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

A. CLASSIFICATION:

Division – Sphenophyta

    Class – Sphenopsida

       Order – Equisitales

           Family – Equisitaceae

              Genus – Equisetum

Equisetum commonly known as horsetails comprises about 25 species and is world-wide in distribution. Equisetum can grow in wide variety of habitats. Some species grow in damp and shaded places, others grow in ponds and marshes or in sandy soil, near the banks of rivers, etc. Some others are found to grow in exposed dry habitats, along the road side and railway embankments.

Some of the common Indian species of Equisetum are – Equisetum debile, E. arvense, E. Diffusum, E. elongatum, 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 – The stem is underground, horizontal and much branched, which often penetrates more than a meter into the soil. The rhoizome is jointed and provided with nodes and internodes. It gives off two types of branches – sterile and fertile branches. Sterile branches are green in colour, persistent and have a whorl of lateral branches at each node. Sterile branches are vegetative in function. Fertile branches are generally unbranched, non-green in colour, short lived and bears a single strobilus at its tip.

Leaves – Leaves are small, slender, scale-like and each is provided with a single midrib. They are arranged in whorls at each node. They are fused at the base and free at the tip which forms a teeth like structure. They form a cup like structure at the base of the internodes.

Roots - Roots are slender, much branched and adventitious (fibrous type). They occur only at the nodes of the rhizomes or stem bases.

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

Epidermis – Epidermis is single layered and its cell wall is heavily silicified. Stomata are present in the cells of the epidermis.

Cortex – Cortex is broad and differentiated into four zones – (a) an outermost cortex made of sclerenchymatous cells, (b) middle cortex made of chlorenchymatous cells and (c) an inner cortex made of thin walled parenchymatous cells. The innermost layer of the cortex is the single layered endodermis with casparian strips.

Stele – Beneath the endodermis lie the stele. Stele is dissected siphonostele. Vascular bundles are arranged in ring. Each bundle has three small strands of xylem between which lies the phloem strands. The centre is occupied by a hollow pith containing water.

           (b) T.S of Rhizome: The rhizome in transverse section shows similar structure like that of aerial stem except the following differences such as, (a) absence of stomata in the epidermis, (b) cortex is devoid of photosynthetic chlorenchyma and sclerenchyma is poorly developed, (c) pith may be solid or hollow. 

                (c) T.S of Root: The root in transverse section shows – epiblema, cortex and stele.

                Cortex is divided into two layers – an outer thick walled zone forming exodermis and an inner thin walled parenchymatous zone forming endodermis. Endodermis is two-layers of cells thick.

                Stele varies from triarch to tetra-arch. Metaxylem lies in the centre.

                 C. REPRODUCTION:

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

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

(a) By tubers – In some species, a round or ovoid tubers are developed in the rhizome. When detached from the parent rhizome each tuber grows into a new plant and thus serves as a means of vegetative propagation of the sporophyte.

(b) By Branch Primordia – Every branch of the rhizome bears preformed branch primordial which can develop into new sub-terranean and aerial branches at any time, after the decay of older rhizome.

2. Spore Formation:- In Equisetum, spores are formed on a specialized spore bearing structure known as strobili (singular : Strobilus), or cone.

Strobilus, i.e., cone arises singly at the apices of fertile shoots. The primitive type of cone is sub-sessile and is apiculate while the advanced type is stalked with a rounded apex. In some species, there is a ring like outgrowth at the base of the cone, called annulus.

Each cone or strobilus consists of a thick central axis upon which several densely crowded peltate appendages called sporangiophores are arranged in whorls. Each sporangiophore (=sporophyll) is a stalked peltate structure projecting at right angles from the cone axis. Each sporangiophore bears on the underside an elongated sac-like structure called sporangia. The number of sporangia per sporangiophore varies from 5-10.

Mature sporangium is an elongated, cylindrical structure with a rounded apex. It consists of a delicate, one cell thick jacket layer. Inside the jacket layer is a mass of sporogenous tissue, from which some of the cells differentiates into spore mother cells and some gets degenerated. The spore mother cells by meiotic division forms haploid spore tetrad. Eqiusetum is normally homosporous, and with the formation of spores, gametophytic generation begins.

            D. STRUCTURE OF THE GAMETOPHYTE:

            Spore is the first cell of the gametophyte. A mature spore is globular in shape and provided with four concentric wall layers, such as – (a) innermost delicate layer of cellulose known as endospore (intine), (b) next to the endospore lies externally exospores (exine), (c) exospores is externally surrounded by a delicate layer known as middle layer and (d) the outermost thick epispore or episporium. The epispore is differentiated into four narrow, spirally wounded bands with flat and spoon like tips. These bands are commonly called elaters. The spiral bands remain tightly coiled about the spore until the sporangium dehisces. These elaters are extremely hygroscopic and respond quickly to changes in air moisture. Their expansion possibly assists in dehiscence of the sporangium and also in spore dispersal.

            Under favourable condition, the spore germinates and gives rise to normally monoecious gametophytic plant body.

            The mature gametophytes are long lived, green, thalloid and branched structures which grow on the wet soil and shaded regions. They show two well marked regions – (a) cushion-like compact and colourless basal region and (b) the vertically erect, green photosynthetic, irregularly shaped lobes. Rhizoids arise on the lower surface of the basal portion.

            If conditions for growth are favourable, the first sex organs are formed when the gametophytes are of 30-40 days old. Archegonia appear first in the meristematic margin of the gametophyte, in the regions where the upright lobes are developed. Antheridia generally begin to develop in large numbers, on the marginal regions of the gametophyte after the development of the archegonia. They generally develop on the photosynthetic lobes or besides the archegonia.

            1. Archegonium:- The mature archegonium has a venter embedded in the gametophyte and a projecting neck. The neck is short and consists of four vertical rows of cells. The neck consists of 1-2 neck canal cells. The venter consists of a ventral canal cell and an egg cell.

2. Antheridium:- The antheridium is more or less rounded structure. It consists of an outer jacket layer, one celled in thickness and an inner androgonial cell. The primary androgonial cell divides and redivides forming a group of androgonial cells which forms androcyte mother cells. Each androcyte forms two androcytes (sperm mother cells). Each androcyte, i.e., sperm mother cell is metamorphosed into a large, flattened, spirally coiled and multiflagellate sperm i.e., spermatozoid. Sperms are set free at maturity by the breaking down of the jacket layer.

Fertilization:- At the time of fertilization one of the sperm swims towards the neck of the mature archegonium, then passes through the passage established by the dissolution of the neck canal cell and the ventral canal cell and finally fuses with the egg. As soon as the fusion takes place, zygote (2n) develops.

With the formation of zygote the diploid generation (2n) begins.

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