Class 12 CBSE Chapter 2 Sexual Reproduction in flowering plants (PART-2) -

FEMALE REPRODUCTIVE WHORL-GYNOECIUM

The female reproductive organ of a flower is called the gynoecium, which is also referred to as the pistil or carpel. The gynoecium may consist of a single pistil, known as monocarpellary, or multiple pistils, known as muticarpellary. The carpel or pistil is equivalent to the megasporophyll and is differentiated into three distinct regions: stigma, style, and ovary.

The stigma is the free end of the carpel that serves as the landing platform for pollen grains. The style is a long, narrow tubular structure present between the stigma and ovary. The ovary is the basal swollen part of the carpel that contains the ovarian cavity or locule. The ovules, also known as integumented megasporangia, are borne on a cushion-like tissue called the placenta in the ovarian cavity. The placenta is located inside the ovarian cavity, and arising from it are the megasporangia or ovules.

The number of ovules in an ovary may be one, as in wheat, paddy, and mango, or more than one, as in papaya, watermelon, and orchids. Gynoecium can be apocarpous, which means free carpels or free pistils, as in rose, lotus, and Michelia, or syncarpous, which means fused carpels or fused pistils, as in papaver and hibiscus.

Digram

STRUCTURE OF OVULE OR MEGASPORANGIUM

Ovule is also known as integumented megasporangium.
Each ovule is attached to the placenta by means of a thin stalk called funicle (or funiculus/funiculum).
The body of the ovule fuses with funicle in the region called hilum. Thus, hilum represents the junction between ovule and funicle.
The main region of the ovule is composed of mass of parenchymatous cells (with abundant reserve food materials) which is called nucellus. Nucellus is the main part of ovule. Located in the nucellus is the embryo sac or female gametophyte.
The nucellus is covered by one or two coats or protective envelopes which are called integuments.
Integuments encircle the ovule except at the tip where a small opening called the micropyle is organized.
In ovule of most of the plants, funicle is attached to the main body of ovule for some distance (at lateral side) to form a ridge-like structure known as Raphe.
Vascular tissues are present inside the funiculus which supply food material from the placenta to the body of ovule.
Opposite the micropylar end, is the Chalaza, representing the basal part of the ovule.
Occasionally in some seeds such as black pepper, beet, and castor remnants of nucellus are also persistent. This residual, persistent nucellus is the perisperm.
Some filaments are attached with funicle [sometimes with placenta] are known as “Obturators”.
The function of obturators is to guide the passage of pollen tube towards the micropyle inside the ovary.

OVULE INTEGUMENTS AND TYPES OF OVULES

Integuments are protective layers that encircle the ovule except for a small opening called the micropyle. In some plants, special types of integuments are formed. The ovules can also be classified based on the number of integuments they have:

SPECIAL INTEGUMENTS:

ARIL: This is a type of third integument that develops from the funicle at the base of the ovule. Examples include Myristica, Asphodelus, and Litchi.
CARUNCLE OR STROPHIOLE: This structure is formed due to the proliferation (outgrowth) of outer integuments over the micropyle. Examples of plants with caruncles are Ricinus communis (castor). It is composed of sugary contents and helps in the absorption of water during seed germination. Dispersal of seeds occurs by ants, which is called myrmecochory.

TYPES OF OVULES ON THE BASIS OF INTEGUMENTS:

UNITEGMIC OVULE: A single integumented ovule is called a unitegmic ovule. Examples include members of Gamopetalae and most gymnosperms.
BITEGMIC OVULE: A bitegmic ovule has two integuments. Examples include most angiosperms such as polypetalae, Capsella, and monocots.
ATEGMIC OVULE: The ovule in which integuments are absent is called an ategmic ovule. Examples include Olax, Liriosma, Loranthus, and Santalum.

