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.