Sexual reproduction in plants.

To understand how reproduction occurs in ferns, it is important to identify their main parts. Although fern species vary in appearance, they all share three major parts:

1. Rhizome – The horizontal stem from which fronds emerge.
2. Fronds – The leafy branches made up of smaller leaflets called pinnae.
3. Sporangia – The reproductive structures where spores are produced.

On the underside of the pinnae, tiny spore-producing structures develop, often appearing as small brownish dots. These are called sporangia.

It's important to note that not all fronds carry spores.

• Fertile fronds are the ones that bear spores and are involved in reproduction.
• Other fronds carry out photosynthesis, helping to produce food for the plant.

New fronds continuously grow from the rhizome, allowing the fern to expand and reproduce both vegetatively and sexually.

1. The diploid sporophyte produces haploid spores through the process of meiosis. This is the same cellular division that produces eggs and sperm in animals and flowering plants.

2. Each haploid spore undergoes mitosis and grows into a small, photosynthetic structure called a prothallus, also known as the gametophyte. The gametophyte contains cells that are haploid and is significantly smaller than the sporophyte fern.

3. The gametophyte (prothallus) produces gametes (sperms and eggs) through mitosis—not meiosis, since the gametophyte is already haploid.

   • Sperms are produced in a structure called the antheridium.
   • Eggs are produced in a structure called the archegonium.

   The gametophyte is composed of leaflets and rhizoids, while the mature sporophyte has fronds and rhizomes.

4. When water is available and conditions are favourable, the motile sperm use their flagella to swim through the water film toward the egg in the archegonium to fertilize it.

5. The fertilized egg (now a diploid zygote) remains attached to the prothallus. This zygote forms when DNA from the sperm and egg combine.

6. The zygote grows into a diploid sporophyte by mitosis, completing the life cycle of the fern.

A flower is the reproductive structure of angiosperms (flowering plants). It develops when the plant reaches maturity, as some of the leaves in the shoot system are modified into specialized reproductive structures called flowers.

A typical flower is composed of four main parts:

• Pistil (carpel) – the female reproductive part, located at the center of the flower. It consists of the ovary, style, and stigma.
• Stamen – the male reproductive part, made up of the anther (which produces pollen) and the filament.
• Calyx – the outermost whorl, made up of sepals which protect the flower bud. The number of sepals is typically three (or multiples of three), four, or five (or multiples of five).
• Corolla – the whorl of petals, usually brightly colored to attract pollinators. The number of petals often matches the pattern found in the sepals.

Other supporting parts include:

• Pedicel – the stalk of the flower.
• Receptacle – the part of the stem that holds the floral organs.

The carpel produces the female gametes (ovules), while the stamen produces male gametes (pollen grains). During fertilisation, the male gamete fuses with the female gamete to form a zygote.

Flowers can be classified based on the presence of reproductive organs:

• Bisexual (perfect) flowers contain both stamens and carpels.
• Unisexual (imperfect) flowers contain either stamens or carpels, but not both.

Sepals form the outermost ring of floral leaves and are collectively known as the calyx. They are typically small, green, and leaf-like in structure, positioned at the base of the flower. The main function of sepals is to protect the flower bud before it opens.

Petals form a ring of floral parts just inside the calyx. The collective term for all petals is the corolla. In insect-pollinated flowers, petals are often brightly colored to attract pollinators. At the base of the petals, there are glandular swellings known as nectaries, which secrete a sugary substance called nectar. Nectar serves to attract insects, aiding in the pollination process.

Together, the sepals and petals are referred to as the perianth.

The stamen is the male reproductive organ of a flower and is collectively called the androecium. Each stamen consists of:

• An anther, which produces pollen grains (male gametes).
• A filament, which is a stalk that supports the anther.

The pistil is the female part of the flower and is collectively referred to as the gynoecium. A flower may contain one or more carpels. When it contains only one carpel, it is called monocarpous; when it has two or more carpels, it is referred to as polycarpous.

The pistil is made up of three main parts:

1. Stigma – This is the sticky, glandular tip of the pistil that functions to receive pollen grains during pollination.
2. Style – This is a slender, tube-like structure that connects the stigma to the ovary. It is hollow to allow the pollen tube to grow during fertilisation.
3. Ovary – Located at the base of the pistil, it contains ovules, which develop into seeds after fertilisation.