Let us make in-depth study of the reproduction in flowering plants (with diagram).
Parts of a Flower
The majority of flowers have both male and female reproductive organs, while others only have male or female sex organs. These flowers are referred to as unisexual flowers (e.g., watermelon, cucumber, etc.) Bisexual flowers are described as having both sex organs (e.g., Hibiscus, pea, etc.) Generally, a flower has a long or short axis.
This axis has two parts: the flower stalk, known as the pedicel, and the swollen top, known as the thalamus (Figure 1). The flower’s parts are arranged on the thalamus. A typical flower has four whorls of floral parts: calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (ovary) (carpels). Calyx and corolla do not participate directly in reproduction.
Figure 1: Diagram of Flower
Androecium and gynecium relate directly to sexual reproduction. The androecium consists of stamens and is the male part of the flower. The gynoecium (or pistil) is the female reproductive organ and is composed of carpels. On the thalamus of a flower, the whorls are arranged in a specific sequence.
The Calyx: The outermost whorl is the calyx. It is made up of sepals. Most of the time, the sepals are green, but sometimes they are other colours. In the bud stage, the calyx protects the flower whorls.
The Corolla: The corolla, the following inner whorl, is made up of petals. Flowers’ petals can be white or vividly coloured. They aid in pollination by luring insects to the flower. In the bud stage, the corolla shields the reproductive whorls.
Figure 2: Germination of pollen grains on stigma
Androecium: Androecium, which refers to the group of stamens as a whole, is the third whorl (Figure 2). A filament and an anther make up each stamen. Two chambers on each anther are referred to as pollen sacs. Your hands might become covered in a yellowish powder if you touch a flower’s stamens. These numerous grains of pollen, which are yellowish, are produced by anthers and contain male gametes.
Gynoecium: The centre of the flower contains the gynoecium (or pistil). The fourth whorl is it. It contains the carpel, the female reproductive organ. Each carpel is made up of three parts: an ovary at the bottom, a style in the middle, and a stigma at the top, which is a flattened disc-like sticky structure with one to many lobes.
Pollination is the process of moving pollen grains from a flower’s anthers to its stigma or that of another flower (Figure 3). Self-pollination is the transfer of pollen from the stigma of a single flower or flowers produced by the same plant (as in pea and Hibiscus). Cross-pollination is the movement of pollen from the anthers of one species of flower to its stigma on another plant. The majority of flowering plants exhibit it commonly.
Figure 3: Pollination Process
Pollen can be transported by numerous agents, including insects, birds, wind, and water. Flowers and pollen grains are altered in order to facilitate cross-pollination. For instance, insect-pollinated flowers are brightly coloured to attract pollinators. Wind-pollinated flowers generate pollen grains that are light enough to be carried by the wind.
After pollination, the pollen grains germinate on the stigma. The inner wall of the pollen grain transforms into a pollen tube, which then descends the style and reaches the ovule. A zygote is formed when a male gamete fuses with a female gamete inside the ovule. This process is called fertilisation.
In the ovule, the zygote divides repeatedly to form the embryo (future plant). The embryo contains a future root (radicle), a future shoot (plumule), and food-storing cotyledons. The ovary rapidly develops into the fruit. The ovary wall matures and develops into the fruit wall. The flower’s sepals, petals, stamens, style, and stigma degenerate and typically fall off.
Occasionally, the sepals may remain on the fruit. The ovule transforms into the seed. The ovule’s wall thickens to form the seed’s protective coat. The seed loses moisture and hardens and dries out. In this state, seeds can withstand drought and other adverse conditions. This is advantageous for plants that produce seeds.
Under favorable environmental conditions, the embryo in the seed becomes active and germinates to form a small seedling. The radicle produces the root, whereas the plumule produces the shoot. The food stored in the cotyledons is utilised by the root and shoot as they develop.