg cell.

Fertilization occurs thanks to the intervention of the water that carries the anterozoids to the archegonium where the egg cell is located. During the second haploid sporophytic generation, the sporangia grow horn-shaped on the thallus from which they feed and attach.

These sporangia contain stomata similar to higher plants, and unlike liverworts, they do not have mushrooms or carpophores. Hornbills are characterized by uninterrupted growth; the sporangium grows actively if the environmental conditions are suitable.

Anthoceros showing a thallus gametophyte with numerous sporophytes. Source: Jason Hollinger

The spores develop inside the capsule, when the capsules mature they open according to the environmental conditions. The mature spores are released and spread thanks to the intervention of the wind and rain until they are anchored in a substrate.

Asexual reproduction

Asexual reproduction occurs by mitosis when a part of the thallus detaches and reproduces a plant similar to its parent.

Examples

Anthoceros sp.

A genus of the Anthocerotaceae family comprising 118 species identified and 57 in the approval process. They are characterized by the particular shape of the sporangium and the dark brown or black color of the spores.

The word Anthocero etymologically comes from the Greek “ανθος” (anthos) flower and from “κηρας” (keras) horn. It has a global distribution.

Anthoceros agrestis

It is a horn known as "field horn" that has the particularity of presenting the cinnamic acid 4-hydroxylase compound. Cinnamic acid 4-hydroxylase is one of the first mono-oxygenases and hydroxylases of cytochrome P ^450, frequent in higher plants.

Anthoceros agrestis. Source: BerndH

Folioceros sp.

A non-vascular plant genus belonging to the Anthocerotaceae family, comprising some 38 identified and 21 recognized species. They are located in tropical and subtropical regions of the Asian continent, on wet rocks, water sources and fallow fields.

The gametophytic thallus has small pinnate branches that are yellowish green, crisp and transparent. Very small plants are 3 cm long by 1 cm wide; they are dioecious or monoecious.

Leiosporoceros dussii

It is the only species of the genus Leiosporoceros of the Leiosporocerotaceae family, morphologically and genetically different from the other members of the Anthocerotophyta group. They are characterized by producing tiny spores and maintaining cyanobacteria in symbiosis in their longitudinally oriented schizogenic channels.

Nothoceros sp.

A genus of hornworts of the family Dendrocerotaceae located in the neotropical zone and eastern North America, South America and New Zealand. It is a genus of non-vascular plants that comprises 16 described species of which only 8 have been accepted.

Phymatoceros sp.

Phymatoceros is the only genus in the Phymatocerotaceae family that includes two known species. They are dioecious species that have smooth, curly and stipulated thallus, and develop in calcareous and humid soils.

References

1. Anthocerotaceae (2017) Wikipedia, The Free Encyclopedia. Recovered at: es.wikipedia.org
2. Antocerotas (2018) Biodiversity and Taxonomy of Cryptogamic Plants. Complutense University of Madrid. Recovered at: escala.bio.ucm.es
3. Delgadillo-Moya, C., & Juárez-Martínez, C. (2014) Biodiversity of Anthocerotophyta and Marchantiophyta in Mexico. Mexican Journal of Biodiversity, 85, 106-109.
4. Gómez Agudelo, S. (2014). Anthoceros (Anthocerotophyta) life cycle, characteristics and reproduction. Recovered in: Naturaleza.paradais-sphynx.com
5. Wikipedia contributors (2018) Hornwort. In Wikipedia, The Free Encyclopedia. Recovered at: es.wikipedia.org