ALTERNATION OF GENERATIONS IN PTERIDOPHYTES

In the life cycle of Pteridophytes, two distinct individuals can be observed i.e., there is a typical heteromorphic alternation of sporophytic and gametophytic generations. These two generations alternate with each other in regular succession one after the other, i.e. from sporophyte to gametophyte and from gametophyte to sporophyte. The sporophytic or asexual generation is diploid (2n), while the gametophytic or sexual generation is haploid (n).

The actual phenomenon responsible for bringing about alternation in generation is the “Periodic Reduction of Chromosomes” as a result of meiosis. Due to meiosis a reduction in the chromosome number takes place which leads to the formation of haploid individuals. The haploid individuals in turn by produce haploid gametes which proceed the process of fertilization to produce diploid individuals.

1. Gametophytic Generation:- The haploid individual bears sex organs, antheridia and archegonia. Antheridia produce male gamete i.e., antherozoids or sperms and archegonia produce female gamete oosphere or egg, which is concerned with the sexual reproduction. These individuals are named as gametophyte or prothallus and represents gametophytic generation. Both the haploid gametes (male and female) come together and unite (i.e., fertilization or syngamy) to produce a diploid (2n) zygote. Zygote is the pioneer structure of the sporophytic generation. It germinates to form embryo which develops into new sporophytic individual.

The gametophyte in the homosporous forms is short lived, independent and may be surface living and green (autophytic) in nature. They are always exosporic and thus are not enclosed by a spore wall, e.g., Lycopodium. In heterosporous forms, they have separate male and female prothalli. The male prothallus is extremely reduced and is represented only by a single prothallial cell. The female prothallus is well developed and larger in size. Both male and female prothalli are endosporic and are enclosed inside a spore wall, e.g., Selaginella.

2. Sporophytic Generation:- Zygote is the pioneer structure of the sporophyte. It develops into embryo which in turn develops into diploid individual. This diploid individual is known as sporophyte and the generation is termed as sporophytic generation. The sporophyte bears a specialized spore bearing structure known as strobilus. Inside the strobilus numerous haploid spores are formed after meiosis which are known as meiospores. There are two types of sporophytic individuals – homosporous and heterosporous. Homosporous individual produce only one type of spores, whereas heterosporous individuals produce two types of morphologically distinct spores i.e., microspores and megaspores. The spores are the pioneer structure of gametophytic generation. It germinates to form a new haploid gametophytic individual.

In pteridophytes the sporophytic individual is complicated, independent and dominant generation.  It is no doubt independent, but it has to depend upon the gametophyte during earlier stages of development. It achieves its complexity after establishing its independence.

Both the homosporous and heterosporous pteridophytes exhibit heteromorphic type of alternation of generations, because the sporophyte and gametophyte individual present marked morphological and anatomical differences.

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HETEROSPORY AND SEED HABITAT IN PTERIDOPHYTES WITH RESPECT TO SELAGINELLA

Heterospory is the condition that interprets the production of spores of two different sizes and two different development patterns. The two different sizes are smaller spores also known as microspores and the larger spores also known as megaspores. Hetrrospory can be observed in some of the pteridophytes such as Selaginella, Marsilea, etc.

They have differential developmental patterns because the microspores germinate to produce male gametophytes or microgametophytes that bear male sex organs called antheridia, and the megaspores germinate to produce female gametophytes or megagametophytes that bear female sex organs called archegonia.

The two kinds of spores are produced in two kinds of sporangia. The microspores are produced in microsporangia and the megaspores in megasporangia. The microspores are produced in large numbers and are comparatively smaller than megasporengia which are produced in lesser numbers and larger in size.

Importance of Heterospory

(a) The most important aspects of heterospory is that it is an expression of sex determining process of the plant. It has brought about along with its onset, the sex determining capacity from the gametophyte to sporophyte. In all the homosporous individuals, sex can be determined in their gametophytic phase, during the formation of antheridia and archegonia. But in the heterosporous individuals sex can be determined in their sporophytic phase during sporogenesis i.e., during the formation of microspores and megaspores.

(b) Heterospory is the most important evolutionary development in pteridophytes because it has ultimately led to seed developments. It is rather a pre-requisite to seed habit. Heterospory has brought about a number of changes in the characteristic of spore development which is the pioneer characters of seed habits in higher plants.

Heterospory in Selaginella

Selaginella, no doubt, illustrates an example of heterosporous pteridophyte that approach seed habit because of the following notable characteristics –

(a) It is heterosporous.

(b) The megaspore starts germination within the megasporangia and their time of release from the megasporangia varies with species.

(c) The number of megaspores in S.rupestris and S. monospora is reduced to one.

(d) In S. rupestris the megaspore is never shed and fertilization and development of the embryo takes place while the megaspore is still within the megasporangium, which retains its connection with the parent plant. This condition can be linked with the vivipary in some angiosperms.

