Class Anthoceropsida (Hornworts) – Detailed Notes
1. Introduction
Anthoceropsida is a distinct class of Bryophyta, commonly known as hornworts. The name is derived from their elongated, horn-like sporophyte. Among bryophytes, Anthoceropsida shows several advanced features and occupies an important evolutionary position.
2. General Characteristics
- Plant body is a gametophytic thallus
- Thallus is dorsiventral, flat and lobed
- Each cell contains a single large chloroplast with a pyrenoid
- Air chambers and air pores are absent
- Mucilage cavities are present, often containing Nostoc (cyanobacteria)
- Rhizoids are smooth-walled
- Sporophyte is green, photosynthetic and long-lived
• Plant body is a gametophytic thallus
The dominant plant body in Anthoceropsida is the gametophyte, which is haploid and thalloid in nature. It lacks true roots, stems and leaves and represents the main photosynthetic and reproductive phase of the life cycle. The sporophyte always remains attached to and nutritionally dependent on this gametophyte.
• Thallus is dorsiventral, flat and lobed
The thallus shows clear dorsiventral differentiation, with an upper (dorsal) photosynthetic surface and a lower (ventral) surface bearing rhizoids. It is flat, thin and irregularly lobed, forming a rosette-like structure that lies closely attached to the soil, which helps in water absorption and anchorage.
• Each cell contains a single large chloroplast with a pyrenoid
A characteristic feature of Anthoceropsida is the presence of one large chloroplast per cell, which contains a pyrenoid. The pyrenoid is involved in carbon fixation and starch synthesis, a feature resembling green algae and indicating a primitive evolutionary condition.
• Air chambers and air pores are absent
Unlike liverworts such as Marchantia, the thallus of Anthoceropsida lacks specialized air chambers and air pores. Gas exchange occurs directly through the general surface of the thallus, which remains thin and moist, allowing efficient diffusion of gases.
• Mucilage cavities are present, often containing Nostoc (cyanobacteria)
The thallus contains mucilage-filled cavities that frequently harbor colonies of Nostoc, a nitrogen-fixing cyanobacterium. This represents a symbiotic association, where Nostoc fixes atmospheric nitrogen for the plant, while the plant provides shelter and nutrients.
• Rhizoids are smooth-walled
Rhizoids in Anthoceropsida are unicellular, smooth-walled and non-septate. They arise from the ventral surface of the thallus and function mainly in anchorage and water absorption, unlike the pegged rhizoids found in some liverworts.
• Sporophyte is green, photosynthetic and long-lived
The sporophyte is elongated, horn-like and chlorophyllous, capable of performing photosynthesis. It is long-lived due to the presence of a basal intercalary meristem, which allows continuous growth. Although partially independent, it remains attached to the gametophyte for water and nutrients.
3. Habitat and Distribution
Anthoceropsida commonly grows in:
- Moist and damp soils
- Shady and humid places
- Distributed worldwide, from tropical to temperate regions
Moist and damp soils
Anthoceropsida commonly grows on moist, damp and water-retentive soils, especially those rich in organic matter. Such soils provide the continuous moisture required for thallus growth, fertilization, and spore germination, as water is essential for the movement of antherozoids.
• Shady and humid places
Hornworts prefer shaded, cool and humid environments, such as the banks of streams, moist fields, forest floors and shaded gardens. High humidity prevents desiccation of the thin thallus and facilitates efficient gaseous exchange and reproductive processes.
• Distributed worldwide, from tropical to temperate regions
Anthoceropsida has a cosmopolitan distribution, occurring in tropical, subtropical and temperate regions of the world. Their wide distribution is due to their simple body organization, efficient spore dispersal, and ability to survive in diverse moist habitats.
