Embryology of Angiosperms

M.Sc. Botany Semester -II/ B.Sc. Botany Semester -IV

Structure and Development of Ovule

Compiled by: Dr. Ankit Samuel Singh

Department of Botany Ewing Christian College

Structure and Development of Ovule

Gynoecium is the female reproductive structure of flower, the unit of gynoecium is called pistil or carpel. The carpel is also known as megasporophyll. The carpel is differentiated into three parts: Ovary, Style and Stigma. The basal part of carpel is called ovary. The cavity of ovary contains the ovules; each ovule encloses an embryo sac.

Ovule or Megasporangium  The ovary contains one or more circular or oval structures known as the ovule or megasporangium. It is attached to the placenta by a stalk called, funiculus.

 The mature ovule consists of nucellar tissue enclosed by one or more protective coverings called integuments.

 These integuments surround the whole nucellus, leaving a small opening called the micropyle.  The point of attachment of funiculus to the ovule is called chalaza, which represents the basal part of the ovule.  The nucellus is a mass of cells, enclosed within the integuments. The embryo sac or female gametophyte is present in between nucellus.  Embryo sac is develops inside the ovule, by functional megaspore. In most of the plants, it contains 7 cells (3 antipodal cells, 1 egg cell, 2 synergids and 1central cell).

The ovule is typically consists of following parts: Integument: The Nucellus is enclosed by protective coverings called integuments, except the opening region called micropyle. The ovule with only one integument is called unitegmic and the ovule with two integuments, is called bitegmic. The ovule, which lacks integuments is called, ategmic and it is mainly found in the members of family Olacaceae.

Micropyle: The small narrow opening in the integument of ovule is called micropyle. It is generally formed by the inner integument or by both inner and outer integuments.

Nucellus: The central, bulk part of ovule is called nucellus. Depending on the development of the nucellus, the ovules is of following types- (1)Tenuinucellate- The archesporium or archesporial cell do not divides and directly functions as megaspore mother cell. The sporogenous cell or megaspore mother cell is hypodermal due to single layered nucellar epidermis.

(2)Crassinucellate- The archesporium divides into outer parietal cell and inner sporogenous cell, which latter functions as megaspore mother cell. The sporogenous cell is sub-hypodermal due to the formation of parietal cell, which remains undivided or divide to form parietal tissue.

One another type of ovule, can be found – (3)Pseudo-crassinucellate – The archesporium do not divides and the parietal cell is absent. The sporogenous cell is sub-hypodermal due to the formation of nucellar tissue by the division of nucellar epidermis.

Obturator: Any structure present in ovule, which is associated with directing the growth of pollen tube towards the micropyle, is generally referred to as obturator.

Endothelium or Integumentary tapetum: In unitegmic, tenuinucellate type of ovules, where only single layer of integument is present. The nucellus degenerates at an early stage of ovule development and the inner most layer of integument becomes specific for nutritive function, for embryo sac and is called endothelium. It shows similarities with the tapetum of anther, so it is also referred as integumentary tapetum. It is a nutritive layer which serves as a medium of transportation of food materials from integument to embryo sac.

Types of ovule

1. Orthotropous ovule:

This type of ovule is erect or straight. The funiculus, chalaza and the micropyle lie in a straight line example, Piperaceae, Polygonaceae.

2. Anatropous ovule:

This type of ovule becomes completely inverted and the micropyle and chalaza lie along one line and the funiculus lies parallel to it. This type of ovule is common in Sympetalae and other families.

3. Hemi-anatropous ovule:

This is a rare type of ovule, in which the body of ovule is bending at 900 to the funiculus example, Ranunculus.

4. Campylotropous ovule:

In this type of ovule, the body of ovule is curved. This type is common in the family Fabaceae.

5. Amphitropous ovule:

In this type the curvature of the ovule is very much prominent and the embryo sac also becomes curved or horse-shoe shaped. This type of ovule is found in the families, Alismaceae, and Butomaceae.

6. Circinotropous ovule:

This type of ovule is generally found in Opuntia and Plumbago. The axis and nucellus are in the same line firstly, but due to continuous growth on one side only, the ovule becomes anatropous. The curvature continues further and the long funiculus partially encircles the body of ovule.

