The Megasporangium: Types, Parts and Functions

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
• Anatropous
• Camphylotropous
• Amphitropous
• Hemianatropous

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  . 

Function 

•  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.

Function-

• 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.

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 .

Functioning Megaspore 

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 .