LYCOPODIUM - CLASSIFICATION, STRUCTURE OF SPOROPHYTE, REPRODUCTION, STRUCTURE OF GAMETOPHYTE AND FERTILIZATION

A. CLASSIFICATION:

Division – Lycophyta

     Class – Eligulopsida

         Order – Lycopodiales 

              Family – Lycopodiaceae

                   Genus – Lycopodium

The genus Lycopodium is commonly known as “Club Moss” or “Ground Pine”. It is a large genus comprising about 200 species, growing mainly in sub-tropical and tropical forests. Some species are distributed in arctic and temperate regions also. All the species grow in moist and shady places, rich in organic compounds and humus.

B. STRUCTURE OF THE SPOROPHYTE:

1. External structure:- The sporophyte, i.e., the plant body is well differentiated into – stem, roots and leaves.

Stem – The stem is weak, slender and rhizomatous. In some species (L. clavatum and L. cernuum), etc., the stems are creeping on or below the ground. In others (L.selago and L. plegmaria), etc., the stems are erect and pendent. The branching of the stem is dichotomous. Stems and branches are covered with small leaves.

Roots – Roots arise from the underside of the creeping stem. They arise in clusters or singly. Branching of the root is dichotomous.

Leaves – Leaves are small, sessile, simple and lanceolate in shape with broad base. They arise spirally along the main axis of the stem. Mature leaves are provided with un-branched mid-vein.

 

2. Internal structure:- (a) T.S of Stem: The internal structure of aerial shoot shows the following tissue systems –

Epidermis – Epidermis is one-celled in thickness and covered with cuticle. Stomata are present in the epidermal layer.

Cortex – Cortex is massive and divided into 3-zones – (a) outer zone composed of thick-walled sclerenchymatous cells, (b) middle zone composed of thin-walled parenchymatous cells and (c) an innermost zone of one-celled thickness known as endodermis.

Stele – Internal to the endodermis lie the pericycle composed of 3-6 layers of thin-walled cells. The stele is protostelic with exarch xylem. The xylem core has radiating ribs forming a star-like mass, or it may be in the form of isolated transverse strands, (actinostele and plectostele respectively). Phloem lies in space between the xylem rays.

(b) T.S of Root: The root in transverse section shows –

Epidermis – Epidermis is single layered.

Cortex – Cortex is massive and composed of 2-zones – (a) outer zone with thick-walled sclerenchyma cells and (b) inner zone with thin-walled parenchyma cells.

Stele – Stele is protostelic with monoarch xylem vessel in young roots and diarch to deca-arch in older roots. The phloem cells lie in between the xylem rays.

(c) T.S of Leaf: The internal structure of leaf shows a single layered epidermis covered with thick cuticle. Beneath the epidermis lies one type of mesophyll tissue, composed of rounded or angular parenchyma cells. There is a single median, concentric vascular bundle.

C. REPRODUCTION:

Sporophyte of Lycopodium reproduces both by vegetative means and by production of spores –

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

(a) By Gemmae or Bulbils – These are modified vegetative structures that arise as lateral out-growths near the stem apex. Each bulbil consists of short axis surrounded by a number of thick and fleshy leaves. The gemmae fall on the ground and grow into a new plant.

(b)Fragmentation – In this case, the branches gets separated from the parent plant due to dead and decay of the older parts. The separated branches grow into new individual plants.

(c) By Resting Buds – In some species of Lycopodium, the tip of the rhizome or branches store food material and becomes thick with crowded leaves. These are resting buds. During unfavorable condition, the whole plant dies except the resting buds. It resumes growth at the advent of favorable condition, and produces a new individual.

(d) By Root Tubercles – Tubercles originated from the parenchymatous region of the cortex. It consists of a group of cells with stored food material and protected by thick walls and has the capacity to germinate into a new plant individual.

(e) By Adventitious Buds – Buds are developed from isolated bulbil leaves. It also develops in the stem near the apex. Such buds can produce a new plant.

2. Spore Formation:- In Lycopodium, the spores are formed in a specialized reproductive structure known as strobili (singular : Strobilus) or cone.

Each strobilus is a slender structure, sessile or stalked, simple, un-branched or dichotomously branched, arising at the apex of the stem or branches. It is a cylindrical structure measuring 2.5 cm and consists of central axis in which fertile leaves or sporophylls are spirally arranged. Each sporophyll bears a solitary sporangium on the upper side at the basal portion.

Each sporangium is yellow or orange coloured and provided with sterile jacket layer of 2-3 layers of cells thick. Within the jacket layer is the fertile sporogenous tissue provided with nutritive tissue known as tapetum. The sporogenous tissue later differentiates into spore mother cells, each of which by meiotic division produces spore-tetrad.

Lycopodium is homosporous, i.e., it produces only one type of spores. As soon as the spores are developed, haploid (n) gametophytic generation begins.

D. STRUCTURE OF THE GAMETOPHYTE:

Spore is the first cell of the gametophyte. Spores are very small, tetrahedral and provided with two thin walls – outer exine and an inner intine. Each spore contains a single nucleus and fats and oils as reserve food materials.

Lycopodium is homosporous, hence the germination of spore produces homothallic gametophytic plant body or prothalli (singular : Prothallus). Depending upon their nature, the prothalli of Lycopodium is of 3-types –

First Type – In this type, the prothallus is very small (2-3 mm long), cylindrical or ovoid in shape, short lived, green in colour and develops on the surface of the ground. Such type of prothallus is found in tropical species.

Second Type – In this type, the prothallus is much larger (1-2 cm long), more or less tuberous or carrot shaped, long lived, yellowish in colour or almost colourless and sub-terranean. Such type of prothallus is found in creeping species.

Third Type – This type of prothallus is intermediate between first and second types. This type of prothallus have irregularly shaped tuberous body (about 2mm in diameter), colourless and sporophytic in nature. Such type of prothallus is commonly found in epiphytic species.

Since the prothallus is homothallic, it bears both male and female sex organs, i.e., antheridia and archegonia in a single gametophytic plant body.

1. Antheridia:- Antheridia arise in several numbers in a gametophytic plant body. They remain either wholly embedded in gametophytic tissue or projected slightly. They are generally oval in shape. Each antheridium is surrounded by a jacket layer of one-celled in thickness. Inside the jacket layer lies numerous sperm mother cells, which directly metamorphosed into small, cubical, biflagellate sperm.

2. Archegonia:- Archegonia also arise in numbers in the gametophytic plant body . They also remain sunken with only their neck projecting outwardly. A mature archegonium consists of a neck, composed of 6-13 neck canal cells, and a narrow venter, composed of one ventral canal cell and an egg cell.

         3. Fertilization:- At maturity, the neck cells of the archegonium separate and the neck canal cells and ventral canal cell disorganize, leaving a passage for the entry of sperm. The sperm after liberation from the antheridium makes its way through the neck and finally reaches the egg. On reaching t he egg, one sperm fuses with the egg to complete the fertilization.

            As a result of fertilization, a diploid zygote (2n) is formed. With the formation of zygote, diploid sporophytic generation begins.

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