Considering the above points we can reach to the conclusion that had reached to the level of seed habit but fail to develop seeds because of the following shortcomings –

(a) They have no protective structures like the integuments surrounding their megasporangia.

(b) The permanent retention of megaspores within the megasporangia is not established.

(c) Histological union between the megaspore and the megasporangium is absent.

            (d) Lack of resting period after the development of embryo.

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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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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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SELAGINELLA - CLASSIFICATION, STRUCTURE OF SPOROPHYTE, REPRODUCTION, STRUCTURE OF GAMETOPHYTE, FERTILIZATION, MORPHOLOGY OF RHIZOPHORE OF SELAGINELLA

A. CLASSIFICATION:

Division – Lycophyta

     Class – Lycopsida

         Order – Selaginellales

             Family – Selaginellaceae

                 Genus – Selaginella

The genus Selaginella is commonly known as “Club-moss” or “Spike-moss”. Selaginella is a large genus comprising about 700 species and is world-wide in distribution. Some species of Selaginella are found to grow in temperate regions but majority of them are found to occur in the rain forests of tropical countries. The species of Selaginella is found to grow on the ground, on damp, shaded and humid conditions. Some species are also occurs in arid regions of the world. Temperate species are found to grow on damp shaded sides of the hills.

About 55 species are found to occur in India. Of these the common species are S. rupestris, S. chrysocaulos, S. pallidissima, S. jacquemonth, S. megaphylla, S. pentagona, etc.

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 long, slender, usually dorsiventral and prostrate with erect branches. In some species the stem is erect. The stem may be un-branched or dichotomously branched. From each ramification of the stem, colourless, leafless, elongated and cylindrical appendages known as rhizophores develop. The rhizophore develops downwardly into the soil and gives rise to small tuft of adventitious roots at their free ends.

Leaves – Stems and branches bear numerous small, lanceolate, ovate to filiform leaves which are arranged in spirals, decussate pairs or four longitudinal rows. Leaves are generally thin and delicate in texture, and are provided with unbranched mid-vein.

Roots – First root i.e., primary root is short lived, later delicate adventitious root arise from the underside of the stem and from the tip of the rhizophores also. Roots are delicate and branching is dichotomous.

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 it consists of parenchymatous cells. Stomata are absent in the epidermal layer.

Cortex – Cortex is thick and composed entirely of thin-walled, green, parenchymatous cells without intercellular spaces, or partly sclerenchymatous cells forming hypodermis and parenchymatous cells. True endodermis is absent, instead endodermal cells are modified into radially elongated cells known as tuberculae, by means of which stele or steles are attached to the cortex. The tuberculae cells contain casparian strips.

Stele – The stellar organization varies in different species. The stele is protostelic in nature with exarch xylem, the number of which varies from one (monostelic) to several i.e., 1, 2, 3, 4, etc. (polystelic). Each stele is limited externally by a layer of pericycle.

(b) T.S of Root: The root in cross section shows one cell layer tick epidermis. Cortex is like that of stem but is provided with endodermis. Stele is protostelic, which is monoarch and exarch.

(c) T.S of Leaf: The leaf in cross section shows a distinct upper and lower epidermis, each of one-celled in thickness, an undifferentiated mesophyll and a central vascular bundle. The mesophyll tissue is composed of more or less elongated and similar cells with intercellular spaces. Vascular bundle is concentric. Phloem surrounds the xylem.

C. REPRODUCTION:

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

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

(a) Fragmentation – It is affected only in species that grow under humid conditions (S. rupestris). In this case the trailing branches of the stem develop adventitious branches and later get disconnected from their parent plant and grow into separate individual plants.

(b) Tubers – Formation of tubers have been reported in S. chrysorrhoizos and S. abyssinica. The tubers bear rudimentary scales and appear towards the end of the growing season at the tip of the underground branches that arise from the base of the stem. During unfavourable condition the aerial parts of the plant die and the tubers enable the plant to perennate.  At the advent of favourable conditions the tubers germinate to produce new plant.

(c) Resting Buds – Resting buds have been reported to develop at the ends of some aerial branches in S. chrysocaulos. It is formed as a result of compact arrangement of the leaves. The buds give off rhizophores that bear roots at their tips and fix them to the soil. The resting buds survive the unfavourable periods when the rest of the plant dies. They grow into new individuals at the return of the favourable conditions.

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

The cone i.e., strobilus varies in size from 5mm to 7cm . They are cylindrical or quadriangular and are borne at the apices of the main stem or on lateral branches. Each strobilus consists of an axis upon which two types of sporophylls viz., megasporophylls and microsporophylls are arranged spirally. The megasporophylls appear at the base and microsporophylls at the above portion of the strobilus.

Each megasporophyll bears a single stalked megasporangium in its axil on its upper side. Simillarly, microsporophyll bears a single microsporangium in its axil on its upper side. Megasporangia are larger in size than the microsporangia. Both the types of sporangia are provided with a jacket wall of sterile cells of two-celled thickness. Within the jacket wall lies the sporogenous tissue, which is surrounded externally by a prominent layer of nutritive tissue known as tapetum.

