Plant Reproduction

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Gamete Development

  • Male:
    -In the anther, diploid cells undergo meiosis to form haploid pollen grains, surrounded with a tough wall which enables it to be transferred from one plant to another without drying out.
    -Inside the pollen grain the haploid nucleus undergoes mitosis to produce two nuclei, a generative nucleus and a tube nucleus; the first produces both male nuclei later on.
    -When the pollen matures, the anthers dry out and the pollen grains are released - which can be carried away by insects in certain flowers.
  • Female:
    -The ovules are produced in the ovary with the female gamete developing in the ovule.
    -In the ovule a mother cell undergoes meiosis to produce haploid embryo sac, within eight nuclei form by mitosis - the ovule stays in the ovary. 
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Fertilisation

  • Where the male and female gamete fuse to form a zygote, the male reaches the female through a pollen tube into the ovary:
    -When a compatible pollen grain lands on the stigma, it produces a sugary solution which causes it to germinate and form a pollen tube.
    -The pollen tube grows down the style and secretes an enzyme as it does; digesting its way into the ovary and collecting nutrients along the way.
    -The pollen tube nucleus is positioned at the tip of the tube, with the other two behind.
    -It grows through the gap in the integuments called the micropyle and passes into the embryonic sac, where the pollen tube nucleus disintegrates.
    -The other two male nuclei are released into the embryonic sac.
    -One of them fuses with the female gamete to form a zygote.
    -The other fuses with both polar bodies to form a triploid endosperm nucleus.
    -Thus double fertilisation occurs, which is unique to flowering plants. 
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Fertilisation

  • Where the male and female gamete fuse to form a zygote, the male reaches the female through a pollen tube into the ovary:
    -When a compatible pollen grain lands on the stigma, it produces a sugary solution which causes it to germinate and form a pollen tube.
    -The pollen tube grows down the style and secretes an enzyme as it does; digesting its way into the ovary and collecting nutrients along the way.
    -The pollen tube nucleus is positioned at the tip of the tube, with the other two behind.
    -It grows through the gap in the integuments called the micropyle and passes into the embryonic sac, where the pollen tube nucleus disintegrates.
    -The other two male nuclei are released into the embryonic sac.
    -One of them fuses with the female gamete to form a zygote.
    -The other fuses with both polar bodies to form a triploid endosperm nucleus.
    -Thus double fertilisation occurs, which is unique to flowering plants. 
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Development

  • Following fertilisation:
    -The seed develops from the fertilised ovule and contains the food store.
    -The diploid zygote divides by mitosis to form the embryo, consisting of a plumule (developing shoot) and radicle (developing root), and one or two leaves.
    -The triploid endosperm develops into a food store for the embryo.
    -The integuments become the seed coat.
    -The ovule becomes the seed.
    -The ovary becomes the fruit. 
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Structure Of The Seed

  • Broad bean is classed as a dicotyledon, as it has two seed leaves, whereas maize is a monocotyledon as it only has one seed leaf. In the first the food source has been absorbed into the leaves but the second it surrounds the seed leaf.
  • Germination of Vicia faba:
    -After dormancy and when environmental factors are favourable, the stored food will mobilise and the seed will germinate, it requires:
    1. A suitable temperature for the enzymes to operate at.
    2. Water for the mobilisation.
    3. Oxygen for respiration.
  • The food reserves must be broken down into small molecules to be mobilised, and transported to the growing plumule and radicle. Water is taken up initially to mobilise enzymes and causing the seed to swell; enzymes such as amylase, and protease are essential.
    If the depth of plantation is correct then the plumule will push up in a hook shape to protect itself and then unfurl to photosynthesise as the food reserve will be gone. 
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Dry Mass Of Seed Over Time

  • Total mass first decreases with use of food store, then increases with growth.
  • Endosperm is used up and thus decreases in mass.
  • And the shoot/root grow and gain mass.
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