Q.1. Describe the life cycle of fern (Pteridium) with labelled diagrams.
Related Questions-
Q. Describe the structure of fern sporangium with the help of diagram. (2011)
Q. With the help of diagram, explain the sporangium of sporophyll of any fern. (2008)
Ans. Systematic position: -
Division - Filicophyta or pterophyta
Class - Leptosporangiopsida
Order - Filicales
Family - Polypodiaceae
Genus - Pteridium
Occurrence: - Plants grow luxuriantly on moist and shady places both in the hills and in the plains. They are widely distributed occurring in both temperate and tropical regions but are most abundant in the humid parts of the tropics, especially at higher altitudes. In India, the plants of Pteridium are found growing at altitudes between 1000 to 3000 meters. The plant body of Pteridium is sporophytic. Its life cycle shows very clear alternation of generation.
Spore Producing Organs: -
The Pteridium plants, a member of family Polypodiaceae. The spores are formed in sporangia born in sori on the sporophyll. The sori may be marginal or superficial. They may be covered by an inducium. The inducia are of two types true and false. A true inducium is a specially developed structure whereas a false inducium is formed by covering of sporophyll margin.
Sporangium: - The development of sporangium is of leptosporangiate type. Each sporangium develops from a single marginal cell of placenta or superficial intial. In eusproangiate types the sporangium arises from a group of initials.
A sporangium is consists of a stalk and a capsule. The stalk is multicellular and biseriate. The capsule is oval or elliptical and like biconvex in shape. Wall followed by double layered tapetum that encloses the archesporium. The archesporial cells divide and redivide to form a mass of sporogenous tissue. Most of the sporogenous behave as spore mother cells. They undergo meiosis to form tetrahedral tetrads of spores. As a result 32-62 spores are formed in each capsule. The tapetal layer is nutritive. It degenerates at maturity of the sporangium.
As the capsule matures, about four lower median cells of the jacket stretch tangentially. Of these two median ones opens. This is called as stomium. The remaining cells of the, same median row of the jacket covering about three fourths of the perimeter become specialised. They develop a thickening along their radial and inner tangential walls. This layer is called as annulus.
At the maturity the inducium dries exposing the sorus. The cells of the annulus loose water.
Due to presence of thickening along the radial and inner tangential walls. Their upper walls shrink and the inner ones straight and the annulus coils, thus, it exerts pressure on the wall resulting in breaking of the capsule between the cells of stomium throwing out the spores.
Gametophytic Phase: -
Spores: - It is the first cell or unit of gametophytic generation. The spores of Pteridium are about 0.30 mm in diameter and all alike i.e. homosporous. Each spores has usually two wall layers ; a thin endospore (intine) and a hard, brown and irregularly thickened exopore (exine). The spore germinates to form a filamentous gametophyte which develops into a heart shaped prothallus.
Prothallus: - The mature prothallus is green and heart-shaped structure. It shows polarity and dorsiventrality. The dorsal surface is smooth and the ventral is provided with rhizoids and sex organs. The apical notch identifies the anterior end. The prothallus is 4-6 cells thick in region of median axis but only one cell thick in the region of wings. The gametophyte is made up of thin walled chlorenchymatous cells.
Sex Organs - The prothalli of Pteridium are monoecious. The sex organs are sessile. They remain partly embeded in the region of sagittal axis. While the archegonia lie near the apical notch, the antheridia lie away from them.
Antheridium: - It consists of a three celled jacket enclosing a mass of androgonial cells. The two lower jacket cells are ring like and the terminal cell is called as opercular cell or cover cell or cap cell. Sometimes their may be two cap cells and in that case the jacket is four celled.
The last generation of androgonial forms the Androcytes, each of which is metamorphose into a spirally coiled, multiflagellate Antherozoid.
Fig : Development of antherozoids in pteridium
Archegonium: - It consists of a neck like structure which is bent towards the posterior side.
It is 5-7 cells high and made up of four vertical rows of cell, capped by four cover cells. It contains a binucleate neck canal cell. There is no venter. The egg and single ventral canal cell remain surrounded by the cells of prothallus.
Fig. : Development of archegonium and entry of antherozoids in pteridium.
Fertilization: - Before fertilization the walls of androgonial cells disorganize to form a mucilagenous mass. Water medium is essential for fertilization. When the archegonium is mature, the neck canal cell and the venter canal cell disorganise and a passage is formed for the entry of the antherozoids. The antherozoids reach near the neck of the archegonium wriggling through the water film. Malic acid is secreted at the mouth of archegonial neck, attracts antherozoids. The fusion of one antherozoid nucleus takes place with egg of a archegonium resulting in the formation of oospore of zygote which is diploid in nature.
Sporophyte: - The zygote is the first cell of sporophyte. After fertilization the zygote enlarges in size inside venter of the archegonium. The zygote them divides vertically into epibasal and hypobasal cell. One more division takes place at right angle to the first quadrant is formed. The four cells further divides into eight and redivides to form a multicellular embryo. The half epibasal cells form the ‘stem’ and ‘leaf’ while the half hypobasal cells form the ‘root’. More and more growth takes place in the embryo resulting in the formation of young sporophytic plant. In the young condition for a very short period the sporophytic plant remain attached with prothallus and is dependent, but soon after the prothallus (gametophyte) get disorganise and sporophyte become independent.
Thus the life history of Ferm shows distinct alternation of sporophyte and gametophyte.
Q.2. Describe circinat vernation. (2010, 13)
Ans. Circinate Vernation: -
Circinate vernation is found in young leaves of ferns and Cycas. In both the plants young leaves are found in coiled condition. This condition is known as circinate vernation. In Cycas, like the ferns the rachis is incurled and the leaflets are inrolled in the bud stage. Circinate vernation is the distinguishing feature in ferns and Cycas.
Q.3. Explain the Mechanism of spore discharge in Pteridium. (2002, 08)
Related Questions -
Q. Describe the dehiscence of Fern sporangium. (2010)
Ans. Dehiscence of fern Sporangium: -
On the maturity of sorous the inducium dries and shrivels, thus, exposing the sporangia to dry air. The wall of sporangium begins to lose moisture and get dryness, alongwith other cells of annulus also lose water. The drying of the annulus brings about the unequal tension of its cell walls on account of the fact that in structure of the cells of the annulus, their inner tangential walls and radial wall being thick do not bend or collapse, so easily when the pressure is brought to bear on the outside of the cells, but the thin outer walls bend inwards and become concave. This drags on the ends of each of the radial walls drawing them together and thus shorten the outer circumference of the annulus. On account of the annulus tend to straighten and in doing so the sporangium open, and the spores get liberated free.
