Structure, Reproduction, Life Cycle, Diagram, Uses microbiologystudy

Classification of Anthoceros (Source: USDA, NRCS. 2024. The PLANTS Database)

Kingdom	Plantae (Plants)
Division	Anthocerotophyta (Hornworts)
Subdivision	Anthocerotae
Class	Anthocerotopsida
Order	Anthocerotales
Family	Anthocerotaceae
Genus	Anthoceros

Habitat and Distribution of Anthoceros

Anthoceros is cosmopolitan in nature and commonly occurs in the Tropical and Temperate regions of the world. There are about 200 species all over the world. Anthoceros grows on moist, clayey soil, or wet rocks in very moist, shady places, usually in dense patches. They are mostly found growing on the sides of moist slopes, along the hill side roads, along banks of streams etc.

Morphology of Anthoceros

The adult plant is gametophyte. It is a small, yellowish green or dark green dorsi-ventrally flattened thallus. The lobes with divided margins overlap. The lobes are thick and fleshy body with no distinct margin. The ventral surface lacks scales, tuberculate rhizoids and mucilage hairs. However, it bears unicellular, smooth walled rhizoids. In addition, small, rounded bluish green areas can be spotted on the ventral surface. These are the nostoc colonies.

Anatomy of Anthoceros (thallus)

The thallus of Anthoceros is several layers thick but without a midrib. There is no tissue differentiation and cell differentiation. The assimilatory region and storage regions are not differentiable. The cells are uniform and compact with no air pores. The cells contain lens shaped chloroplast. There is no organized epidermis. In some species, there is stoma like pores or slits on the ventral surface that contains slimes. They are also known as slime pores which leads to large intercellular spaces filled with mucilage. These intercellular spaces are inhabited by Nostoc colonies.

Nostoc-Anthoceros Relationship

Ridgeway (1967) and Rodgers and Stewart (1977) reported the presence of Nostoc colonies in Anthoceros. Rodgers and Stewart (1977) demonstrated that this association is highly specialized form of symbiosis in which both partners are beneficial to each other. Both the organisms are modified physiologically, morphologically and biochemically which permits rapid exchange of metabolites between them. Fixed nitrogen apparently in the form of ammonia is transported to Anthoceros from Nostoc, while living symbiotically the Nostoc has reduced carbon-dioxide fixation capability and is thus dependent on Anthoceros for fixed carbon (Rodgers and Stewart, 1977).

However, in some species like A. himalayensis, the mucilage cavities are absent.

Reproduction in Anthoceros

Anthoceros reproduces both asexually and sexually. The methods of reproduction in Anthoceros are as follows-

Asexual/Vegetative Reproduction in Anthoceros

Anthoceros reproduce asexually by various means-

• Fragmentation: The cells in the basal portion of thallus die and disorganize and when the progressive decay reaches the branching region, the thallus lobes becomes separated. Each separated lobes give rise to new individual. This method is not so common in Anthoceros but some species may reproduce by this method.
• Gemmae: Some bryologists reported the formation of gemmae on short stalks and along the margin of thallus in A. glandulosus, A. formosae and others. This gemma detaches and gives rise to new thallus.
• Tubers: Certain species of Anthoceros which are exposed to drought develops marginal thickenings called tubers. With the onset of favourable condition, these tubers give rise to new individual. Tuber formation has been reported in species such as A.himalayensis, A. tuberosus, etc. The tubers primarily functions as the organ of perennation and secondarily functions as the means of asexual reproduction.
• Persistent growing apices: During unfavourable season, all the plant dries up excluding some cells on the growing point of thallus lobes. These are known as persistent apices which resumes growth with the return of favourable condition. This type of reproduction is seen in A. pearsoni and A. fusiformis.
• Apospory: The phenomenon of production of gametophyte thallus directly from vegetative cells of the sporogonium is called apospory. Some species of Anthoceros may show this type of reproduction. Genetically, the aposporously produced thalli are diploid; however they are normal in appearance.

Sexual Reproduction in Anthoceros

Most of the species like A. gollani, A. fusiformis, A.punctatus are monoecious while species like A.erectus, A. crispus are dioecious. A. himalayensis are monoecious but protrandus i.e. antheridia develops before archegonia.

The sex organs in Anthoceros are immersed in the thallus tissue on the upper surface. 

Antheridia

They are endogenous and occur singly or in groups on the upper surface of the thallus within the antheridial chambers. Each antheridial cavity is roofed over by the thallus tissue to cell layer in thickness. The ripe antheridia are bright orange in colour.

Structure of antheridium

Each antheridium has ovoid or pouch like body with a multicellular stalk. The body of antheridium consists of jacket layer of cells enclosing a mass of androcytes. Each androcyte forms single sperm.

Dehiscence– when antheridia reach maturity the roof of antheridial chamber brusts. The antheridium absorbs water and swells which leads to rupture and the jacket layer separates. Finally the sperms are discharge in presence of water.

The sperm is biflagellate curved structure. 

