Plant reproduction is a complex biological process that enables the propagation and continuity of species. This set of notes delves into the intricate sequential processes of pollination, fertilisation, and seed dispersal, highlighting the key structures involved.
Pollination
Pollination is the transfer of pollen grains from the male part of the plant to the female part, enabling fertilisation.
Types of Pollination
- 1. Self-pollination:
- Description: Pollen is transferred within the same flower or to another flower on the same plant.
- Advantages: Guarantees reproduction in isolated areas, and maintains specific traits within a population.
- Disadvantages: Reduced genetic diversity may lead to inbreeding depression over time.
- 2. Cross-pollination:
- Description: Pollen is transferred between flowers of different plants of the same species.
- Advantages: Enhances genetic diversity, may lead to traits that enhance adaptability and survival.
- Disadvantages: Often requires specific pollinators or conditions, and may be less reliable.
Agents of Pollination
- 1. Animals (Biological vectors):
- Insects: Bees, butterflies, and beetles often transfer pollen.
- Birds: Hummingbirds and sunbirds may pollinate flowers.
- Mammals: Bats and some rodents also contribute to pollination.
- 2. Wind (Anemophily): Wind carries pollen in plants with small, unattractive flowers.
- 3. Water (Hydrophily): Some aquatic plants use water currents for pollination.
Mechanisms to Ensure Cross-pollination
- Dichogamy: Temporal separation of male and female reproductive organs maturing.
- Herkogamy: Structural barriers preventing self-pollination.
- Self-incompatibility: Genetic mechanisms that prevent self-pollen from fertilising.
Fertilisation
Fertilisation follows pollination and involves the fusion of male and female gametes.
Process of Fertilisation
- 1. Pollen Grain Germination: Pollen grain forms a pollen tube through the style.
- 2. Pollen Tube Growth: It grows towards the ovule containing the female gamete.
- 3. Sperm Nucleus Movement: Two sperm nuclei travel down the pollen tube.
- 4. Double Fertilisation:
- Formation of Zygote: One sperm nucleus unites with the ovum, forming a zygote.
- Endosperm Formation: The other sperm nucleus forms a triploid cell that becomes the endosperm.
- 5. Seed Formation: The ovule develops into a seed, containing an embryo.
- 6. Fruit Formation: Ovary walls thicken to form fruit around the seed(s).
Seed Dispersal
Seed dispersal allows plants to spread offspring into new areas.
Methods of Seed Dispersal
- By Wind (Anemochory):
- Example: Dandelion seeds have parachute-like structures for wind dispersal.
- By Water (Hydrochory):
- Example: Coconuts can float and are carried by water.
- By Animals (Zoochory):
- External (Epizoochory): Seeds cling to fur or feathers.
- Internal (Endozoochory): Seeds are ingested and later excreted.
- By Explosion (Ballochory):
- Example: Touch-me-not plant pods burst open when touched.
Importance of Seed Dispersal
- Reduces Competition: Minimizes competition with parent plants and siblings.
- Facilitates Colonisation: Spreads species to new, suitable habitats.
- Promotes Genetic Diversity: Supports diversity, adaptation, and evolution.
Human Influence on Seed Dispersal
- Agriculture: Humans have developed methods for large-scale seed dispersal in farming.
- Invasive Species: Human activity can unintentionally spread seeds of invasive species, impacting native flora.
FAQ
Bees play a crucial role in the pollination process of many flowering plants. As they forage for nectar and pollen, bees brush against the reproductive organs of flowers, collecting pollen on their bodies. When they visit another flower, some of this pollen is transferred to the stigma of the new flower. This transfer of pollen from one flower to another is essential for fertilisation in many species. By aiding in the cross-pollination of different plants, bees help increase genetic diversity and the production of seeds and fruits.
Self-incompatibility is a mechanism in some plants that prevents self-pollination, thereby encouraging genetic diversity. When pollen from the same plant lands on the stigma, a biochemical reaction occurs, identifying the pollen as "self." This recognition triggers a response that inhibits the growth of the pollen tube, preventing the pollen from reaching the ovules and fertilising them. It's a genetically controlled system that ensures cross-pollination by actively preventing the pollen from fertilising the eggs of the same plant.
Wind-pollinated flowers typically have small, inconspicuous petals and produce large amounts of lightweight pollen that can be easily carried by the wind. They often have exposed stamens and large, feathery stigmas to capture the windborne pollen. In contrast, insect-pollinated flowers tend to be brightly coloured and have attractive scents or nectar to lure insects. Their stamens and stigmas are usually positioned to ensure contact with the insect's body, facilitating pollen transfer. Wind-pollinated flowers generally do not invest in features to attract insects, while insect-pollinated flowers have adaptations to encourage insect visitation.
Ovules are structures found within the ovary of a flower that contains the female gametes, or egg cells. During fertilisation, pollen grains land on the stigma, grow a pollen tube down to the ovule and deliver the sperm cells. One sperm cell fuses with the egg cell within the ovule, forming a zygote. After fertilisation, the ovule develops into a seed, containing the embryo of the new plant, while the surrounding ovary forms the fruit. The ovule's structure and function are essential for sexual reproduction in flowering plants.
Fruits are developed from the fertilised ovary of a flower and contribute to seed dispersal by appealing to different agents. For example, fleshy fruits attract animals that eat them, and the seeds are then dispersed through the animal's faeces. Wind-dispersed fruits may have wing-like structures to be carried by the air, while some have spines or burrs to attach to animal fur. The structure of the fruit directly aids in the method of dispersal, allowing seeds to spread to different areas where they can germinate.
Practice Questions
Double fertilisation is a process unique to angiosperms, where two fertilisation events occur simultaneously. Upon the arrival of a pollen grain on the stigma, a pollen tube grows down to the ovule. Two sperm nuclei travel down this tube. One sperm nucleus fuses with the egg cell to form a diploid zygote, initiating the development of the embryo. The other sperm nucleus fuses with two additional nuclei in the ovule to form a triploid cell, which develops into the endosperm. The endosperm provides nourishment to the growing embryo, aiding in the formation of a viable seed. This process ensures efficient usage of the plant's reproductive resources.
Self-pollination occurs when pollen from a flower fertilises the same flower or another flower on the same plant. This leads to genetic uniformity and can preserve favourable traits, but it may also result in inbreeding depression. Cross-pollination occurs between flowers of different plants of the same species, enhancing genetic diversity and adaptability. A mechanism that ensures cross-pollination is dichogamy, where male and female reproductive organs mature at different times. This temporal separation prevents the pollen from fertilising the ovules of the same flower, promoting genetic diversity and reducing the chance of self-pollination. Other mechanisms include herkogamy and self-incompatibility.
