Megasporangium is the structure of a plant body which contains female reproductive organ . It can be called ovule . It consists of nucellus and integument . The ovule can be formed in different size and shape . Many times it changes its form during the course of its development.
Mature megasporangium can be classified under 5 types .
- Orthotropous or atropous
Orthotropous – The micropyle lies directly in line with the hilum. Example -Polygonaceae, Piperaceae
Anatropous– The ovule becomes completely Inverted so that the micropyle and hilum lie close to each other . Example – all members of Sympetale and several family in monocot and dicot.
Camphylotropous- The ovules are curved . Example – Leguminosae.
Amphitropous- When the curvature of ovule is more pronounced , it affects the embryo sac , it bents like a horse – shoe. Example- Alismaceae, Butomaceae.
Hemianatropous- When the integument and nucellus lie at right angle, example- Ranunculus, Tulbaghia.
A very peculiar type ovule is seen in some members in Plumbaginaceae, here the curvature does not stop rather continue until the ovule turn completely . This type is called Circinotropous. Example- Opuntia.
Integument – Nucellus is surrounded by a cover, except in the micropilar region is called integumen . Generally the ovule has one or two integument . Most genera in monocot have two integument.
There is a evidence that in some genera by a fusion of two separate primordia, a single integument has originated.
- In Opuntia a peculiar condition occur where the extremely long funicles surrounds the ovule n looks like a third integument . The 3rd integument can arise by splitting of outer integument , example – Ulmus . But most other cases it is a new structure arising from the base of the ovule , example – Asphodelus . In Euphorbiaceae, 3rd integument arise by a proliferation of the integumentary cells at the micropylar region , called Caruncle.
- In overy if there is one integument, it is called Unitegmic and if there are two integument , it is called Bitegmic. In Sympetale ( 1 petal is present ) flower one integument is present except – Plumbaginales , Primulales. If there is no integument, it is called Ategmic , example – Lauranthus , Santalus .
- In many plants it can be in different structure at young stages . In mature stage it can be absent or remain in a complicated structure .
- Some integument can possess chlorophyll , example – Morigna olifera , Gladiolus communis .
- Stomata can also found in integument , example – Gossypium, Curvularia .
- Normally forms seed coat in mature stage
- Sometimes, supply nutrition
Sometimes remain as a thin layer
Micropyle- It may be formed either by the inner integument, in Centrospermales or by both inner and outer integument , in Pontederiaceae where there is two integuments are present.
When both integument take part in formation , a passage is formed by the outer integument, as a result the micropylar canal has a zigzag outline, example- Melastomaceae . In Ficus, the integumentary cells come in contact with each other so that the canal of micropyle is extremely narrow and imperceptible.
Nucellus- Depending on the development of the nucellus , the ovules can be two type-
Crassinucellate – A well developed parietal tissue is present and the microspore mother cell is separated from the nucellar epidermis by one or several layer . The nucellus may enlarge by an increasing number of parietal cell or by periclinal division, example- Zizyphus . Several members in Salicaceae, Polygonaceae has a beak- shaped nucellus reaching out the micropyle.
Tenuinucellate- The parietal cell is absent and megaspore mother cell lies directly below the necellar epidermis, example- Rubiaceae, Orchidaceae.
When the embroyo sac matures, the cell of nucellus becomes used up.
There is another type of ovule can be found –
Pseudo – crassinucellate – No partial cell is present . The epidermis is divided and form many layer of cell. Example – Nigella damascena
Integument tapetum- Where the nucellus is disorganized, the embryo sac comes in contact with the inner layer of seed coat, they show a pronounced radial elongation and sometimes binucleate . Having the similarities with the anther tapetum, this layer is known as integument tapetum or endothelium . It is a nutritive layer whose main function is to serve as an medium of transportation of food materials from integument to embryo sac . In later stages, at the time of embryo’s maturity, the inner surface of endothelium become cutinized and becomes a protective layer.
- Gives nutrition to embryo sac
- Works as a medium
- Gives protection
Hypostase- At the level of origin of two integuments and directly below the embryo sac , a well- defined but irregularly outlinend group of nucellar cells with poorly cytoplasmic contents but have partially lignified walls composed of a highly refractive material.
Epistase-A similar , well marked tissue in micropylar part of ovule. Usually originates from the apical cell of nucellar epidermis.
Archesporium- It has a hypodermal origin. Cells which lay directlly below the epidermis become more conspicous , large in size and have prominent nucleus, they are called archesporial cell . The archesporial cell may divide to form primary parietal and sporogenous cell or may function as megaspore mother cell .
So these are the parts of megasporangium or ovule . Later , the ovule undergoes meiosis cell divison to produce spores which are haploid through megasporogenesis.
The megaspore mother cell is dipolid ( 2n ) and forms haploid megaspores . The 1st divison is transverse and two cells are formed . Both the cells again transversely divides and four cells are formed which are haploid . This spores are arranged in a row which is axial and called linear tetrad . Only of megaspore out of all of them remain at chalazal end and works as functional megaspore and other cells are gradually degenerate .
Normally it is the functional chalazal megaspore but in Elytranthe , Balanophora the micropylar megaspore give rise to embryo sac and others are degenerate . A similar condition occour in Onagraceae family . In Rosa sometimes 2nd micropylar megaspore functions . There can be so many various in it .