Sporogenous tissue of each megasporangium differentiates into megaspore mother cells, but all of them except one degenerates. The surviving megaspore mother cell by reduction division gives rise to four megaspores. In some cases, out of the four megaspores only one or two survive, others degenerate.

Within the microsporangiun sporogenous tissue later on differentiates into microspore mother cells, all of which except a very few, by reduction division gives rise to spore-tetrads. Thus, each microsporangium contains numerous microspores.

D. STRUCTURE OF THE GAMETOPHYTE:

 Selaginella is heterosporous, hence it produces two types of gametophytes, viz., microgametophyte i.e., male gametophyte from microspore and megagametophyte i.e., female gametophyte from megaspore. Thus gametophytes are dioecious (heterothallic).

1. Male gametophyte:- Microspore is the first cell of the male gametophyte. Each microspore is small, spherico-tetrahedral and provided with two coats, viz., outer thick ornamental exine and an inner delicate intine.

Germination of microspore takes place within the microsporangium. Microspore nucleus first divides to form small lense-shaped prothallial cell at one side and the larger antheridial initial. The prothallial cell divides no further but the antheridial initial divides and re-divides forming 12-celled structure, the so called antheridium. Now the male gametophyte consists of 13 cells (12 cells from the divisions of antheridial initial and 1 prothallial cell). Of these 13 cells, the central four cells constitute the primary spermatogenous cells, the eight cells surrounding the primary spermatogenous cells constitute the sterile jacket cells. The primary spermatogenous cells divide and re-divide forming 128 or 256 sperm mother cells i.e., androcytes. Each sperm mother cell is then metamorphosed into biflagellate sperm.

2. Female gametophyte:- Megaspore is the first cell of the female gametophyte. Megaspore are larger and tetrahedral in shape and consists of outer sculptured thick exine and inner thin intine.

The female gametophyte also begins to germinate while the megaspore is still within the megasporangium. The germinating megaspore first enlarges in size and now consists of three wall layers and a thin layer of peripheral cytoplasm enclosing the nucleus. Its nucleus divides into two. Then the two nuclei, by free nuclear divisions, divide continuously until the cytoplasmic layer contains many free nuclei, surrounding the large central vacuole. As the nuclei increases in number, the cytoplasmic layer becomes thicker and the vacuole becomes smaller, and ultimately the vacuolar region is filled up with cytoplasm. Now wall formation begins about the nuclei in the apical region. As a result a cushion of tissue is formed which extend inwards filling the megaspore completely before fertilization.

Shortly, after the formation of apical tissue, the spore wall cracks along the tri-radiate ridge and the apical cushion of tissue becomes exposed. This tissue of the gametophyte may become green and rhizoids may develop from the gametophyte after they have fallen into the soil.

Most of the superficial cells of the apical tissue are potential archegonial initial and several of these develop into archegonia. Archegonia are developed from the centre of the cushion. They are small and shunken in the gametophytic tissue.

Each archegonium consists of neck, composed of two tiers of four cells each, one neck canal cell, ventral canal cell and an egg.

            3. Fertilization:- It may takes place while the female gametophyte is still within the magasporangium or after the megasporangium has fallen to the ground. The sperm after liberation swim to the archegonia in dew or in rain water and of them ultimately fertilize the egg. As a result, a zygote (2n) is formed. With the formation of zygote, diploid sporophytic generation begins.

MORPHOLOGY OF RHIZOPHORE OF SELAGINELLA

Rhizophores are the colourless, leafless, elongated and cylindrical appendages which arise from each ramification of the stem of Selaginella.

There are three different views regarding the morphological nature of the rhizophore. These are –

1. Rhizophores are regarded as capless roots, as they look like root, positively geotrophic, leafless and have the same anatomical characteristic as that of a root (Van Tieghem and Harvey Gibson, 1902; Uphof, 1920).

2. Rhizophores are regarded as leafless shoots because rhizophores like the stems are exogenous in origin and develop from angle meristem one above the other below the junction of two branches (Bruchmann, 1871 and Worsdell, 1910).

3. Rhizophore is neither shoot nor root, but exhibit some of the characters of both (Sporne, 1966; Goebel, 1905; Bower, 1908, 1935).

Some of the stem-like characteristics of rhizophores are –

(a) Exogenous origin.

(b) They develop from special meristem called angle meristem that are present in between the              two branches of the stem.

(c) They lack root caps.

(d) They have no root hairs.

(e) Experimental evidence proved that under certain environmental conditions the rhizophores

develop into leaf bearing shoots.

Some of the root-like characteristics of rhizophores are –

(a) They are positively geotrophic.

(b) They bear no leaves.

(c) Their internal structure resembles that of a root.

            (d) Their stellar organization is always monostelic even if the stems are polystelic.

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