4. Vegetative Structure (Gametophyte)
External Structure
- Thallus is thin, green and rosette-like
- Midrib is absent
- Ventral surface bears rhizoids and mucilage cavities
Thallus is thin, green and rosette-like
The gametophytic thallus of Anthoceropsida is thin, soft and green, forming a rosette-like structure that spreads flat over the soil surface. The green color is due to abundant chlorophyll, enabling efficient photosynthesis, while the thin nature of the thallus allows easy diffusion of gases and water.
• Midrib is absent
Unlike some liverworts, the thallus of Anthoceropsida lacks a distinct midrib. The tissues are uniformly distributed throughout the thallus, giving it a simple and undifferentiated appearance, which is considered a primitive feature among bryophytes.
• Ventral surface bears rhizoids and mucilage cavities
The ventral surface of the thallus bears numerous smooth-walled rhizoids that anchor the plant to the substratum and assist in water absorption. In addition, mucilage cavities are present, many of which contain Nostoc, forming a symbiotic association that contributes to nitrogen fixation.
Internal Structure
- Upper epidermis: protective layer
- Photosynthetic tissue: loosely arranged parenchyma cells
- Mucilage cavities: houses Nostoc in symbiotic association
- Cells are thin-walled and parenchymatous
Upper epidermis: protective layer
The thallus is covered by a single-layered upper epidermis which serves as a protective covering. It reduces excessive water loss while still allowing gaseous exchange due to the thin and delicate nature of the cells.
• Photosynthetic tissue: loosely arranged parenchyma cells
Below the epidermis lies the photosynthetic tissue, composed of loosely arranged parenchymatous cells. These cells contain chloroplasts and perform photosynthesis, while the loose arrangement creates intercellular spaces that facilitate diffusion of gases.
• Mucilage cavities: houses Nostoc in symbiotic association
The thallus contains mucilage-filled cavities that commonly harbor colonies of Nostoc, a nitrogen-fixing cyanobacterium. This represents a mutualistic symbiosis, where Nostoc supplies fixed nitrogen to the plant, and the plant provides shelter and nutrients.
• Cells are thin-walled and parenchymatous
All cells of the thallus are thin-walled, living and parenchymatous, which helps in rapid absorption of water, easy transport of nutrients and efficient metabolic activities. This simple tissue organization reflects the primitive nature of Anthoceropsida.
5. Vegetative Reproduction
Vegetative reproduction occurs by:
- Fragmentation of thallus
- Tubers (in some species) for perennation
- Death and decay of older thallus regions
Fragmentation of thallus
Vegetative reproduction commonly occurs by fragmentation, in which the thallus breaks into smaller pieces due to aging or mechanical injury. Each fragment, when it reaches favorable conditions, is capable of developing into a new independent gametophytic plant.
• Tubers (in some species) for perennation
In some species of Anthoceropsida, tubers are formed as specialized vegetative structures. These tubers serve as perennating organs, helping the plant survive unfavorable conditions such as drought or extreme temperatures. When conditions become favorable again, tubers germinate to produce new thalli.
• Death and decay of older thallus regions
As the thallus grows, the older central portions gradually die and decay, while the younger peripheral regions remain active. This natural separation leads to the formation of new individuals, thus aiding in vegetative propagation.
6. Sexual Reproduction
Sexual reproduction is oogamous. Most species are monoecious, bearing both sex organs on the same thallus.
a) Antheridium
- Antheridia are sunken and embedded in the thallus
- Develop from superficial cells
- Androcytes divide to form biflagellate antherozoids
Antheridia are sunken and embedded in the thallus
In Anthoceropsida, antheridia are completely sunken and embedded within the tissue of the gametophytic thallus. They are not borne on special stalks but lie inside antheridial chambers, which open to the exterior by a small pore. This sunken position provides protection against desiccation and mechanical injury.
• Develop from superficial cells
Each antheridium originates from a single superficial (dermal) cell of the dorsal surface of the thallus. This cell divides periclinally to form:
- An outer sterile jacket layer, and
- Inner androgonial cells.
- The jacket layer becomes one-cell thick and encloses the developing spermatogenous tissue.
