Sexual Reproduction in Flowering Plants 

Flowers hold significant aesthetic, ornamental, and cultural value and are key organs of angiosperms, playing a crucial role in sexual reproduction. They are seen by biologists as morphological and embryological marvels, with flowers being modified shoots.

1.0Flower Structure

A typical flower has four kinds of whorl and calyx—Corolla, androecium and gynoecium. Generally, flowers have a stalk called a pedicel; the upper swollen part of the pedicel is called the thalamus. Calyx (whorl of sepals) and corolla (petals) are accessory organs, while androecium and gynoecium are reproductive organs or essential whorls.

Reproductive Structure of A Flower

Androecium:The androecium is the male reproductive whorl of a flower, consisting of units called stamens. Each stamen typically has a filament (a stalk) that holds the anther, which is where pollen is produced.

Filament: The filament is a thin stalk that supports the anther; its primary function is to provide the necessary support to the anther and to ensure that it is positioned to maximise the chances of pollination. 

Anther: A typical angiosperm anther is bi-lobed with each lobe having two thecae, i.e., dithecous. The anther consists of four microsporangia at the corners, two in each lobe. The microsporangia develop further and become pollen sacs. 

Structure of Anther:

In the transverse section of the anther, it is seen as almost tetragonal (4 sided); the microsporangium is enclosed by the anther wall, which consists of several layers from the outside which are as follows: 

Epidermis: This is the outermost layer of the anther wall. It serves as a protective layer.

Endothecium: The endothecium contributes to the mechanical support of the anther.  Endothecium is fibrous (𝛂-cellulose fibres) and hygroscopic in nature help in pollen release. 

Middle Layers: Middle layers consist of parenchymatous cells. This layer is one to three-cell thick structures. Food is stored by parenchymatous cells in this layer. 

Tapetum: The innermost layer, crucial for supplying nutrients and substances (sporopollenin) for pollen wall formation. Tapetum are initially diploid but become polyploid and multinucleate due to endomitosis and nuclear division, respectively.

2.0Development of Anther and Microsporogenesis

In the initial stage, the anther is a spherical or oval structure composed of meristematic cells, enclosed by a single-layered epidermis. First, vascular tissues form in the centre, while cells below the epidermis at the four corners, called archesporial cells, enlarge. These cells split into primary parietal and primary sporogenous cells. Primary parietal cells further divide to form the anther's layers: endothecium, middle layers, and tapetum. Meanwhile, primary sporogenous cells multiply to become sporogenous tissue,  eventually differentiating into microspore mother cells. 

Microsporogenesis

Each microspore mother cell or cell of sporogenous tissue divides to form four haploid microspore or pollen grains by meiotic division or reduction division. The process of formation of microspores from pollen mother cells through meiosis is called microsporogenesis. 

At the initial stage all four microspores are attached together with the help of a callose layer. The microspore is arranged in a cluster of four cells - the microspore tetrad. After some time, this callose layer is dissolved by the callase enzyme, which is secreted by the tapetum. 

Microgametogenesis (Development of Male Gametophyte) 

In flowering plants, a microspore or pollen grain is considered the first cell of the male gametophyte.  Partial germination or development of pollen grain starts before dehiscence of anther (before pollination). Development of pollen also takes place at the mother's place [means inside pollensac of anther] it is called In-situ development. Pollen grains are generally spherical, measuring about 25-50 micrometers in diameter. It has a prominent two-layered wall. The outer wall layer is thick, rigid, and ornamented, and it is called exine. This layer is formed mainly by sporopollenin. The inner wall of the pollen grain is thin, continuous, soft, and elastic. It is called intine. It is made up of pectin and cellulose or pectin-cellulose.

3.0Pre-Pollination Development

In the initial stages, the pollen grain's nucleus divides unequally through mitosis, producing two distinct nuclei: the smaller generative nucleus, located near the wall, and the larger, irregularly shaped tube or vegetative nucleus, inside the cytoplasm. Following this, the cytoplasm densifies, and unequal cytokinesis occurs, resulting in the formation of two cells of differing sizes: the larger vegetative cell, containing the tube nucleus, and the smaller generative cell, containing the generative nucleus. As the pollen matures, it holds these two cells, known as a partially developed male gametophyte or mature pollen grain. The generative cell then transforms into a spindle-shaped structure, detaches from the wall, and floats within the vegetative cell's cytoplasm. Most angiosperms (over 60%) release pollen grains at this two-celled stage. In others, the generative cell divides once more to form two male gametes before pollen release, resulting in a three-celled stage.

4.0Gynoecium

The gynoecium, the female reproductive part of a flower, consists of units called pistils or carpels. A gynoecium can have a single pistil (monocarpellary) or multiple pistils (multicarpellary), with each carpel acting as a megasporophyll. A carpel is divided into three parts: the stigma, the style, and the ovary. The stigma, at the carpel's top, catches pollen grains. The style is a slender tube connecting the stigma and ovary, which is the bulged base of the carpel containing the ovarian cavity or locule. Inside the ovary, ovules or integumented megasporangium are found, attached to a tissue called the placenta. 

5.0Structure of Ovule

The ovule, referred to as an integumented megasporangium, is connected to the placenta via a slender stalk known as the funiculus. At the hilum, the funiculus and the ovule's body merge, marking the attachment point. The ovule's core consists of the nucellus, a tissue rich in reserve food materials, which houses the embryo sac, or female gametophyte. Surrounding the nucellus are one or two protective layers called integuments, leaving a small opening at the tip called the micropyle. The base opposite the micropyle is the chalaza, forming the ovule's basal region.

