Q.15. Give an account of the life history of Selaginella.
Related Question-
Q. With the help of diagram. Describe the structure of cone (strobili) of selaginella. (2012, 13)
Q. Describe the structure of female prothallus of Selaginella. (2006,11, 12)
Q. With the help of diagrams describe the mature male gametophyte of Selaginella. (2003)
Q. With the help of diagrams only describe the development of archegonium in Selaginella. (2002)
Ans. Systematic Position: -
Division - Pteridophyta
Class - Lycopsida
Order - Selaginellales
Family - Selaginellaceae
Genus - Selaginella
Selaginella is a widely distributed genus containing about 700 species. It is very common in damp forests of tropical regions. It is also found in temperate regions of the world. It prefers to grow in damp and shady places. A few species such as S.rupestris, S. lepidophylla grow under xerophytic conditions. S.oregana is the epiphytic species growing on tree trunks. About to species have been reported from Himalayas and the hills of south India by Alston (1945). S. pallidisima, S. adunca, S. kraussiana are common species.
The Sporophyte: -
Plant body is sporophytic which is differentiated into root stem, leaf, and rhizophore. It is herbaceous. In some xerophytic species such as S. lepidophylla and S. pilifera, the entire plant during the dry periods assumes a shape of a tight ball which in the presence of water opens up into normal green plant. Such plants are commonly known as the resurrection plants.
Stem: - In the species referred to the subgenus Homoeophyllum, the stem is generally erect and radially constructed. In the species of the subgenus Heterophyllum, the stem is usually dorsiventral and prostrate with erect branches, but it may be sub-erect or sub-erect scandant.
The branching of the stem in all the species is typically dichotomous but often appears to be monopodial because of rapid growth of the main stem.
Leaves: - Leaves are microphyllous, simple, small lanceolate to ovate in shape. The leaves are narrow at stem attachment, Each leaf has unbranched vein. Each leaf bears on its upper surface near its base a small flaplike outgrowth, known as ligule. In mature leaves the ligule shrinks and becomes inconspicuous.
Rhizophore: - They are peculiar leafless colourless and prop-like branches. They grow downwards from the leafy stem and branches out to form roots when they tuches the soil.
Roots: - The roots are adventitious and come out from the tips of the rhizophore.
Reproduction: -
The sporophytic plant body of selaginella reproduces vegetagively and by spores.
(A) Vegetative reproduction: - It occure by following ways
(a) Fragmentation: - It is common in species growing in humid conditions such as S. rupestris. Some of the prostrate branch develop roots and break into small pieces. Each piece or fragment forms a new plant.
(b) By tubers: - In S.chrysorhizos tubers develop underground at the tips of modified vegetative branch whereas in S. chrysocaulos they are formed at the end of aerial vegetative branches. Under favourable conditions tubers form new plants.
(B) Sexual Reproduction: - The reproductive structures of Selaginella are compact and develop at the terminal portions of the branches which are called as strobili or cones. Each strobilus consists of an axis having spirally arranged, ligulate sporophylls. In the axils of sporophylls develop the stalked sporangia which are called megasporangia and microspornagia. They possess the large sized megaspores and of the small size microspores respectively.
The length of strobilus varies from 9.6 cm to 5.7 cm in different species of Selaginella. Selaginella is a heterosporous plant and strobili are also heterosporangiate. Each strobilus is made up of central axis bearing compactly spirally arranged sporophylls. The sporophyll which bears megasporangium is called mega sporophyll while those bearing microsporangium are called microsporrophylls. Each sporophyll bears a basal sporangium.
In the L.S. of the strobilus the microsporophylls and megasporophylls are found spirally arranged on the central axis. Each sporophyll bears single stalked sporangium attached in its axis. The sporophyll possess ligule between the sporangium and the upturned part of the sporophyll. The strobilus shows the presence of both microsporangia and megasporangia. Usually the megasporangia on the megasporophyll are located in the basal portion and the microsporangia on the microsporophyll in the upper portion. The microspornagium is small, attached in the axis of microsporophyll. It has two layered wall. Outer layer is thick, whereas inner layer made up of thin walled cells.
Development of Sporangium: - The development of both the sporangia is eusporangiate type. In both cases the first intial division is a periclinal division forming an outer tier of cells jacket initial and an inner tier archesporial cell. The archesporial cell undergoes repeated vertical and transverse divisions resulting into a mass of sporogenous tissue. Side by side jacket initial also divide and redivide to form a two called thick jacket layer. The cells of outer jacket layer become thick walled while inner jacket layer remain thin walled. Outermost layer of the sporogenous tissue gets differentiated into tapetum.