• Androcytes divide to form biflagellate antherozoids
The inner androgonial cells divide repeatedly to form androcytes (sperm mother cells). Each androcyte metamorphoses into a single antherozoid, which is biflagellate, spirally coiled and motile. At maturity, the jacket layer ruptures and the antherozoids are released into water, enabling them to swim toward the archegonia for fertilization.
b) Archegonium
- Archegonia are embedded in the thallus
- Simple structure with:
- Short neck
- Venter containing egg and ventral canal cell
- Neck canal cells disintegrate before fertilization
Archegonia are embedded in the thallus
In Anthoceropsida, archegonia are sunken and embedded within the gametophytic thallus. Unlike some liverworts, they do not sit on stalks but develop directly from the dorsal surface cells. This sunken position protects the developing egg from desiccation and mechanical injury.
• Simple structure with short neck
The archegonium is structurally simple and consists of:
-
Short neck – a few cells thick, leading to the external opening.
-
Venter – basal swollen portion housing the egg (oosphere) and the ventral canal cell.
- Neck cells act as a channel for the sperm to reach the egg.
- The ventral canal cell later dissolves to facilitate fertilization.
• Neck canal cells disintegrate before fertilization
Before fertilization, the neck canal cells degenerate and release mucilage. This chemical and physical pathway guides the motile antherozoids toward the egg. The egg remains at the base of the archegonium and is haploid, ready to fuse with the male gamete to form a zygote.
7. Fertilization
- Fertilization requires water
- Antherozoids swim toward the archegonium
- Fusion of gametes results in the formation of a zygote
Fertilization requires water
Fertilization in Anthoceropsida is aquatic-dependent. The motile, biflagellate antherozoids require a thin film of water on the thallus surface to swim toward the archegonia. Without water, the male gametes cannot reach the female gamete, making moist habitats essential for sexual reproduction.
• Antherozoids swim toward the archegonium
The biflagellate antherozoids are chemotactically attracted to the archegonium by mucilage secreted from the neck canal cells. They enter the neck of the archegonium and move along the channel formed by the degenerated neck canal cells to reach the egg in the venter.
• Fusion of gametes results in the formation of a zygote
Once an antherozoid reaches the egg, plasmogamy and karyogamy occur, resulting in the formation of a diploid zygote (2n). This zygote remains attached to the gametophyte and will develop into the sporophyte, which is partially independent and photosynthetic.
8. Sporophyte
The sporophyte of Anthoceropsida is considered the most advanced among bryophytes.
Main Features
- Differentiated into foot and capsule
- Seta is absent
- Capsule has a basal intercalary meristem
- Shows continuous growth
- Sporophyte is partially independent and photosynthetic
Differentiated into foot and capsule
The sporophyte of Anthoceropsida is elongated and horn-like. It is differentiated into two main regions:
-
Foot – embedded in the gametophyte tissue, functions in anchorage and absorption of water and nutrients from the gametophyte.
-
Capsule – the elongated, green structure above the foot, which contains spore-producing tissues.
• Seta is absent
Unlike mosses, the sporophyte of Anthoceropsida lacks a seta (stalk). The capsule arises directly from the foot region, making the sporophyte appear as a continuous, elongated horn-like structure.
• Capsule has a basal intercalary meristem
The intercalary (basal) meristem located at the base of the capsule is responsible for continuous growth of the sporophyte. New cells are produced at the base, pushing the older parts upward, which is a unique and advanced feature among bryophytes.
• Shows continuous growth
Due to the basal meristem, the sporophyte grows continuously throughout its life, unlike the short-lived sporophytes of liverworts and most mosses. This allows longitudinal elongation of the horn-like capsule and prolonged spore production.
• Sporophyte is partially independent and photosynthetic
The sporophyte is chlorophyllous and green, capable of photosynthesis, making it partially independent. However, it still relies on the gametophyte for water, minerals, and support, unlike fully independent sporophytes in vascular plants.