Development of antheridium

• The antheridia develop endogenously. Each antheridium develops from antheridial initial.
• The antheridial initial divides by two vertical divisions at right angle to each other and form four cells. Each of these undergoes transverse division and forms eight cells. The four cells of the lower tier forms the stalk cells. 

They divide and re-divide to form stalk.

• The four cell of the upper tier divides transversely to form octant cells. Each cell of the octant divides periclinally which leads to formation of eight outer primary jacket cells and eight inner primary androgonal cells. 
• The primary jacket cells divides to form antheridial wall.
• The primary androgonal cells divides to form androcytes mother cells.
• Each androcytes mother cells give rise to two androcytes. Each androcytes undergoes modification to form sperms.

Archegonium

The archegoniun of Anthoceros are sunk deep in the fleshy thallus of upper surface. They are developed in acropetal order. 

Each archegonium consists of four to six neck canal cells, a ventral canal cell and egg cell. There is no sterile jacket layer but four cover cells at the top of an archegonium.

Development of archegonium

• Each archegonium arises from archegonial initial. The archegonial initial divides transversely to form primary archegonial cell and primary stalk cell. 
• The primary archegonial cell divides to form jacket intial and primary axial cell. The primary axial cell divides transversely into outer cell and central cell. 
• The central cell gives rise to ventral canal cell and ovum. 
• The outer cell undergoes transverse division to form terminal cover initial and primary neck canal cell.
• Cover initial divides to form cover cells and primary neck canal cell divides to form neck canal cells. The archeginoum is embedded inside the thallus and has no stalk.

Fertilization

Fertilization takes place in presence of water. The sperms are transferred with the help of water and in the meantime, the neck canal cells disintegrate to form mucilage and absorb water to swell which leads to opening of cover cells. Thus a passage is formed. The mucilage in archegonium contains chemical substances that attract the sperms and due to chemical interaction, they enter the venter and the most compatible one penetrates the egg and unites to form zygote.

Sporophytic phase

The zygote is the pioneer structure of this phase which undergoes repeated division to form elongated embryo. The embryo by further cell division and differentiation gives rise to elongated spindle shaped structure called the sporogonium or the sporophyte.

The sporophyte is differentiated into three regions- The foot, intercalary zone, and the capsule.

The foot– It is a rounded bulbous structure deeply embedded in the thallus. It serves as the organ of anchorage. The surface of the foot contains rhizoid like outgrowths that helps in absorbing nutrients from the thallus. Thus the foot functions as haustorium. 

Intercalary zone- It is a narrow zone of meristematic cells located between foot and capsule. It helps in adding new cells to the capsule. It enables the capsule to grow for a long period and form spores.
The capsule- It is long, slender, upright and cylindrical structure. The capsule is differentiated into columella, archesporium and capsule wall. It is endothecial in origin. Around the collumela there is a double layer of elongated sporogenous tissue. The capsule wall is the outermost layer that consists of epidermis and stomata. The epidermis is photosynthesis in function. Thus the sporophyte of Anthoceros is semi-independent.

Development in Anthoceros

The embryo divides into two equal daughter cells which undergo transverse divisions to form four cells. The four cells again divide by a vertical wall at a right angle to form octant state. The four cells of upper tier undergo another transverse division which results in formation of three tiers of four cells each. The uppermost tier forms the capsule. It divides periclinally to form central group of four cells and the surrounding cell. The central group of four cells forms the endothecium and the surrounding cells forms the amphithecium. The endothecium gives rise to coloumella. The amphithecium divides periclinally to form archesporium from the inner cells and jacket initial from the outer cells. The jacket initial gives rise to capsule wall. The archesporium give rise to sporogenous tissue. The sporogenous tissue differentiates into sterile and fertile cells. The sterile cells give rise to pseudoelater mother cells which give rise to pseudoelater. The fertile cells give rise to spore mother cells. The spore mother cell undergoes meiosis to form four haploid tetrahedral spores.

Economic Importance of Anthoceros

Nitrogen fixation

Anthoceros harbors symbiotic cyanobacteria (e.g., Nostoc) that fix atmospheric nitrogen, converting it into ammonia, a form usable by plants. This natural fertilization enhances soil fertility, reducing the need for synthetic nitrogen fertilizers, which are costly and environmentally damaging. This benefit supports sustainable agriculture and lowers input costs for farmers. 

Erosion control and moisture retention

Anthoceros helps in stabilizing the soil and retain moisture which helps in preventing erosion and maintaining soil health. These functions are vital for land management, conservation efforts, and maintaining the productivity of agricultural lands.

Medicinal use

Anthoceros and other bryophytes like Riccia are of high medicinal use. Watt (1891) reported the use of Anthoceros, Marchantia and many other bryophytes. 

Maintaining ecosystem

Anthoceros contributes in maintaining healthy ecosystem. This indirectly supports ecosystem services such as clean water, air, and fertile soil, which are essential for human well-being and economic activities.

References

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