Development of Microsporangia: - All sporocytes, except 10-20 parcent sporocytes which are the last cell generation of the sporogenus tissue, degenrate and serve as nourishing fluid for young developing spores. The sporocytes or the potentiel spore mother cells undergo reduction division and form tetrad.
Development of Megasporangium: - All sporocytes except one degenerate. The degenerating sporocytes develop vacuoles and accumulate starch. The functional sporocyte undergoes reduction division and form four megaspores.
Mature Sporangium: - Both micro and megasporangia are shortly stalked structure surrounded by wall of two layers. The cells of outer layer are columner and thick walled and elongated. Mature microspornagia may be yellow, brown or red coloured while the megaspornagia are greenish white or pale coloured. Megasporangia are generally four lobed structures each lobe having one megaspore. On drying the lower boat shaped part of sporangium shrinks and squeezes out the spores throwing them out at some distance. Groups of the microspores are being pushed out in the process when microsporangium is under the process of dehiscence.
Gametophyte: -
The spores are the first cell of gametophytic generation Selaginella is hetertosporous since it produces two kinds of spores: small microspores, borne within microsporangia and large megaspores, borne in megasporangia. On germinating, from a microspore develops a male gametophyte bearing antherozoids and form a megaspore develops a female gametophyte bearing the archegonia.
Male Gametophyte: - The microspore start germinating within the microsporangium. This is called Precocious. The male gametophyte develops within the wall of microspore. The microspore divides to two unequal cells. The smaller cells is called prothallial cell. Prothallial cell is inactive and does not play any role in development. The large cell called antheridial cell undergoes several divisions in various planes to form a group of twelve cells. Out of these twelve cells eight form a jacket surrounding a group of four central cells called primary androgonial cells. (13 celled stage i.e. 8 jacket cells, 4 primary androgonial cells and one prothallial cell) At 13 celled stage usually the male gametophyte is shed out of the microsporangiurn. The four primaryandrogonial cells divide repeatedly to form 128-256 on the antherozoid mother cells or androcytes.
Each androcyte metamorphoses into a single spirally coiled, biflagellate antherozoids on the rhizoids at maturity are liberated by cracking open the spore wall along the triradiate ridge. The antherozoids swim freely till they reach the neck of the archegonium.
Female Gametophyte: - Each megaspore is a large, broad and somewhat triangular 1 to 5 mm in diameter. Megaspore has three walls, outer thick exospore, inner thin endospore and mesospore between the two. The megaspore like microspore also starts germinating within the megasporangium.
During development the megaspore nucleus divides repeatedly forming large nuclei. Fall formation starts at the apex of the spore between the daughter nuclei, resulting 2-3 cells thick cushion of cellular tissue. The female gametophyte gets ,differentiated into upper female prothallus and lower storage tissue. Diaphragm lies between the two regions. A few cells called archagonial initial appear in prothallus. The female gametophyte also develop some, vestigial rhyzoids after liberation from megasporangium. The archegonial initials divide periclinally to form a primary cover cell and a central cell. The central cell divide periclinally to form a primary canal cell and a primary ventral cell.
The primary canal cell does not divide and forms a single neck canal cell. The primary ventral cell again divides by a transverse division to form a Ventral canal cell and an egg cell. The primary cover cell divisions by two anticlinal walls to form four neck initials which ultimately form a short neck of two cells in height.
The archegonium slightly produces out and is embedded in the tissue of female gametophyte. The neck canal cell and ventral canal cell disintegrate at maturity and form mucilage which attracts antherozids.
Fertilization: - Fertilization takes place in presence of water. The antherozoids enters the archegonium and one of them fuses with the egg forming oospore or zygote. Water is essential medium for fertilization. The antherozoid swim in water drops and pass down through the open passage of neck. A single antherozoid fuses with the egg to form zygote.
Development of Embryo: -
The oospore is the first stage of sporophytic generation. It divides by a transverse division into upper epibasal and lower hypobasal cell.
The epibasal cell forms suspensor. The suspensor pushes the developing embryo into the gamtophytic tissue. The hypobasal cell forms the embryonic éell. The embryonic cell divides by two vertical divisions at right angles to each other into four cells. One of these four cells divides by an oblique wall to form an apical cell of future shoot. The other embryonic cell divide by transverse division to form two tiers of 4 cells each. The embryo gets differentiated into stem, apex, enclosed by two cotyledons on either sidely foot and root.
The young sporophyte soon becomes independent of the gametophyte.
The Young Sporophyte: -
Zygote is the first cell of sporophytic generaton. It divides transversel into an upper epibasal and lower hypobasal cell. The upper cell forms the suspensor while the lower cell forms embryo.
The young embryo soon develops into a foot, a rudimentory root, and a rudimentary stem with two cotyledons the first leaves.
The young embryo is embedded in the tissue of the female gametophyte which is still enclosed within the megaspore wall.