Sexual Reproduction in Plants

Sexual reproduction in plants involves the fusion of male and female reproductive cells. Pollen containing sperm cells from the male part of the flower fertilizes the egg cells in the female part, leading to seed formation. This process promotes genetic diversity and the growth of new plants.

Organs of Reproduction: 

Flowers serve as the reproductive structures of plants, housing their reproductive organs. Stamens, the male organs, produce pollen, while carpels, the female organs, develop into fruits containing seeds after fertilization. Most plants bear flowers with both stamens and carpels, making them bisexual or hermaphroditic.

However, some plant species have unisexual flowers, meaning a single flower contains either stamens or carpels exclusively. In certain cases, both male and female flowers exist on the same plant, such as in hazel trees, where male and female catkins grow on the same tree. Conversely, willow trees bear male and female catkins on separate trees.

The male gamete resides within the pollen grain, while the female gamete is housed in the ovule. Pollination is the process by which the male gamete is transferred to the female gamete, typically from the stamen to the stigma. A microscopic tube grows from the pollen grain to carry the male gamete to the female gamete for fertilization. Subsequently, the zygote develops into a seed.

Structure of Flower:

Petals:

Petals, often brightly colored and sometimes fragrant, are arranged either in a circular or cylindrical fashion. Typically numbering from four to ten, they may fuse together to form a tube, obscuring individual petals. Petals play a crucial role in attracting insects for pollination.

Sepals:

Surrounding the petals, sepals are generally smaller and green. They serve a protective function, particularly when the flower is in bud.

Stamens:

Stamens constitute the male reproductive organs. Each stamen comprises a filament supporting an anther, which houses pollen sacs. Flowers like buttercups may possess numerous stamens, while others like tulips may have a smaller, proportional number. When ripe, the anthers release pollen grains.

Pollen:

Insect-pollinated flowers produce relatively small, often sticky pollen grains to adhere to insect bodies. Conversely, wind-pollinated flowers generate larger, smoother pollen grains designed for dispersal by the wind.

Carpels:

The carpels represent the female reproductive organs. Flowers such as buttercups can have multiple carpels, while others like lupins may have a single elongated carpel. Each carpel comprises an ovary containing ovules, which develop into seeds. The ovary eventually transforms into a fruit. The stigma, with its adhesive surface, captures pollen during pollination, while the style varies in length.

Receptacle:

All the aforementioned flower structures are attached to the expanded end of the flower stalk, known as the receptacle. In some cases, particularly after fertilization, the receptacle may become fleshy and edible, as seen in fruits like apples and pears.

Lupin:

Lupins feature five fused sepals forming a short tube. The five petals exhibit varying shapes and sizes, with the uppermost petal (standard) held vertically, flanked by two wing petals. Two additional petals form a boat-shaped keel inside the wings. The single elongated carpel contains approximately ten ovules, with the long style terminating in a stigma positioned within the pointed end of the keel. Ten stamens, comprising five long and five short ones, encase the ovary within a basal sheath formed by their fused filaments.

Pollination

Pollination is the process by which pollen is transferred from the anthers, where it is produced, to the stigma, where it can fertilize the ovules. Typically, this transfer involves the anthers splitting open to release microscopic pollen grains. These grains can be transported by various means, such as being carried by insects or blown by the wind. Once airborne, the pollen grains may land on the stigma of another flower, initiating the process of fertilization.

Pollen is transported from the anther to the stigma, the female reproductive part of the flower. This transfer can occur in several ways:

• Insect Pollination: Pollinators, attracted by the flower’s features, collect nectar and inadvertently brush against the anther, picking up pollen. When they visit another flower, some of this pollen is deposited on the stigma.

• Wind Pollination: In some plants, especially those with inconspicuous flowers, the wind carries pollen from one flower to another.

Insect pollination vs Wind pollination

Self Pollination and Cross Pollination      

Cross-pollination and self-pollination are two different mechanisms by which plants achieve fertilization, the process of transferring pollen from the male reproductive organ (anther) to the female reproductive organ (stigma) for the production of seeds.

Cross-Pollination

Cross-pollination occurs when pollen is transferred from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species.

This process often involves external agents, such as wind, water, insects, birds, or other animals, that carry the pollen from one flower to another. It promotes genetic diversity within a population because it combines genetic material from two different parent plants.

Cross-pollination has the advantage of introducing genetic variability, which can enhance the adaptability and fitness of a population. It also ensures a higher chance of successful fertilization in species that depend on external agents for pollination.

Self-Pollination

Self-pollination occurs when pollen is transferred from the anther to the stigma of the same flower or to another flower on the same plant.

In self-pollinating plants, the pollen can be transferred by various means, such as wind, gravity, or physical contact. Some plants have mechanisms that promote self-pollination, like flowers that are structurally designed to bring the anther and stigma in close proximity.

Self-pollination ensures reproductive success in environments where pollinators are scarce. It also maintains the genetic characteristics of a particular plant since it involves the transfer of pollen within the same genetic individual.

Fertilization in Plants – in detail

Fertilization in plants is the process by which male and female gametes (reproductive cells) fuse to form a zygote, initiating the development of a new individual. In flowering plants (angiosperms), fertilization involves the fusion of male gametes, contained within pollen grains, with female gametes, located in the ovule within the ovary of a flower. This process is essential for the production of seeds, which develop into new plants.

The transfer of pollen (containing male gametes) from the anther of a flower to the stigma of the same or another flower. This can occur through various means, such as wind, water, or animal pollination.

• Once the pollen reaches the stigma, it germinates, forming a pollen tube.

• The pollen tube grows down the style of the flower, reaching the ovary where the ovules are located.

• This unique feature in angiosperms involves the fusion of two sperm nuclei with two different nuclei in the ovule:

• One sperm nucleus fuses with the egg cell, forming a diploid zygote.
• The other sperm nucleus fuses with two other nuclei in the female gametophyte, forming a triploid cell that develops into the endosperm.

• The fertilized ovule develops into a seed, containing the embryo (zygote) and surrounded by protective seed coats.

Fertilization ensures genetic diversity by combining genetic material from two parent plants and is a crucial step in the plant life cycle, contributing to the continuity of plant species.

Germination of seeds

The germination of seeds is influenced by a combination of environmental factors. These factors can vary depending on the plant species, but some common conditions that affect seed germination include

Water

Adequate water is crucial for germination. It helps soften the seed coat and activates enzymes that initiate germination. However, excessive water or waterlogged conditions can lead to fungal growth and seed rotting.

Temperature 

Different plants have specific temperature requirements for germination. Some seeds germinate best in cool temperatures, while others require warmer conditions. Temperature influences the speed of germination and the metabolic processes involved.

Oxygen

Oxygen is essential for aerobic respiration, which provides the energy needed for germination. Seeds require oxygen to break down stored nutrients and support early growth. Poor aeration, such as waterlogged soils, can limit oxygen availability and hinder germination.