Chapter 20: Reproduction in Plants

Asexual Reproduction

Is the production of new individuals that does not involve fertilization, i.e. there is no fusion of two reproductive cells called gametes.

Natural vegetation

• Is a form of asexual reproduction.

• Involves the growth of a new plant (daughter) from any part of an old plant.

• Advantages

  • The process does not need external agencies.

  • Since food is usually present in the vegetative structures, buds can develop rapidly into daughter plants.

  • The daughter plants resemble the parent plant in every way. Beneficial or desirable qualities in the parent plant are passed on directly to the young plants without any change.

  • The process involves only one parent, and no fertilization is required.

  • Since plants are already in a suitable habitat, they can colonise the area more rapidly.

• Disadvantages

  • Lack of any dispersal mechanism may lead to overcrowding, causing the new plants to compete for light and food.

  • The new plants are less varied compared to those produced by seeds, thus they may be less adaptable to changes in environmental conditions.

Following are artificial methods of vegetative reproduction

• Cutting, layering and marcotting.

• Budding and grafting.

Sexual Reproduction in plants

Involves the fusion of cells called gametes. In plants, sexual reproduction takes place in flowers. Two mechanisms involved are pollination and fertilization.

• Parts of flower

  • Pedicle

    • Is a flower stalk.

  • Receptacle

    • Is the enlarged end of flower stalk.

    • Other parts of flower are borne.

  • Sepals

    • Are modified leaves which enclose and protect the other parts of the flower in the bud stage.

  • Petals

    • Are modified leaves forming the visible part of the flower.

    • Are usually brightly coloured in insect-pollinated flowers.

    • Petals have two important functions:

      • They are brightly coloured to attract insects for pollination.

      • They provide a landing platform for insects.

  • Stamens

    • Is the collective name for the stamens of a flower which produce pollen grains.

    • Is the male part of the flower.

    • Consists of a filament bearing an anther.

    • The anther is usually made up of two lobes, each containing two pollen sacs (contain pollen grain).

      • Pollen grains are produced meiosis so they contain the haploid number chromosomes.

      • Each pollen grain gives rise to two haploid male gametes which are the reproductive nuclei of the flower. When anther matures, its two lobes split, setting free the pollen grains.

  • The pistil

    • Is the female part of the flower.

    • Consists of one or more units called carpels.

      • A carpel consists of an ovary.

Pollination

Is the transfer of pollen grains from the anther to the stigma.

Method of Pollination

• Insect pollination

  • Insects are attracted to the plant, where pollen sticks to them.

  • They then move to another plant and drop the pollen, causing the second plant to be pollinated.

  • Pollen that is carried by insects is adapted to be sticky and have hooks which attach to the insect.

• Wind pollination

  • The wind blows pollen off the first plant and carries it to the second plant.

  • Pollen carried by the wind does not need to stick to anything, thus its smooth. It is also very light.

Fertilization

1. Pollen grains land on a plant stigma.

2. If the pollen is of the same species, a pollen tube begins to grow.

3. The pollen moves down the pollen tube to the ovule.

4. Fertilisation occurs when the nucleus from the pollen cell fuses with a nucleus in an ovule. This produces a zygote.

5. The zygote develops into an embryo plant.

Dispersal of fruits and seeds

• Why is it important

  • Avoid overcrowding and competition for food and light with the parent plants.

  • Enable plants to colonise new and favourable habitats.

  • Reduce the spread of diseases.

• Dispersal by wind

  • Small and light so that they can float in the air and be readily be blown.

  • Large, flattened wing-like structures or a parachute of fine-hairs.

  • Large surface area which increases air resistance.

• Dispersal by animals

  • Are brightly coloured, scented and sticky to attract insects.

  • Some have hook like structure which hook onto them.

• Dispersal by water

  • Adapted to float.

Germination

Is the process during which the food reserves present in a seed are broken down and the embryo starts to grow into a plant.

• Conditions

  • Sufficient supply of water, suitable temperature and adequate oxygen supply.

• Types of germination

  • Epigeal germination where the cotyledons are carried above the ground.

  • Hypogeal germination where the cotyledons remain below the surface of the soil.