9. Structure of Capsule
The capsule consists of:
- Epidermis – protective outer layer
- Chlorophyllous tissue – photosynthetic
- Columella – central sterile tissue
- Spore sac – contains spore mother cells
- Pseudoelaters – help in spore dispersal
Epidermis – protective outer layer
The capsule is covered externally by a single-layered epidermis that acts as a protective barrier, shielding the developing spores and internal tissues from desiccation and mechanical damage.
• Chlorophyllous tissue – photosynthetic
Beneath the epidermis lies the chlorophyllous tissue, which is green and performs photosynthesis. This allows the sporophyte to produce organic nutrients, contributing to its partial independence from the gametophyte.
• Columella – central sterile tissue
The columella is a sterile, central axis of cells within the capsule. It provides structural support to the spore-producing tissue and sometimes aids in nutrient transport from the gametophyte to the developing spores.
• Spore sac – contains spore mother cells
Surrounding the columella is the spore sac, which contains spore mother cells (sporocytes). These cells undergo meiosis to produce haploid spores, which are the dispersal units of the plant.
• Pseudoelaters – help in spore dispersal
Interspersed among the spores are pseudoelaters, long hygroscopic sterile filaments that twist and coil as they dry. Their movement helps in pushing the spores out of the capsule and aids in effective dispersal by wind.
10. Spore Formation and Dispersal
- Spore mother cells undergo meiosis
- Haploid spores are formed
- Capsule splits longitudinally
- Pseudoelaters assist in spore dispersal
Spore mother cells undergo meiosis
Within the spore sac of the capsule, the diploid spore mother cells (sporocytes) undergo meiosis, reducing the chromosome number from 2n to n. This produces genetically haploid spores, ensuring the continuation of the haploid gametophytic generation in the life cycle.
• Haploid spores are formed
Each sporocyte divides meiotically to produce four haploid spores. These spores are the dispersal units of Anthoceropsida and can germinate into new gametophytic thalli under favorable conditions. The spores often have thick walls, which help them survive short periods of adverse conditions.
• Capsule splits longitudinally
At maturity, the capsule splits along its length (longitudinal dehiscence) to release the spores. The splitting occurs gradually, allowing continuous spore release over time rather than all at once, which increases the chances of successful dispersal.
• Pseudoelaters assist in spore dispersal
The pseudoelaters, sterile elongated cells interspersed among spores, respond to changes in humidity. They twist and coil when dry, helping to push and scatter the spores into the environment, facilitating wind dispersal and colonization of new habitats.
11. Life Cycle of Anthoceros
- Shows alternation of generations
- Dominant phase is gametophyte
- Sporophyte remains attached and dependent
- Life cycle is haplodiplontic
Shows alternation of generations
Anthoceropsida exhibits alternation of generations, meaning the life cycle alternates between:
-
Haploid gametophyte (n) – sexual phase that produces gametes
-
Diploid sporophyte (2n) – asexual phase that produces spores
This alternation ensures genetic recombination and continuity of the species.
• Dominant phase is gametophyte
The gametophyte is the long-lived, photosynthetic, and nutritionally independent phase of the life cycle. It forms the main plant body, while the sporophyte grows attached to it.
• Sporophyte remains attached and dependent
The sporophyte, though partially photosynthetic, remains attached to the gametophyte for water, minerals, and support. It develops from the zygote and grows from a basal intercalary meristem, producing spores over time.
• Life cycle is haplodiplontic
The life cycle of Anthoceropsida is haplodiplontic, meaning it alternates between a haploid gametophyte phase and a diploid sporophyte phase.
- Haploid gametophyte → produces gametes by mitosis
- Diploid sporophyte → produces spores by meiosis
This feature is typical of all bryophytes, but in Anthoceropsida the sporophyte is more advanced and long-lived compared to liverworts.
