Thallus of Anthoceros: Morphology and Anatomy

• Division: Bryophyta
• Class: Anthocerotae
• Order: Anthocerotales
• Family: Anthocerotaceae
• Genus: Anthoceros

Anthoceros is a cosmopolitan bryophyte. Of the 25 species reported in India, the most common ones are Anthoceros himalayensis, A. erectus, and A. chambensis are commonly found species. All the species grow on moist, clayey soil or wet rocks in moist, shady places.

Morphology of Thallus of Anthoceros

The plant body of Anthoceros is a haploid gametophyte. 

• The thallus is dorsiventral with branching at the apex. 
• It is yellowish-green or dark green. 
• There is no midrib, and the thallus is thicker at the margins. 
• Due to the mucilaginous ducts, the thallus is spongy. 
• The dorsal surface is velvety, smooth, or has ridges and spine-like projections. 
• Only smooth-walled rhizoids are seen on the ventral side.
• There are no scales or tuberculated rhizoids in Anthoceros. 
• Due to the overlapping organization of the thallus, it appears rosette.
• On the ventral side, there are blush or dark-green spots where endophytic blue-green algae such as Nostoc reside. 
• These blue-green algae grow in the mucilaginous cavities. 

Anatomy of Thallus of Anthoceros

Anthoceros thallus does not show much tissue differentiation. 

Cellular Organisation

The multilayered thallus of Anthoceros is thicker in the middle and tapers as it goes towards the margin. On average, there are 6-40 layers of cells in the middle. 

• Every cell in Anthoceros has a single chloroplast, and some species have 2-4 as well. 
• Each chloroplast in Anthoceros is discoid and large. 
• They have a plate-like lamellae and a large pyrenoid. 
• Every cell has a single nucleus near the chloroplast.

Internal Organisation

• It has a single layer of small cells containing one chloroplast on the dorsal side. 
• This layer consists of air pores as well. 
• Right under the peidermal layer, there are uniform cells containing a single small chloroplast. 
• Random intercellular cavities with mucilage are present between these cells. 
• Blue-green algae reside in these cavities. 
• Mucilage in these cavities is secreted by the inhabiting algae. 
• This algae (Nostoc) help with nitrogen fixation for Anthoceros. In return, the bryophyte supplies organic compounds for the algae. 
• These cavities open through narrow slits on the ventral side. These openings are called slime pores. 
• Slime pores are guarded by kidney-shaped cells.
• Rhizoids arise from the lower epidermis.  

Vegetative Reproduction

The Anthoceros thallus, the gametophyte, reproduces vegetatively by fragmentation, tubers, and by persistent-growing apices. Gemmae formation and apospory are also seen. 

Sexual Reproduction

Sexual reproduction in Anthoceros occurs through the development of embedded antheridia and archegonia. The sex organs develop within the thallus tissue. Thalli can be monoecious or dioecious. 

Monoecious species	Dioecious species
Anthoceros gollani A. longii A. fusiformis A. crispus A. himalayensis  A. punctatus	A. erectus A. chambensis A. hallii  A. leavis

Antheridia

• The antheridia develop from the hypodermal cells. 
• Usually, one antheridial initial cell gives rise ot multiple antheridia.  
• The antheridia are unique, developing within a closed endogenous cavity called the antheridial chamber.
• Each chamber may have multiple antheridia, and the chamber will be closed by a double-layered roof with no pores. 
• The antheridial body has a jacket layer that encloses a mass of androcytes. 
• Each androcyte forms into a sperm or an antherozoid.
• The roof of the matured antheridial chamber bursts open irregularly in the presence of water to release the sperm. 
• The released sperm are tiny, bi-flagellated. 

Archegonia

• Archegonia are flask-shaped structures. 
• Each of them has 4 cover cells, a row of 4 – 6 neck canal cells, a venter canal cell, and an egg or oosphere. 
• The unique feature of Anthoceros archegonium is that its archegonium is immersed in the thallus and in contact with the surrounding vegetative cell. 
• On maturity, neck canal cells and venter canal cells degenerate to form mucilage.
• Swelling of this mucilage separates the cover cells to form a path that leads to the oosphere. 

Fertilization

• Sperms are chemotactically attracted to the archegonia. 
• One sperm fuses with the egg in each archegonium to form the zygote. 
• Each zygote undergoes mitotic divisions, forming a needle-like, narrow upright sporophyte or sporogonium. 

Sporophyte

• These sporogonia are often bent and twisted to appear as hornworts. 
• Sporogonium is differentiated into a capsule, an intercalary or intermediate zone, and the foot. 
• Seta is absent in Anthoceros. Instead, the space is occupied by the intercalary zone made of meristematic cells. 
• The capsule forms the major and conspicuous part of the sporogonium. In form, it is one to about 15 cm long, narrow, slender, smooth, and upright. Potentially, the capsule (sporogonium) of Anthoceros is capable of unlimited growth in length because of the presence of a meristematic zone at its base. 
• The capsule of Anthoceros shows great internal elaboration and complexity of the structure.
• The centre of the capsule is a slender solid core made of 16 vertical rows called the columella. 
• These cells are sterile and are conducting in function. 
• Columella is covered by a double-layered sporogenous tissue. 
• The sporogenous tissue extends over the top of the columella, resembling Sphagnum moss. 
• The sporogenous tissue has pseudoelaters.
• The capsule wall is usually 4 – 6 layers thick. 
• The outermost layer, the epidermis, has several stomata.
• The capsule wall is photosynthetic, which is an advanced characteristic of Anthoceros. The photosynthetic nature makes the sporophyte partially parasitic on the gametophyte. 
• The basal haustorial foot absorbs nutrition from gametophytic thallus tissue. 
• On maturity, the capsule wall breaks longitudinally and releases the spores. 
• On germination, each spore will grow into a new thallus. 
• Thus, the life cycle of Anthoceros shows alternation of generations.

References

• Abraham P C. Bryophytes, Pteridophytes, Gymnosperms & Paleobotany. 2000. St. Mary’s Books & Publications.
• Soni, N. K. (2010). Fundamentals of Botany (Vol. 1). Jodhpur: Vishal Publishing Co.

Additional Topics

• Classification of Bryophytes
• Riccia Thallus Structure & Life Cycle
• Gametophyte of Marchantia