Asexual and Sexual Reproduction in Plants

Artificial Propagation: Cuttings

Humans often assist in asexual reproduction through a process called cuttings. A section of a parent plant is removed and encouraged to grow into a new individual. This process is driven by auxin, a plant hormone that accumulates at the base of the stem to trigger root formation.

Exam Tip: Remember that cuttings produce clones. If a question asks about the genetic makeup of a plant grown from a cutting, the answer is that it is genetically identical to the parent.

Sexual Reproduction in Flowering Plants

While asexual reproduction is efficient, sexual reproduction provides a significant evolutionary advantage: genetic variation. By combining genetic material from two parents, the resulting offspring are unique, making the population better equipped to survive environmental changes or diseases.

Anatomy of a Flower

To achieve sexual reproduction, flowering plants utilize specialized male and female organs:

The Male Parts (Stamen): The stamen is the male reproductive unit. It consists of a stalk called a filament, topped by an anther. The anther is responsible for producing pollen, which contains the plant's sperm cells.

The Female Parts (Carpel): The carpel is the female reproductive unit. It consists of three main parts:

1. Stigma: The sticky top part designed to capture pollen.
2. Style: The stalk that connects the stigma to the ovary.
3. Ovary: The base of the carpel which houses the ovules (the eggs).

Pollination: The Transfer of Pollen

Pollination is the process of transferring pollen from an anther to a stigma. This can occur in two ways:

• Self-pollination: Pollen is transferred from a plant's own anther to its own stigma.
• Cross-pollination: Pollen is transferred from the anther of one plant to the stigma of a different plant of the same species.

Cross-pollination is highly desirable because it increases genetic variation. Many plants have evolved mechanisms to prevent self-pollination, such as having their male and female parts ripen at different times. Pollination is often facilitated by wind, water, or animal pollinators like bees, which pick up pollen while feeding on nectar.

Fertilisation and Seed Development

Once pollen reaches the stigma, the process of fertilisation begins. This is a multi-step biological sequence:

1. Pollen Tube Growth: The pollen grain produces a pollen tube that grows down through the style toward the ovary.
2. Sperm Delivery: The pollen's generative cell travels down this tube and divides to form two sperm cells.
3. Fusion: The sperm cells enter an ovule within the ovary. One sperm cell fertilises the egg to create a zygote. The second sperm cell helps form the endosperm, which serves as a food source for the developing offspring.
4. Development: The zygote develops into an embryo. As the embryo grows through cell division (mitosis), the ovule matures into a seed. Eventually, the entire ovary develops into the fruit of the plant.

Summary Table: Comparison of Reproductive Outcomes