Diagram

TYPES OF OVULES ON THE BASIS OF POSITION OF DIFFERENT PARTS –

ATROPOUS OR ORTHOTROPOUS OVULE: The body of the ovule is upright in position. The micropyle, chalaza, and hilum lie in one straight line, making this type of ovule straight or upright. This is the most primitive and simplest type of ovule found in both angiosperms (Betel, Piper, Polygonum) and gymnosperms. It lacks a raphe.
HEMITROPOUS OR HEMIANATROPOUS OVULE: In this type of ovule, the body of the ovule bends on the funicle at a 90-degree angle, making the ovule intermediate between ortho and anatropous ovules. The body of the ovule is horizontal on the funiculus, making it also known as a horizontal ovule. The micropyle and chalaza are in the same line, but the micropyle is situated away from the hilum. Examples include Ranunculus and Primula.
ANATROPOUS OVULE: In this type of ovule, the body of the ovule is completely turned at a 180-degree angle due to the unilateral growth of the funiculus, making it an inverted ovule. The chalaza and micropyle lie in a straight line, and the hilum and micropyle are side by side and very close to each other. This is the most common type of ovule in angiosperms and is considered the “typical ovule” of angiosperms. It is also called a resupinate ovule. Examples include members of Malvaceae, Cucurbitaceae, Solanaceae, Compositae, and Pea.
AMPHITROPOUS OVULE: In the amphitropous ovule, the curvature affects the nucellus, causing the embryo sac to become horseshoe-shaped. The micropyle is located near the hilum. Examples include Mirabilis, Lemna, and poppy (Papaver)
CAMPYLOTROPOUS OVULE: The body of the ovule is curved in the campylotropous ovule, and the micropyle and chalaza are not in a straight line. The nucellus is also curved, but the embryo sac remains straight. The micropyle is close to the hilum, and this type of ovule is also known as the curved ovule. Examples include plants from the Leguminosae and Cruciferae families such as Capsella.
CIRCINOTROPOUS OVULE: The body of the ovule becomes inverted and then returns to a straight position due to the growth of the funiculus in the circinotropous ovule. The entire ovule is surrounded by the funiculus, and it is also called the coiled ovule. The micropyle is situated away from the hilum. Examples include Opuntia from the Cactaceae family.

DIAGRAM

DEVELOPMENT OF OVULE

During the development of the ovule, nucellus initially develops from the placenta in the form of a small rounded growth-like structure. At this stage, all the cells of the nucellus are undifferentiated, homogenous, and meristematic, and finally, they become parenchymatous. This mass of cells is surrounded by a single layer of epidermis.

One hypodermal cell of the nucellus differentiates and increases in size. It becomes different from the rest of the cells due to the presence of a distinct nucleus and is called the archesporial cell.

The archesporium (archesporial cell) divides mitotically to form a primary parietal cell and a primary sporogenous cell.

The primary sporogenous cell directly acts as a megaspore mother cell (MMC) at the micropylar region, which is a single MMC that is differentiated in the micropylar region of the nucellus during ovule development. The MMC is a large cell with dense cytoplasm and a prominent nucleus. The MMC divides meiotically to form four haploid megaspores.

Megasporogenesis

Megasporogenesis is the process of formation of megaspores from megaspore mother cell (MMC). l The four haploid megaspores are generally arranged in a linear tetrad.

In the majority of flowering plants, the lowermost or chalazal megaspore remains functional out of the four megaspores while the other three, which lie towards the micropyle, degenerate.

The functional megaspore produces the female gametophyte (embryo sac).

In most angiosperms, such as Capsella, the chalazal megaspore remains functional.

This method of embryo sac formation from a single megaspore is termed monosporic development.

The ploidy of the cells involved in this process are: Nucellus (2n), MMC (2n), Functional megaspore (n), Female gametophyte (n).

DEVELOPMENT OF EMBRYOSAC OR FEMALE GAMETOPHYTE MEGAGAMETOGENESIS:

The megaspore, which is the first cell of the female gametophyte, grows in size and obtains nutrition from the nucellus. l The nucleus of the functional megaspore divides mitotically to form two nuclei. Each nucleus moves towards the opposite pole forming the 2-nucleate embryo sac. l Sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac. l It is important to note that these mitotic divisions are strictly free nuclear, meaning that nuclear divisions are not immediately followed by cell wall formation. l Out of the four nuclei, one nucleus from each pole migrates towards the center, one from the chalazal side and one from the micropylar side. These nuclei are known as polar nuclei and are present in the center. After the 8-nucleate stage, cell walls are laid down, leading to the organization of the typical female gametophyte or embryo sac. l Six of the eight nuclei are surrounded by new cell walls and organized into cells. l Towards the micropylar end, three cells are formed, out of which one cell is larger and more distinct. This is called the egg cell, and the remaining two smaller cells are known as synergids. These three cells are collectively known as the egg apparatus. [1 Egg cell + 2 Synergids] l Towards the chalazal end, three cells are formed, which are called antipodals or antipodal cells.

The large central cell of the female gametophyte contains both polar nuclei below the egg apparatus. Just before fertilization, the polar nuclei fuse to form a diploid secondary nucleus or definitive nucleus. After three rounds of mitosis in the megaspore, a seven-celled and eight-nucleated structure is formed, which is called the female gametophyte or embryosac of angiosperms. This type of embryosac is referred to as the “polygonum type” because it was first discovered by Strasburger in the Polygonum plant. The polygonum type of embryosac is the most common type in angiosperms like Capsella. It develops from a single megaspore and is thus also known as a monosporic embryosac. The synergids located at the micropylar tip have finger-like structures called filiform apparatus. These structures help the synergids to absorb nutrients from the nucellus and transfer them to the embryosac. Additionally, the filiform apparatus secretes chemicals that attract and guide the pollen tube towards the synergid during fertilization.