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

A. CLASSIFICATION:

Division – Pterophyta

    Class – Leptosporangiopsida (Pteropsida)

        Order – Marsileales

            Family – Marsileaceae

                Genus – Marsilea

The genus Marsilea comprises about 58 living species. The genus is worldwide in distribution and is very common in warmer parts of the world. The species of Marsilea are aquatic or amphibious plants – they grow either completely sub-merged in water, or partly or wholly out of water with their roots embedded in the muddy soil.

In India, only 9 living species are found to occur. The common Indian species are – M. quadrifolia, M. aegyptica, M. coromandelica, M. brachypus, M. condenseta, etc.

B. STRUCTURE OF THE SPOROPHYTE:

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

Stem – Stem consists of an elongated stolon-like rhizome with distinct nodes and internodes. Rhizome grows either on the surface of the soil or slightly embedded in the muddy soil. The stem is freely branched.

Leaves – Leaves are long-petioled and compound, developing at the nodes. They are arranged alternately in two rows along the upper side of the rhizome. Petiole of each leaf bears 4 terminal leaflets which are apparently quadrifoliate. The veins of the leaflets are dichotomously branched.

Roots – Roots arise from the underside of the stem at the nodes. The roots are unbranched, adventitious type and arise in one or two clusters.

2. Internal structure:- (a) T.S of Stem: The T.S of stem, i.e., rhizome shows the following structures – 

Epidermis – Epidermis is single layered. The epidermal layer lacks stomata.

Cortex – Cortex is differentiated into three regions – (a) outermost region composed of parenchymatous cells with large radiating air spaces. The air spaces are traversed by single row of cells, (b) middle region composed of thick walled sclerenchymatous cells, and (c) innermost region composed of thin walled parenchymatous cells containing starch.

Stele – Stele of stem is actinostelic type. It is bounded on both outer and inner sides by single row of endodermis. It consists of a ring of single row of xylem vessel, bounded on both sides by phloem cells.

Pith – The centermost portion of the stem is occupied by central pith. The pith is generally parenchymatous, but become sclerotic in the species growing in dry or wet soils.

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

Epidermis – Epidermis is single layered, beneath which lies the single-layered hypodermis.

Cortex – Cortex is massive and composed of 2-regions – (a) outer region composed of parenchymatous cells with prominent air spaces, and (b) inner region made of sclerotic cells.

Stele – Stele is monoarch or diarch protostele, with exarch xylem. The phloem lies in the form of bands on either side of the xylem plate. The stele is surrounded by single layered pericycle and endodermis on outer side.

C. REPRODUCTION:

The sporophyte of Marsilea reproduces both by vegetative means and by production of spores –

1. Vegetative Reproduction:- In M. hirsuta, a special reproductive structures known as tubers are developed from the plant body. These tubers remain viable during unfavorable condition, and each germinates into a new plant under favorable condition.

2. Spore Formation:- Marsilea is heterosporous. Two types of spores (Microspores and Megaspores) develop within their respective sporangia (Micro-sporangia and Mega-sporangia respectively). These sporangia are borne within the specialized reproductive structure known as sporocarp.

**Sporocarps are bisporangiate, i.e., they contain both micro and mega-sporangia. The sporocarps are borne either singly or in clusters on short lateral branches of the petiole, known as pedicels. Young sporocarps are soft, green and covered with hairs. Mature sporocarps are brown or dark-brown in colour, hard and nut-like.

The mature sporocarp is covered by 3-layered, thick and resistant wall – (a) outer wall known as epidermis in which stomata are present, (b) next to the epidermis is the middle wall, known as hypodermis and (c) the innermost wall made of parenchymatous cells.

Beneath the wall, the sori are arranged in alternating rows in the cavity of the sporocarp. Each sorus is bounded by an indusium, of 2-celled thickness. Each sorus bears both micro and mega-sporangia.

Micro and mega-sporangia are more or less globose, long and short stalked respectively. Both micro and mega-sporangia are provided with single layer of jacket cells without annulus. Within the jacket layer, 2-3 layers of sporogenous cells are present.

Within the microsporangium, 16 microspore mother cells are developed from the sporogenous cells. All the microspore mother cells by two successive divisions form 64 microspores.

In case of megasporangium, all the megaspore mother cells, developed from the sporogenous cells disintegrate except one remains functional megaspore mother cell. This functional megaspore mother cell by meiotic division gives rise to 4-megaspores. Of these 4-megaspores only one megaspore survives. The surviving megaspore becomes large and develops a very thick wall.

At this stage of spore development, the sporocarp gets detached from the plant and remains in water or muddy soil. Soon, the sporocarp opens into two halves and the sporangia are set free.

D. STRUCTURE OF THE GAMETOPHYTE:

Both the types of spores are liberated from their respected sporangia when the walls of the sporangia and indusia undergo gelatnization. Germination of microspores and megaspores start as soon as they come in contact with water, resulting in the starting of the gametophytic phase of the life cycle.

1. Male gametophyte:- Microspore is the first cell of the male gametophyte. Germination of microspore starts as soon as it is liberated from the microsporangium.

At first the microspore nucleus divides into two, forming small prothallial cell and the larger apical cell. The apical cell then divides transversely, forming two cells, each representing antheridial initial. The nucleus of each antheridial initials divide and re-divide forming a jacket of three external cells and one spermatogenous cell, or primary androgonial cell. The primary androgonial cells by four successive divisions give rise to 16 androcytes or sperm mother cells. The sperm mother cells metamorphosed into spirally coiled, multiflagellate sperms. Mature sperms are set free when spore wall bursts. 

2. Female Gametophyte:- Megaspore is the first cell of the female gametophyte. Germination of megaspore starts as soon as it is liberated from the megasporangium.

At first, the megaspore nucleus divides into two, forming a smaller apical cell and a larger basal cell. The gametophyte proper develops from the smaller apical cell.

Mature female gametophyte is much reduced structure and it consists of single archegonium embedded in a cap of tissue attached over the apex of large nutritive cell.

The archegonium is short but broad and consists of a neck and venter. The neck consists of a neck canal cell and venter consists of a ventral canal cell and an egg cell.

        3. Fertilization:- It takes place when the sperm swim towards an archegonium, and only one sperm enters through the neck and fuses with the egg. As a result of fusion, a diploid zygote (2n) is formed.

            As soon as the zygote is formed, it develops into diploid sporophytic generation.

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