Plant Reproduction Notes

Plant Reproduction

Topic 2: Plant Reproduction

  • Anther: Male gametes
  • Stigma: Part of the female reproductive system
  • Ovule: Female gametes

Sexual & Asexual Reproduction

Asexual Reproduction

  • Involves only one parent.
  • Offspring are genetically identical to the parent and each other (no variation).
  • Gametes are not produced.
  • Cell division is only mitotic.
  • A large number of plants are produced in a very short time.

Sexual Reproduction

  • Involves two parents and the fusion of gametes.
  • Male parent produces sperm, and the female parent produces eggs.
  • Sperm and egg fuse during fertilization to form a zygote, which develops into an embryo.
  • Offspring are different (variation) from parents and each other.
  • Gametes are produced by meiotic divisions, and the zygote develops by mitotic division.
  • Comparatively, the number of offspring produced is less.

Vegetative Reproduction

  • Name given to asexual reproduction taking place naturally in plants.
  • Can occur by means of:
    • Stolons (runners)
    • Rhizomes (e.g., grass)
    • Tubers (e.g., potatoes)
    • Bulbs (e.g., onions)
  • Certain plants can also make new plants from cuttings.

Advantages & Disadvantages of Asexual Reproduction

Advantages

  • Not a lot of time or energy invested in the production of flowers, seeds, or fruit.
  • Can produce many plants quickly.
  • Genetically identical offspring can be advantageous if genes are 'good'.

Disadvantages

  • Lack of variation means plants are less adaptable to changing environments.
  • Genetically identical offspring are susceptible to the same diseases or pests.
  • Cannot bring together gametes from different parent plants, limiting variation.

Flowers as Reproductive Structures

  • Flowers are the organs of sexual reproduction in angiosperms.

  • A typical flower is made up of concentric rings (whorls) of parts.

    • Calyx: Made of green, leaf-like sepals (protect bud).
    • Corolla: Petals.
    • Androecium: Male parts, stamens (filament + anther).
    • Gynoecium: Female parts, carpels (ovary + style + stigma).

Sexual Reproduction in Angiosperms

  • Sexual reproduction involves the following processes:
    • Formation of gametes
    • Pollination
    • Fertilisation
    • Development & dispersal of fruit and seed
    • Germination of seed into new plant

Formation of Female Gametes - Egg Cells

  • Formed in the ovary.
  • Each ovary contains 1 or more ovules.
  • Ovule is made of parenchyma tissue (called nucellus) surrounded by a protective covering (integument).
  • Center of each ovule – megaspore mother cell. This divides to form several cells; one is the female gamete.

Formation of Male Gametes

  • Formed inside anthers.
  • Each anther has 4 pollen sacs.
  • Mother microspore cells undergo MEIOSIS to form microspores.
  • Microspores undergo MITOSIS to form haploid pollen grains.
  • Each pollen grain contains two male gametes/sperms.
  • If a pollen grain lands on the stigma, only one sperm will fertilize the egg to form a zygote.
  • Pollen grains have tough, waterproof coats to prevent drying out.

Anthers \rightarrow Microspores \rightarrow Microspore \, mother \, cell \xrightarrow{MEIOSIS} Microspores \xrightarrow{MITOSIS} Microgametophyte \, (pollen) \rightarrow Generative \, cell + Tube \, cell

Double Fertilisation

  • Pollen grain lands on stigma and grows a pollen tube.
  • Pollen tube grows down the style towards the ovary.
  • Tube pushes through the ovary wall to reach the ovule.
  • When the tube enters the ovule, the tip of the tube bursts to release the male gamete.
  • One male gamete fertilizes the egg nucleus to form the zygote.
  • The zygote develops into an embryo by mitosis.
  • The second male gamete fuses with other cells produced by the mother cell to form the endosperm.
  • Endosperm tissue stores food for the developing embryo.
  • The process is called double fertilization because two male gametes fuse with two female cells.
  • The ovary becomes the fruit.
  • The ovule becomes the seed.
  • The fruit is the mature ovary.

Pollination

  • The transfer of pollen grains from the anther to the stigma.
    • Self-pollination: Transfer happens in the same flower or between flowers of the same plant.
    • Cross-pollination: Transfer from the anther of one plant’s flowers to the stigma of the flower of a different plant (but the same species).
  • Cross-pollination brings variation (different combinations of genetic material).
  • Plants have adaptations for ensuring cross-pollination:
    • Separate male & female plants (dioecious).
    • Anthers and carpels that mature at different times.
    • Stigmas are pushed out of flowers far above anthers.

Insect Pollination

  • Examples: Daisy, Orchid, Salvia, legumes, etc.
  • Scented flowers producing lots of pollen.
  • Conspicuous flowers, brightly colored.
  • Markings & patterns (honey guides) on petals to guide insects to nectaries.
  • Petals modified into landing pads.
  • Pollen is large, sticky, or spiny to cling to insects.
  • Anthers inside flowers; insects brush against them.
  • Stigmas have sticky surfaces.

Bird Pollination

  • Examples: Ericas, Proteas, Aloes, etc.
  • Brightly colored flowers.
  • Little or no scent.
  • Nectaries producing large amounts of nectar.
  • The size & shape of the flower matches the size and shape of the beak of the bird.
  • Flowers are large, with a strong stem and flower stalks as a perch for birds.

Development of Seeds

  • After fertilization, petals, stamens, and sepals of the flower dry out and drop off.
  • The ovary wall becomes the fruit, and ovules develop into seeds.

Seed Dispersal & Germination

  • For species survival, it is important for seeds to be carried away from the parent plant.
  • Avoids overcrowding & competition for resources.
  • The fruit is a unique feature in angiosperms that helps with seed dispersal.
  • Animals eat the fruits and deposit the seeds with their droppings.
  • Beans and peas develop within pods, which self-disperse when the ripe pods split open and scatter the seeds.
  • Under favorable conditions, seeds germinate to form a new generation of plants.

The Significance of Seeds

  • The type of food reserves stored in seeds varies.
    • In sunflowers, seeds mainly store lipids.
    • Maize and rice seeds store mostly starch.
    • Seeds of legumes (beans & peas) store mostly protein.
  • Seeds are a good source of protein, fat, and carbohydrates; therefore, they are a good source of food for animals.
  • Maize, wheat, and rice make up the staple diet for over half of the people in the world.

Seed Dormancy and Seed Banks

  • In the final stages of seed development, water is withdrawn & metabolic processes slow to a minimum.
  • This hardens the seed, making it resistant to cold and adverse conditions – the seed is dormant.
  • Dormant seeds are able to survive for a very long time.
  • When conditions are favorable, the seed will germinate and develop into a new plant.
  • This is a way of preserving the DNA of a plant species over time.
  • Scientists are able to store seeds in seed banks (called Doomsday vaults); this ensures that important crops are not lost and rare or endemic species don’t die out.