6.0Development of Ovule and Megasporogenesis 

In ovule development, the process begins with the nucellus forming from the placenta as a small, rounded structure. Initially, all nucellus cells are undifferentiated, homogeneous, and meristematic, eventually becoming parenchymatous and enclosed by a single epidermal layer.

A hypodermal cell in the nucellus then differentiates, enlarges, and becomes the archesporial cell, identifiable by its distinct nucleus. This cell divides mitotically, creating a primary parietal cell and a primary sporogenous cell. The primary sporogenous cell serves as the megaspore mother cell (MMC) at the micropylar end of the nucellus. This MMC, a large cell with dense cytoplasm and a prominent nucleus, undergoes meiotic division to produce four haploid megaspores, a process known as megasporogenesis.

Typically, these megaspores arrange into a linear tetrad. In most flowering plants, the chalazal megaspore (the one farthest from the micropyle) remains functional, while the other three degenerate. This functional megaspore then develops into the female gametophyte or embryo sac.

7.0Megagametogenesis (Development of Female Gametophyte)

The megaspore, initiating the female gametophyte development, enlarges and nourishes from the nucellus. Its nucleus undergoes mitotic divisions, first splitting into two, which move to opposite ends, forming a 2-nucleate embryo sac. Further divisions lead to 4-nucleated and eventually 8-nucleate stages, without immediate cell wall formation, a process called free nuclear division.

At the 8-nucleate stage, one nucleus from each end moves to the centre, becoming polar nuclei. Subsequently, cell walls form around six of the eight nuclei, creating the structured female gametophyte or embryo sac.

Three cells form at the micropylar end, with the largest being the egg cell and the other two as synergids, together referred to as the egg apparatus. The opposite end forms three antipodal cells. The central cell houses the two polar nuclei, which fuse before fertilisation to form a diploid secondary nucleus.

8.0Pollination 

"Pollination is defined as the process of transfer of pollen grains from anther to the stigma." In flowering plants both types of gametes (male gamete and female gamete) are non motile and brought together for fertilisation by pollination.

Types of Pollination 

Self Pollination: When the pollen grains are transferred from anther to the stigma of the same flower or another flower of the same plant, then it is called self pollination.

Cross pollination: When the pollen grains are transferred from anther to the stigma of different flower of different plants but of the same species, then it is called cross pollination. 

9.0Fertilisation in Flowering Plants

Pollination leads to pollen grains germinating on the stigma, where they absorb moisture and nutrients, swelling up. The intine extends through a germ pore to form a pollen tube. As this tube extends into the style, the vegetative nucleus (tube nucleus) leads, controlling growth, followed by a generative cell that splits into two male gametes. The pollen tube, attracted by chemicals secreted by synergid cells, enters the ovule specifically through an egg apparatus cell, moving (chemotropic movement) towards the egg apparatus. One male gamete fertilises the egg, forming a diploid zygote (syngamy), while the other fuses with the secondary nucleus, resulting in a triploid primary endosperm nucleus (triple fusion or vegetative fertilisation). This occurrence of both syngamy and triple fusion within the embryo sac is known as double fertilisation, producing a diploid zygote and a triploid endosperm nucleus.

10.0Post Fertilisation Event 

Development of Endosperm

Endosperm development typically precedes embryo development in flowering plants. After fertilisation, the primary endosperm cell (PEC) divides multiple times to form triploid endosperm tissue. This tissue, rich in reserve food materials, provides crucial nutrition to the developing embryo. 

11.0Discover more: Endosperm 

Development of Embryo

The embryo develops from the zygote at the micropylar end of the embryo sac, with zygote division and embryo formation typically occurring after the initial development of endosperm. This sequence ensures the embryo has enough nutrients for early growth. Embryo development, or embryogeny, follows a similar pattern in both monocots and dicots, progressing from the proembryo stage to globular, heart-shaped, and then mature embryo forms. 

Dicotyledonous Embryo Structure

• A typical dicotyledonous embryo, consists of an embryonal axis and two cotyledons. The portion of the embryonic axis above the level of cotyledons is the epicotyl, which terminates with the plumule or stem tip. The cylindrical portion below the level of cotyledons is hypocotyl that terminates at its lower end in the radicle or root tip. The root tip is covered with a root cap.

Monocotyledonous Embryo Structure

• Features a single cotyledon, known as the scutellum in grasses, attached laterally to the embryonal axis.
• The radicle and root cap are at the lower end, enclosed by the coleorrhiza.
• Above the scutellum, the embryonal axis forms the epicotyl, with a shoot apex and leaf primordia covered by the coleoptile.

Seed:

• Seeds are the final production of sexual reproduction and often described as a fertilised ovule located inside the fruits(ripened ovary). 
• A seed typically consists of seed coat(s), cotyledon(s) and an embryo axis.
• The cotyledons of the embryo are simple structures, generally thick and swollen due to storage of food reserves (as in legumes). 

Fruit: A fruit results from the fertilizing and maturing of one or more flowers.A fruit is the seed bearing structure in flowering plants (angiosperms) that is formed from the ovary. The outer layer, often edible, of most fruits is called the pericarp. The pericarp may be described in three layers from outer to inner, i.e., the epicarp, mesocarp and endocarp.