Special ovular structures Hypostase: The group of cell, which is differentiated from the cells of nucellus in the chalazal region of ovule below the embryo sac and above the vascular supply, is known as hypostase. These cells contains little amount of cytoplasmic contents and have lignified walls.

Epistase: A group of cell, which is differentiated from nucellar epidermis and present above the embryo sac, just like a cap in micropylar region of ovule, is known as epistase. These cells are radially elongated and have cutinized cell walls.

Different functions of Hypostase were suggested by workers like, Van Tiegham (1901) suggested that hypostase mainly acts as a boundary for the developing embryo sac and prevents its elongation outside chalaza. Venkat Rao (1953) suggested that it is a tissue, which connects vascular tissue to the embryo sac and plays active role in speedy transportation of food materials to the embryo sac.

Aril: The annular outgrowth that is formed from funiculus and surrounds the ovule like an integument is called aril. According to Maheshwari, aril is a new structure that is formed only in few taxa and sometimes represents a third integument. The aril present in Myristica fragrans, which is leathery, red or orangish in colour and used as a spice.

Aril

Aril of Myristica fragrans (Nutmeg) Operculum: A cap like thickened structure in some plants like Lemna around the micropyle is known as operculum. Caruncle: The caruncle is a soft and pad like tissue, which is produced by outer integument in micropylar region, like in Ricinus. It involves in the absorption of water during seed germination as well as helps in dispersal of seeds.

Ricinus (Castor) seed showing caruncle

Coma: The tuft of hairs, present around the seed of the members of family Asclepiadaceae is known as coma. It also helps in the dispersal of seeds.

Megasporogenesis The process of formation of megaspores from megaspore mother cell (MMC) inside the ovule is called megasporogenesis.

Development of ovule (or) microsporangium: Ovule arises as a small outgrowth of homogenous tissue known as primordium on the placenta. Small placental outgrowth in the ovary

The hypodermal cell below the uppermost cell layer of the placenta

(Divides by Periclinal Division and forms) Primordium (It is conical in shape, rounded and represents starting of nucellar growth)

The archesporium or archesporial cell becomes visible in primordial nucellus

The inner integument arises as a rim like structure from surface cells of nucellus situated at nucellar tip by periclinal division

This rim grows towards, the nucellar tip surrounding it completely, leaving a narrow opening, the micropyle

The central part of ovule represents the nucellus

A single hypothermal cell present in the primordial nucellus function as archesporium or primary archesporial cell (It is a prominent cell in comparison to its surrounding cells due to large size dense cytoplasm and large nucleus)

Archesporium

In Crassinucellate Ovules, it divides by Periclinal division and forms In Tenuinucellate & Pseudo- crassinucellate Ovules, it Outer smaller Inner bigger primary directly functions primary parietal sporogenous cell as cell

MMC

Functions as, Megaspore Undivided (or) Mother Megaspore Mother divided to form Cell Cell (MMC) parietal tissue* (MMC) or *(Parietal tissue is responsible for the Meiosis formation of number 4 Haploid Megaspores of wall layers or a (Functional megaspore gives rise to Embryo Sac or Female massive nucellar Gametophyte) cap)

Megaspore haustoria: Megaspores also functions as haustoria, a rare condition was reported in the members of family Crassulaceae, Rosaceae, and Rubiaceae is formation of aggressive megaspore haustoria. In Sedum chrysanthum each megaspore of the tetrad produces a protrusion which grows towards apical region of the nucellus in the form of tube. The upper part of nucellus shows a tangle of megaspore tubes or haustoria.

References:

Bhojwani, S.S., Bhatnagar, S. P. and Dantu, P.K. (2015) The Embryology of Angiosperms 6th Edition, Vikas Publishing House Pvt. Ltd, New Delhi.

Maheshwari, P. (1950) An Introduction to the Embryology of Angiosperms McGraw-Hill, New York.

Johri, B. M. (1984) Edited, Embryology of Angiosperms, Springer-Verlag, Berlin, Heidelberg, New York, Tokyo.

Dwivedi, J. N. (1990) Embryology of Angiosperm, Rastogi Publications, Meerut.

Reddy, S. M. (2004) Edited, University Botany-3, New Age International Publishers, New Delhi.