12. Economic and Ecological Importance
- Contribute to soil formation
- Help in nitrogen fixation through Nostoc
- Act as pioneer plants in bare habitats
Contribute to soil formation
Hornworts (Anthoceropsida) grow on bare, moist soil surfaces and help in breaking down rocks and organic matter through their thallus growth and decay. As the thallus decomposes, it adds organic matter and humus to the soil, improving soil fertility and structure. Over time, this makes the habitat suitable for other plants, contributing to ecological succession.
• Help in nitrogen fixation through Nostoc
The mucilage cavities of Anthoceropsida often harbor the cyanobacterium Nostoc, which can fix atmospheric nitrogen into ammonia, a form usable by plants. This symbiotic association enriches the soil with nitrogen, enhancing fertility and supporting the growth of other plants in nutrient-poor habitats.
• Act as pioneer plants in bare habitats
Hornworts are early colonizers of disturbed or barren habitats, such as riverbanks, moist slopes, or areas cleared of vegetation. Their growth stabilizes the soil, prevents erosion, and prepares the substrate for subsequent colonization by mosses, liverworts, and vascular plants. This pioneer role is critical in establishing terrestrial ecosystems.
13. Comparison with Other Bryophytes
| Feature | Anthoceropsida | Hepaticopsida |
|---|---|---|
| Chloroplast | Single with pyrenoid | Many, no pyrenoid |
| Sporophyte | Long, horn-like | Short, delicate |
| Meristem | Basal | Absent |
14. Examples
- Anthoceros
- Notothylas
- Megaceros
Anthoceros
- Most common and widely studied genus of Anthoceropsida.
- Thallus is green, flat, and rosette-like, with mucilage cavities containing Nostoc.
- Sporophyte is elongated, horn-like, with a basal intercalary meristem.
- Found in moist soils, shaded areas, globally distributed.
- Example species: Anthoceros agrestis, Anthoceros fusiformis.
2. Notothylas
- Smaller and less common genus compared to Anthoceros.
- Thallus is usually thick, fleshy, and rosette-shaped, often growing in tropical regions.
- Sporophyte is shorter than in Anthoceros and has similar basal meristem growth.
- Commonly grows on damp soil, humus-rich substrates, sometimes in symbiosis with cyanobacteria.
- Example species: Notothylas orbicularis.
3. Megaceros
- Genus with larger, robust thalli and very long horn-like sporophytes.
- Found mostly in tropical and subtropical regions.
- Sporophyte shows continuous growth from the basal meristem and produces spores over a long period.
- Thallus often colonizes moist soil surfaces and can act as a pioneer plant.
- Example species: Megaceros aenigmaticus.
15. Salient / Unique Features
- Single chloroplast with pyrenoid
- Basal meristem in sporophyte
- Horn-shaped capsule
- Symbiotic association with Nostoc
Single chloroplast with pyrenoid
A characteristic feature of Anthoceropsida is that each cell contains a single, large chloroplast which houses a pyrenoid. The pyrenoid is involved in starch synthesis and carbon fixation, a trait reminiscent of green algae. This is unique among bryophytes and indicates a primitive evolutionary link with algae.
• Basal meristem in sporophyte
The sporophyte exhibits a basal (intercalary) meristem at the base of the capsule. This meristem allows continuous elongation of the horn-like sporophyte, making it longer-lived and more advanced than the sporophytes of most liverworts and mosses.
• Horn-shaped capsule
The sporophyte of Anthoceropsida is elongated and horn-like, which is the most distinctive feature of this class. The horn shape facilitates gradual spore dispersal over time as the capsule grows continuously and matures from base to tip.
• Symbiotic association with Nostoc
The mucilage cavities in the thallus often harbor the cyanobacterium Nostoc, which fixes atmospheric nitrogen. This mutualistic association improves soil fertility, provides nitrogen to the hornwort, and allows Anthoceropsida to thrive in nutrient-poor habitats.
: Structure, Reproduction and Life Cycle){kind=link}
0 Comments