Category : 12th Class
The process of formation of megaspore from megaspore mother cell by meiotic division is known as megasporogenesis. This process takes place in ovule.
Megasporogenesis can be studied under following heads :
(1) Structure of ovule (Megasporangium) : Ovule is considered to be an integumented megasporangium. The ovule consists of the stalk and the body. The stalk is called funicle. One end of the funicle is attached to placenta and the other end to the body of the ovule. The point of attachment of funicle with the body is called hilum. Sometimes funicle gets fused with the body of the ovule one side and forms a ridge known as raphe. The body of the ovule shows two ends: the basal end, often called the chalazal end and the upper end is called micropylar end. The main body of the ovule is covered with one or two envelopes called integuments. These leave an opening at the top of the ovule called micropyle. The integuments enclose a large parenchymatous tissue known as nucellus.
The residual part of nucellus in the mature seed is called perisperm. In the centre of the nucellus is situated a female gametophyte known as embryo sac.
Following are the conditions seen in ovule in relation to integuments :
(i) Unitegmic : Ovule with a single integument, e.g., sympetalous or gamopetalous dicotyledons.
(ii) Bitegmic : Ovule with two integuments as in polypetalous (Archichlamydeae) dicotyledons and monocotyledons.
(iii) Aril : This is a collar-like outgrowth from the base of the ovule and forms third integument. Aril is found in litchi, nutmeg, etc.
(iv) Caruncle : It is formed as an outgrowth of the outer integument in the micropylar region. Caruncle is common in the ovules of Euphorbiaceae. e.g., Castor (Ricinus).
(v) Ategmic : In some parasites like Loranthus, Viscum, Santalum etc., there is no integument. Such an ovule is called ategmic.
(2) Kinds of ovules : Depending upon the shape and orientation, the ovules of angiosperms are classified into following types :
(i) Orthotropous or Atropus : The micropyle, chalaza and funicle are in straight line. This is most primitive type of ovules. e.g., Betel, Piper, Polygonum.
(ii) Anatropous : The body of the ovule is completely inverted (turn at 180o angle ) so that micropyle and hilum come to lie very close to each other. e.g., 82% of angiosperm families.
(iii) Hemianatropous : Ovule turns at 90o angle upon the funicle or body of ovule is at right angle to the funicle e.g., Ranunculus.
(iv) Campylotropous : Ovule is circled more or less at right angle to funicle. Micropylar end is bent down slightly. e.g., in members of Leguminosae and Cruciferae.
(v) Amphitropous : Curvature of ovule is more and embryo sac becomes curved like horse shoe e.g. Lemna, Poppy, Alisma.
(vi) Circinotropous : The ovule is initially orthotropous but becomes anatropous due to unilateral growth of funicle. The growth continues till the ovule once again becomes orthotropous. As a result funicle completely surrounds the body of the ovule e.g., Opuntia (prickly pear).
(3) Formation of megaspore : The ovule or the megasporangium develops as a small protuberance of the placental tissue. In the very young ovule a single hypodermal cell is differentiated as archesporium cell. The archesporial cell may directly function as megaspore mother cell (tenuinucellate ovule) or may divide periclinally to form an outer parietal cell and an inner sporogenous cell (crassinucellate ovule). The sporogenous cell directly behaves as megaspore mother cell (or megasporocyte). The diploid megaspore mother cell enlarges in size and divides by meiosis to form a linear tetrad of four haploid megaspores. Occasionally T-shaped or inverted T-shaped \[(\bot )\] tetrads are also formed. Megaspore is the first cell of female gametophyte.
Of the linear tetrad, three megaspores towards the micropyle degenerate. The lowermost, i.e., the chalazal megaspore enlarges and remains functional. It later produces an embryo sac.
(4) Development of female gametophyte (Megagametogenesis) : The process of development of female gametophyte or embryo sac from megaspore is called megagametogenesis.
(i) Monosporic type (Polygonum) : In this type, only one megaspore situated towards chalazal end takes part in the development of embryo sac. The functional haploid megaspore enlarges in size and by means of three successive mitotic divisions, gives rise to an 8-nucleate embryo sac. Of these, four nuclei occur at micropylar end and the other four at the chalazal end. Three nuclei at the micropylar end form egg apparatus and the fourth migrates from the both pole to the centre and form polar nucleus.
A fully developed typical or polygonum type of embryo sac is large and oval structure consisting of seven cells and eight nuclei.
(a) Egg apparatus : This is a group of 3 cells situated at the micropylar end. The centrally located cell is called egg cell. On its sides are present two synergids. Egg cell has a large vacuole at its upper end and a prominent nucleus near its lower end. Synergids show a filiform apparatus attached to their upper wall. It is known to attract and guide the pollen tube. Each of the synergids has a vacuole at its lower end and the nucleus at its upper end.
(b) Polar nuclei : These are situated in the centre of the embryo sac representing a large binucleate central cell. Generally, both the polar nuclei fuse before fertilization and form a single diploid nucleus called secondary nucleus or definitive nucleus.
(c) Antipodals : The three cells situated at the chalazal end are called antipodals. These cells generally degenerate soon after fertilization.
(ii) Bisporic type : In this type two megaspore nuclei take part in embryo sac formation.
(iii) Tetrasporic type : This type of embryo sac develops from four megaspore nuclei.
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