Plant Reproduction

Summary

Asexual Reproduction

• • Asexual reproduction

    • The production of new individuals that does not involve fertilisation.

    • no fusion of 2 reproductive cells (gametes)

    • produces identical offspring because mitosis usually occurs during asexual reproduction

    • examples:

      • budding

      • spore formation

      • vegetative reproduction

    • Mitosis:

      • a cell divides to produce 2 daughter cells, each having the same number of chromosomes as the parent cell.

      • the daughter cells have the same genes as the parent cell

      • the offspring produced asexually are genetically identical to the parents and are called clones

  • Sexual reproduction

    • reproduction that involves fusion of 2 gametes

Natural vegetative propagation in flowering plants

• • involves the growth of a new part of a plant (bud/stem), which eventually becomes separated from the parent plant to form a new individual

  • many flowering plants produce vegetatively by means of underground storage organs such as rhizomes, bulbs,corms,tuburs,runners

  • the food stored enables the plant to survive through unfavourable climatic conditions. When favourable conditions return, the buds use up the stored food and develop rapidly into new plants (perennation)

  • Advantages

    • process does not need external agents eg insects and wind - for pollination and dispersal

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

    • the daughter plants resemble the parent plant completely - desirable qualities in the parent plant are passed on to the young plants

    • process involves only one parent, and no fertilisation 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 food and light

    • the new plants are less varied compared to those produced by seeds - may be less adaptable to changes in environmental conditions

Artifical methods of vegetative reproduction

• Cutting

  • • stems of certain plants are cut just below a node

    • the cutting is planted in suitable soil so the node can develop adventitious roots form a new plant

    • Eg: sugarcane, tapioca, oleander

• • layering:

    • a low branch with a node is bent down and a ring of bark is removed from the node.

    • the ringed section is completely covered with moist soil while the end of the branch is allowed to remain free

    • when roots have developed, the branch is cut off from the parent plant and allowed to grow on its own

    • eg: lime, bougainvillea

• Marcotting

  • • a ring of bark is removed from a branch

    • then, a layer of moist soil is wrapped round the ringed portion of the branch and kept in place

    • the soil is kept moist by watering every day

    • when roots appear, the branch is cut and planted.

    • eg: rambutan, durian, chiku trees

• Grafting & budding

  • • a healthy plant with an established root system is selected - stock

    • a portion of the plant to be propagated (scion) is carefully cut off

    • it is attached to the stock, and the two grow together

    • the stock absorbs water and dissolved mineral salts through its roots and transports them to the growing scion

    • eg: roses, cherry, almond

• Budding

  • • a bud, together with some cambium, is taken from a selected plant - scion

    • a T-shaped cut is made in the bark down to the cambium in the stock

    • the scion is carefully inserted but with the bud still exposed beneath the bark of the stock

    • the thin cambium in the scion is now in contact with the cambium of the stock

    • the scion and stock are tied together and the junction protected with wax

    • the tissues of the two plants soon unite and the bud grows into a shoot

    • eg: lemon, hibiscus

• Grafting

  • • a twig bearing several buds is cut off

    • its lower V-shaped end is inserted into a complementary shaped end of the stock

    • The two are tied as in budding and the buds will develop into shoots

Sexual reproduction in flowering plants

General parts of a flower

• Pedicel

  • • a flower stalk

    • flowers with no pedicels: sessile flowers

• Receptacle

  • • the enlarged end of the flower stalk on which the other parts of the flower are borne

• Sepals

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

    • all the sepals together make up the calyx

    • usually form the outermost whorl of floral leaves but some flowers (eg hibiscus) have another whorl of floral leaves outside the sepals which make up the epicalyx of the flower

• Petals

  • • modified leaves forming the conspicuous part of the flower

    • usually brightly coloured in insect-pollinated flowers

    • form the corolla of the flower

    • functions

      • brightly coloured to attract insects for pollination

      • provide a landing platform for insects

• • Stamens (andromecium)

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

    • the male part of the flower

    • consists of a filament bearing an anther

    • anther: usually made up of 2 lobes, each containing 2 pollen sacs --> inside are pollen grains

      • pollen grains are produced by meiosis and so contain 2 haploid male gametes which are the reproductive nuclei of the flower

      • when the anther matures, its 2 lobes split, setting free the pollen grains

  • Pistil (gynoecium)

    • the female part of the flower

    • consists of one or more units called carpels

      • consists of an ovary, a style, and one or more stigmas

      • stigma: a swollen structure at the end of the style which receives the pollen grains

      • ovary: contains one or more ovules

        • within each mature ovule is a female gamete called the ovum

        • the region in the ovary to which the ovules are attached is called the placenta

        • ovule is attached to the placenta by a funicle

Pollination

• • the transfer of pollen grains from the anther to the stigma

  • self-pollination: if pollen grains are transferred to the stigma of the same flower or a different flower in the same plant

  • cross-pollination: if pollen grains are transferred to a flower in another plant of the same kind

    • Advantages

      • offspring produced may have valuable qualities from both parents

      • abundant and more viable seeds tend to be produced

      • more varieties of offspring can be produced (greater genetic variability) --> increases the chance of survival of the species during changes in the environment

        • As genetic variability is important for the survival of a species, most types of plants possess special devices favouring cross-pollination

Fertilisation and post-fertilisation changes

• • After pollination, the pollen grain germinates in response to the sugary fluid secreted by the mature stigma

  • from each pollen grain, a pollen tube grows out

  • the cytoplasm and the 2 nuclei (vegetative and generative nuclei) pass into the pollen tube

  • the growth of the pollen tube is controlled by the vegetative nucleus

  • enzymes are secreted to digest the tissues of the stigma and style as the tube grows

  • thus, the pollen tube penetrates right through the style as it grows and enters the ovule usually through the micropyle (an opening in the ovule wall)

  • along the way, the generative nucleus divides to form 2 male, non-motile gametes

  • the vegetative nucleus soon disintegrates

  • within the ovule, the tip of the pollen tube absorbs sap and bursts, releasing the 2 male gametes

  • one male gamete fuses with the ovum to form the zygote - fertilisation

  • the other male gamete fuses with the definitive nucleus (secondary nucleus) to form the endosperm nucleus

  • the zygote develops into the embryo of the seed with cotyledons, developing shoot (plumule) and developing root (radicle)

  • the endosperm nucleus divides and gives rise to the endosperm

  • in some seeds, the endosperm is completely absorbed by the embryo which stores the food materials in the cotyledons

  • non-endospermic

  • a fruit is generally formed from the ovary and the ovules

  • the ovary wall ripens to form the fruit wall (pericarp)

  • the ovules become the seeds

  • the funicle or seed stalk is attached to the part of the fruit called the placenta

  • a fruit has one or more fruit chambers called loculi

  • in some fruits, the pericarp becomes hard and dry; in others it becomes fleshy and succulent

MCQ Answers

1. c

2. c

3. a

4. a

5. c