ANTHOCEROS - CLASSIFICATION, STRUCTURE OF THE GAMETOPHYTE, REPRODUCTION, SPOROPHYTE

A. CLASSIFICATION

Division – Bryophyta

     Class – Anthocerotopsida

         Order – Anthocerotales

              Family – Anthocerotaceae

                  Genus – Anthoceros

The genus Anthoceros comprises about 200 species and is widely distributed all over the world. The species of Anthoceros is cosmopolitan, but occurs mainly in temperate and trophical regions. All the species are found to grow in moist and shady places on ditches, rocks, etc.

About 25 species of Anthoceros have been reported from India. Of these the common species are – A. erectus, A. himalayansis, A. khandalensis, A. crispulus, etc.

B. STRUCTURE OF THE GAMETOPHYTE:

1. External structure:- The plant body i.e., the gametophyte is a small yellowish-green or dark-green dorsiventrally flattened, lobed thallus. The lobes with divided margins overlap. The lobes are thick and fleshy and generally more or less divided to form an irregularly lobed and folded margin. The species of Anthoceros exhibit differential habits. Some remains prostrate on the substratum (A. laevis), whereas the thallus of some species remains slightly raised above the substratum (A. erectus). The upper surface of the thallus is smooth and velvety. From the lower surface of the thallus, numerous unicellular, unicellular, smooth-walled rhizoids occur.

2. Internal structure:- Internally the thallus is sevaral layers thick and without a midrib. There is no tissue differentiation. The entire thallus consists of soft parenchyma cells. The cells are uniform and compact. There is no intercellular spaces or air chambers or airpores. In some species, a stomata-like pores open towards the ventral surfece which are known as slime pores. The slime pores leads towards a large cavity, filled with mucilage which is known as mucilage cavity. Very often these mucilage cavities are inhibited by Nostoc colonies. The cavities containing Nostoc colonies are visible to the naked eye as small, deep blue-green rounded spots on the ventral side of the thallus.

C. REPRODUCTION:

Anthoceros reproduces both by vegetative and sexual process of reproduction –

1. Vegetative Reproduction:- Vegetative reproduction takes place by the following process –

(a) Fragmentation:- The cells in the basal older portions of the thallus die and disorganize. When the progressive decay and death of the cells reaches the branching region of the thallus lobes becomes separated. Each separated lobe by continuous apical growth grows into new thallus.

(b) Gemmae:- Gemmae formation is noticed on the upper surface and along the margin of the thallus in A. glandulosus, A. formosae and A. propaguliferus. The detached gemma grows into a new plant.

(c) Tubers:- Certain species of Anthoceros (A. himalayensis, A. tuberosus, A. pearsoni, A. hallii, etc.) exposed to draught regularly develop rounded, marginal thickenings called tubers. With the approach of dry season the thallus dies leaving behind the tubers. The tubers tides over the adverse condition and with the onset of favourable condition for growth, each tuber develops into a new thallus.

(d) Persistent Growth Apices:- In A. pearsoni and A. fusiformis, only the apices persists through the long summer draught  and resume growth only with the return of favourable condition (Campbell). With the continuous growth the cells of persistent apex, a new thallus develops.

(e) Apospory:- Schwarzenbach (1926) and Lang (1901) reported that Anthoceros thallus arise from the unspecialised cells of the various parts of the sporangium particularly the intercalary meristematic zone, sub-epidermal and sporogenous regions of the capsule. The thallus (gametophyte) is produced directly from the vegetative cells of the sporangium. This phenomenon is called apospory. 

2. Sexual Reproduction:- Anthoceros may be homothallic (monoecious) or heterothallic (dioecious). In homothallic species, antheridia mature early i.e., they are protandrous. Sex organs are deeply embedded in the thallus.

(a) Antheredia:- Antheredia are developed in clusters from the hypodermal cells of the thallus and are enclosed in and roofed over by antheridial chamber. Mature antheredium is a club-shaped or pouch-like body borne upon a more or less slender stalk composed of 4 or more rows of cells. Antheridium jacket is composed of a single layer of cells, within the jacket there lies numerous androcytes i.e., sperm mother cells, each of which forms an antherozoid i.e., sperm. Mature sperms are linear and biflagellate.

           On maturity of the antheridia, the roof of the antheridial chamber ruptures exposing thereby the antheridia. Then after absorbing water the antheridial wall cracks, as a result sperms are liberated to the surrounding film of water.

(b) Archegonia:- Archegonia develop singly from the superficial cells of the thallus and are produced in an acropetal succession. There is no stalk and they are embedded in the thallus, and are in direct contact with the vegetative cells lateral to them. The nearly mature archegonium consists of vertical row of 4-6 neck canal cells, a ventral canal cell and an egg cell. At maturity the neck canal cells and ventral canal cell disorganize.

D. FERTILIZATION:

            At the time of fertilization one of the anthreozoid enters through the neck and fertilizes the egg. As a result a zygote i.e., oospore is formed. With the formation of zygote, diploid sporophytic generation begins.

E. SPOROPHYTIC PHASE:

Zygote is the first cell of the sporophyte. By repeated segmentation it develops into an elongated embryo. The later by further cell division, cell differentiation and continued growth rapidly grows into an elongated, spindle-shaped structure with a bulbous base, which is known as the sporangium or sporophyte.

The sporophyte of Anthoceros is differentiated into two regions – foot and capsule. Each sporophyte is surrounded at its base by a tubular involucre.

(a) Foot:- It is a rounded bulbous structure deeply embedded in the tissue of the thallus. By means of foot Anthoceros sporophyte is attached is well anchored upon and attached to the thallus. The foot mainly consists of a mass of parenchymatous cells.

Seta is absent in Anthoceros sporophyte, instead a zone of meristematic is present, by the activity of which the capsule grows.

          (b) Capsule:- It forms the major and conspicuous part of the sporophyte. It is a long, slender, smooth, upright and cylindrical structure measuring about 2–3 cms in length. Internally, capsule is a complex structure and shows differentiation of tissues. (i) The centre of the capsule is occupied by a sterile mass of tissue known as columella. The columella contains trachied-like cells and is usually 16-celled in cross section. (ii) Surrounding the columella thers is a cylinder of sporogenous tissue which is differentiated into alternative blocks of sterile branched pseudoelaters and spore-tetrads towards the apex. (iii) The uppermost layer consists of the capsule wall. It is multilayered consisting of 4 – 6 layers of cells. The outermost layer forms the epidermis with distinct stomata. Beneath the epidermis forms the parenchymatous jacket cells with intercellular spaces among them and contains chloroplasts.

In dry condition, when the capsule looses water, the tip of the capsule gradually shrivels and the capsule dehisces by splitting into two halves exposing spores, thereby shedding off of the spores by hygroscopic movement. Dispersal od the spores takes place by air current.

Spores after liberation from the sporangium undergo a resting period of few weeks or months – then each germinates through germ tube and forms a new Anthoceros thallus.

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