Table of Contents
- Classification of Anthoceros
- Structure of Gametophyte
- Vegetative Reproduction
- Sexual Reproduction
- Antheridium
- Archegonium
- Fertilisation
- Structure of Sporophyte
- Structure of Spores
- Germination of Gametophyte
Anthoceros is a hornwort that belongs to the family Anthocerotaceae. It is the largest genus of hornworts and is found inhabiting globally. It occurs in moist and shaded sub-tropical and temperate regions. They have a small or medium-sized green thallus that has lobed margins.
Classification of Anthoceros
|
Kingdom |
Plantae |
|---|---|
|
Division |
Anthocerotophyta |
|
Class |
Anthocerotopsida |
|
Order |
Anthocerotales |
|
Family |
Anthocerotaceae |
|
Genus |
Anthoceros |
Structure of Gametophyte
The gametophytic plant body is dorsiventral, thalloid, prostrate and dark green in colour. They have a partial dichotomous branching that gives the thallus a rosette like appearance. The thallus is either pinnately branched or bilobed with a large number of spongy bodies called gemma.
The dorsal surface of the thallus is mostly smooth but it may be velvety because of the presence of lamellae or it can be rough because of spines and ridges. It is shiny, a little thick in the middle and does not have a distinguishing midrib.
The ventral surface has unicellular rhizoids that function to attach the thallus to the substratum. They also absorb minerals and water from the soil. Small blue-green colonies of Nostoc are also seen on this surface which has a symbiotic relationship with Anthoceros.
Internally, the thallus is composed of thin-walled parenchymatous cells that have different thickness in different species. The outermost layer of the thallus is epidermis. It contains lens-shaped, discoid chloroplasts. Each chloroplast encloses pyrenoids which is a characteristic feature of the class Anthocerotopsida.
Air chambers or air pores are absent in the genus. However, some species do have cavities on the lower surface of cells that are filled with mucilage and are called slime pores. These cavities are inhabited by Nostoc colonies.
Vegetative Reproduction
Vegetative reproduction in Anthoceros takes place by the following means:
- Fragmentation: The posterior portion of the thallus constantly dies and decays such that the lobes give rise to a new thalli.
- Tubers: Tubers are structures that remain inactive during unfavourable conditions. They have a protective layer that can withstand desiccation. In favourable conditions, they germinate and give rise to a new thallus.
- Persistent Apices: In this type of reproduction, all the parts of the thallus except the apical regions dry out. Upon arrival of favourable conditions, they form a new plant.
- Apospory: In apospory, diploid gametophytic spores are directly formed from sporophytes.
Sexual Reproduction
Sexual reproduction is of oogamous type in Anthoceros. The male and female sex organs are antheridia and archegonia, respectively. Both monoecious and dioecious species are found.
Antheridium
Antheridia develop on the dorsal surface of the thallus in an acropetal fashion inside antheridial chambers. It is a club-shaped structure with a long stalk. The stalk is multicellular and arranged in 4 rows of vertical elongated cells. The club-shaped antheridium has an outer sterile jacket inside which are enclosed androcytes. The androcytes then develop into biflagellated antherozoids.
Archegonium
The archegonia are borne in acropetal succession on the dorsal surface of the thallus. There are no archegonial chambers. The archegonia is a flask-shaped structure that is composed of a neck and venter. The neck cells disintegrate upon maturation and become mucilaginous cells.
Unlike antheridium, no sterile jacket is found on the archegonium. Four cover cells or lid cells are present at the tip of archegonium
Fertilisation
The antherozoids swim in water and reach the mature archegonium. They pass through the mucilaginous neck cells and one of the antherozoids fuses with the egg to form a diploid zygote that gives rise to a sporophyte.
Structure of Sporophyte
The sporophytic body of Anthoceros grows embedded in the gametophytic body. It is divided into three distinct regions:
- Foot: The foot is a bulbous parenchymatous structure that is embedded in the gametophyte. It helps in absorbing water and nutrients from the gametophyte.
- Intercalary Zone: It is a narrow zone of meristematic cells that is found between the foot and capsule. These cells keep the continuous growth of sporophyte.
- Capsule: It is a fertile, major and conspicuous part of the sporophyte. It is green, long and cylindrical when young, but turns brown or grey upon maturity. The capsule has following two parts:
- Columella: It is the central solid core of sterile cells arranged in 16 rows of vertical cells. It extends from the base to almost the tip of the capsule.
- Sporogenous Tissue: It is a group of spore-forming fertile cells that surrounds the columella. It is single-layered at the base of the capsule and thickens as it goes upwards and forms diploid spore mother cells. The spore mother cells divide by meiosis to produce haploid spores.
The capsule wall is 4-6 layers thick, out of which the outermost layer is the epidermis that contains stomata. The inner cells are made up of chlorenchyma cells and are photosynthetic in nature.
Structure of Spores
The spore is haploid, uninucleate and semicircular in shape with a prominent triradiate mark. The spore consists of two walls: the outer wall is thick and ornamented, known as exospore and the inner wall is thin, known as endospore. Between the two layers are present oil globules, colourless plastids and food material.
Germination of Gametophyte
The spores germinate under favourable conditions by absorbing water. The outer layer (exospore) ruptures at the triradiate mark and the endospore comes out in the form of a tube, called germ tube.
All the contents of the spore are transferred into the germinal tube; the colourless plastids now turn green. Two successive transverse divisions at the tip of the tube results in the formation of a three-celled filament. The filaments then grow into a gametophyte.
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