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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