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Plant Reproduction


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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

 Flower partsPost-fertilisation changes
 whole ovuleseed 
 zygote
embryo consisting of the plumule, radicle, and cotyledons 
 endosperm nucleusendosperm (in some cases, completely absorbed by the embryo during seed formation, producing non-endospermic seed) 
 integumentstesta & tegmen 
 ovule stalkseed stalk (funicle) 
 ovaryfruit 
 ovary wallfruit wall (pericarp) 
stigma & style wither, in some cases may persist and modified to help fruit dispersal 
 stamenswither and fall off 
 petalswither and fall off 
 sepalsmay persist and in some cases, may enlarge and be modified to help fruit dispersal 

Dispersal of fruits and seeds

Dispersal is necessary to:
  • avoid overcrowding and competition for food and light with the parent plants
  • enable plants to colonise new and favourable habitats
  • reduce spread of diseases
Dispersal by
  1. wind
    • small and light fruits and seeds so they can float in the air and be readily blown by the wind
    • large, flattened wing-like structures or a parachute of fine-hair to provide enlarged surface area to increase air resistance or buoyancy in air
      • eg: angsana fruit, tridex fruit, kapur fruit, tecoma, shorea, cotton
  2. animals
    • succulent pericarp, scented and brightly coloured skins to attract animals
    • the whole fruit may be eaten by animals. The seeds may be small and hard, indigestible, and are removed in the faeces, away from the parent plant
    • some fruits have hook-like structures by which they can adhere onto the fur or skin of animals passing by. These fruits may later be brushed off the animals' bodies or fall off when the hooks shrivel.
      • eg: urena, xanthium
  3. water
    • adapted for floating and can drift for considerable distances
    • eg coconut has waterproof skin and fibrous husk containing numerous air spaces which lighten the fruit
    • the seed within it contains a store of food and there is sufficient water in the seed to enable its germination even on sandy shores
    • eg. the seeds of water lily have an aril (small float) which holds air. The seeds can float on the water away from the parent plants until the arils decay. Then, they sink to the bottom of the river/pond and germinate.
  4. explosive mechanism
    • these fruits, on drying up, burst open suddenly with great force to throw out the seeds
    • eg. when ripe balsam fruits dry up, they burst open and eject the seeds away from the parent plant
    • eg. rubber fruits, legumes of many plants (peas, beans etc)

MCQ

1. Which structure is not essential in a wind-pollinated flower?
a. anther
b. ovary
c. petal
d. stigma

2. Which statement is true of asexual reproduction in plants?
a. insects are needed to transfer pollen
b. new plants grow from seeds
c. offspring are genetically identical to their parents
d. two types of gamete are involved

3. Many wind-pollinated flowers have
a. feathery stigmas and light pollen
b. feathery stigmas and sticky pollen
c. short stigmas and light pollen
d. short stigmas and sticky pollen

4. Which statement is not true of the offspring resulting from asexual reproduction?
a. they are produced by self-fertilisation
b. they are produced from a single parent
c. their cells have the same alleles
d. their cells have the same number of chromosomes

5. Which features correctly describe a wind-pollinated flower?
 coloured petalsanthers large and hang out of flowerpollen smooth and lightsticky stigma inside flower
ayes
yes no no 
byesno yes yes 
cno yes yes no 
dnoyes no yes 


MCQ Answers

1. c
2. c
3. a
4. a
5. c
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