Introduction
Reproduction in plants is a crucial aspect of their life cycle, ensuring the continuity of species and enabling genetic diversity. In the context of the International Baccalaureate (IB) Biology Higher Level (HL) syllabus, the topic of plant reproduction encompasses various stages and mechanisms, including flowering, pollination, fertilization, and seed dispersal. This study note will delve into these processes, breaking down complex ideas into manageable sections for better understanding.
9.4.1 Flowering
The Life Cycle of a Flowering Plant
Flowering plants, or angiosperms, undergo a life cycle that includes two main phases: the vegetative phase and the reproductive phase.
The Vegetative Phase
- Characteristics: During this phase, the plant focuses on growth, developing roots, stems, and leaves.
- Asexual Reproduction: Some plants can reproduce asexually during the vegetative phase through methods like budding, fragmentation, or vegetative propagation.
The Reproductive Phase
- Characteristics: This phase is marked by the production of flowers, which are the reproductive organs of the plant.
- Sexual Reproduction: The plant can reproduce sexually, involving the formation of gametes and subsequent fertilization.
Flowering and Gene Expression
The transition from the vegetative to the reproductive phase is controlled by gene expression, influenced by external abiotic factors such as temperature and night length.
- Photoperiodism: The response of a plant to the relative lengths of daylight and darkness, which affects flowering.
- Phytochrome: A pigment involved in detecting light changes. It exists in two forms:
- $P_{r}$ (Pr) absorbs red light.
- $P_{fr}$ (Pfr) absorbs far-red light.
Mechanism
- Day Length: It was initially believed that day length controlled flowering, but it is now known that the length of the night is the critical factor.
- Night Length: When the night reaches a certain critical length, genes that control flowering are either activated or inhibited, leading to the production of flowers.
Photoperiodism ensures that plants flower at the most advantageous times of the year.
9.4.2 Plant Reproduction
Sexual Reproduction in Plants
Sexual reproduction in plants involves the transfer of pollen from the male structures to the female structures of flowers.
Methods of Pollen Transfer
- Wind: Pollen is carried by the wind to other flowers.
- Animals: Many plants rely on animals for pollination.
- Water: Some aquatic plants use water to transfer pollen.
Mutualistic Relationships
Most flowering plants use mutualistic relationships with animal pollinators, where both parties benefit:
- Pollinators: Insects, birds, and small mammals like bats visit flowers to obtain food (nectar).
- Pollen Transfer: Pollen sticks to the body of the pollinator and is transferred to the next flower visited.
