Q.9 With the help of labelled diagrams only, describe the life cycle of Marsilea. (2008, 10, 11)
Related Questions -
Q. With help of diagram explain the sporangium of sporophyll of any fern. (2004)
Q. Draw a lebelled diagram of Marsilea female gametophyte. (2002)
Ans. Systematic Position: -
Division - filicophyta
Class - Leptosporangiopsida
Order - marsileales
Family - Marsileaceae
Genus - Marsilea
Distribution and Habitat: -
Marsilea is found all over the world. Most of the species have been reported from the warmer parts of tropical Africa and Australia. Marsilea minuta is a common Indian species. Most of the species grow in mud and shallow waters. Some are xerophytic such as M. rajasthanensis of Condensata. M. minuta and M.quadrifolia are aquatic. M. aegyptiaca is amphibious. The sporocarps are formed under dry terrestrial conditions when water dries.
Sporophyte: -
The sporophytic plant body of Marsilea, which resembles a “four-leaved clover plant”, shows differentiation into stem, leaves, and roots. Stem is modified under-ground rhizome. The rhizome shows dichotomously branching and capable to idefinite growth.
It has nodes and internodes. The leaves and roots are borne at nodes. The leaves are compound and the lamina is divided into four equal leaflets or Pinnae. Each leaf has a long flexible petiole. The young leaves show circinate verenation. Each leaflet and veins dichotomously branched. The sporocarps or spore producing organs are born at the top of petioles just above the soil. The roots are present at the nodes on the underside of the rhizome.
Reproduction: -
It reproduces by vegeatively and asexually :
Vegetative reproduction: - It takes place by formation of tubers. They are small bud like grow on rhizome under dry conditions. Food is stored in side tubers and help them to resist unfavourable conditions. They produce new marsilea - plants under favourable conditions.
Asexual reproduction: -
It takes - place by spores. Marsilea is hetorosporous and, it has two kinds of spores-microspores and megaspores. The spores are produced together in highly specialized structures called- sporocarps. The sporocarps develop on short lateral branches of petiole. Sporocarp is quite large flat and bean shaped structure. They are soft and green in the begining and hard and brown at maturity. It is a bivalved structure. Each valve hollow bearing a row of elogated Sori. Sori both the valves alternate with each other. Each sorus is enclosed by a delicate saclike structure called the indusium. Microporangia and megasporangia are normally present in each sorus on the receptacle.
Megasporangia: -
Each megasporangium is spherical in shape with a short stalk.The development of sporangium is legtosporangiate type. It develops from a superficial cell present at the tip of receptacle and behaves as sporangial initial. It divides by a transverse division into an outer and inner cell. The outer cells forms the sporangium and it divides by three successive diagonal divisions to form a tetrahedral apical cell with three cutting faces. The apical cell cuts of two segments from its three cutting faces. These segments develop into stalk of the sporangium. The apical cell now divides arched periclinal wall to form outer small primary jacket cell and an inner tetrahedral archesporial cell. The jacket initial divides only anticlinally to form a single layered jacket of the sporangium. The archesporial cell divides periclinally to form primary rapetal cell and central primary sporogenous cell.
The primary repetal cell by periclinal and anticlinal divisions produces 2-3 layered tapetum-which finally disorganises to form nutritive fluid. The primary sporangenous cell divides to form a group of 8-16 spore mother cells. All megaspore mother cell degenerate except, one. The functional megaspore mother cell divides by meiosis to form 4 megaspores and out of 4 megaspores only one survives. It grows large. At maturity the megasorangium has one layered wall and a large megaspore.
Microsporangia: -
The microsporangia develop laterally on the receptacle. These are smaller in size with long stalk. The development is similar to megasporangium except that here all the microspore mother cells are functional and after reduction division form 32 - 64 microspores.
The sporangia enclosed in sori come out of the sporocarp due to its partial decay. The soral and sporangial wall gelatinise and the spores are liberated in surrounding water.
Gametophyte Phase: -
Male and female gametophytes are separate and are produced by the germination of microspore and megaspore respectively.
Male Gametophyte: -
Microspore is yellow in colour, globose with a triradiate ridge. Microspore has two layers of wall outer thick exine and inner thin intine. A single haploid nucleus with strach grains in cytoplasm are present. Microspore absorbs water, enlarges in size and divides to form a small prothallial cell and large apical cell. Prothallial cell may divide again to form another prothalilal cell. The large apical cell divides in plain diagonal to prothellial cell to produce two cells called antheridial initials which divide by curved wall to form a jacket. The wedge-shaped sister cell again divides periclinally into a small inner cell called second jacket cell and a large outer cell. The large outer cell again divide to form third Jacket cell and a primary androgonial cell.
The primary androgonial cell undergoes a series of divisions to produce 16 androcytes or antherozoid mother cells. Each mother cell converts itself into coiled and multiflagellate antherozoid. The spore wall bursts and the antherozoid are liberated in surrounding water.
Structure of antherozoid: -
Marsilea antherozoids have many coils, sometimes a dozen or more. They are cork-screw like in shape.
The upper three or four coils usually consists of blepharoplast and do not bear flagella. The posterior coils are broad and bear 50 - 60 flagella. At the posterior end a globular vesicle containing cytoplasm is found.
Female Gametophyte: -
Megaspore is ellipsoidal in shape with, hemispherical papilla at its anterior end. The wall of megaspore is thick except at papilla. Nucleus is flat and lies in the apical papilla. The cytoplasm is granular arround nucleus. The basal portion of megaspore contains starch, oils albumin.
The nucleus becomes spherical at germination. It divides by a transverse wall to form a small apical cell and a large prothallial cell. The small apical cell lies within papilla while large prothalial cell occupies the basal portion of megaspore. It has lot of food material and does not divide further.
The apical cell alone forms the female gametophyte. It divides by a vertical divisions in different planes to form a central cell and numberous surrounding cells. The surrounding cells divide transversely to form a single layered wall of female gametophyte. The central cell functions as archegonial initial. The initial cell divides by a periclinal wall into outer primary cover cell and an inner cell.
The primary cover cell divides by two anticlinal walls to form four neck initials which by oblique walls give rise to a neck of stiers of four cells each. The inner cell by periclinal division forms an outer primary canal cell and inner primary ventral cell.
The primary canal cell may or may not divide to form a single or 2 neck canal cells, the primary ventral cell divides to form a large egg and small ventral canal cell.
The mature female gametophyte of Marsilea is a minute structure. It is surrounded by a broad gelatinous layer with a funnel shaped jacket extending upward from the archegonium. The mature archegonium has a small neck with a neck canal cell and large venter with large egg cell and small ventral canal cell.
Fertilization: -
Water is essential for the act of fertilization. The neck canal cell and ventral, canal cell get disorganize and a chemical substance is secreted from the neck of archegonium. The antherozoids reach near the archegonium, and one of the antherozoids fuses with egg to form the zygote or oospore which is again sporophytic diploid in nature.
Fig. - Life cycle of Marseliea
Young Sporophyte: -
The oospore or zygote is the first cell of sporophyte. The embryo develops from the oospore or zygote formed as a result of the fusion of gametes, but it may also develop parthenogenetically (apogamy). Division of the oospore takes place two or three hours after fertilization. First of all zygote divides by a vertical division and is followed by another transverse division forming quadrum. The outer two segments or upper segment form leaf and root and the inner or lower two segment from foot and stem. The cells of venters of archegonium from calyptra. This embryo later germinates into adult plant.