sexual reproduction in plants

• Characteristic of insect pollinated flowers

  • Large, bright coloured petals, attract insect pollinators.
  • Stigma is sticky to trap pollen grains, they are inside the flower where the insect rubs against it depositing the pollen.
  • Anthers are inside the flower where the insect rubs against them, increasing the chance for pollen to be deposited on the body.
  • Pollen grains are large with spiky walls which stick to the insect body
• Characteristic of wind pollinated flowers

  • Petals are small and green or there are no flowers. Flowers are above the leaves, or produced in early spring to gain an increased chance of being carried by wind currents.
  • Stigma is large and feathery and it hangs outside of the flower to catch airborne pollen.
  • Anther is suspended outside the flower to release pollen grains into the wind currents.
  • Large number of small pollen grains are produced and they are easily carried by the wind.
• Female part of the flower

  Carpel:

  • Stigma
  • Style
  • Ovary
• Male part of the flower

  Stamen:

  • Anther
  • Filament
• Pollen development

  • Pollen grains contain male gametes and are formed inside the pollen sec.
  • Area of dehiscence - When pollen grains are mature, outer layer of the anther dries out. Tension in the lateral grooves causes dehiscence, when the walls of the anther pull apart releasing pollen grains.
• Formation of pollen

  1. Diploid mother cell in the pollen sac undergoes meiosis a tetrad, 4 immature haploid pollen cells.
  2. Each pollen cell undergoes mitosis to form a pollen grain with 2 haploid nuclei.
  3. Pollen grain matures consisting of a generative nucleus, a tube nucleus and a cell wall made up of the inline and exine
• Functions of the cell wall in the mature pollen grain

  Prevents the pollen grain from drying out
• Development of the ovule

  1. The mass of the cell (Nucelleus) is surrounded by 2 integuments. The ovule is carried on a short stalk called the funicle. Once cell in the nucelleus enlarges and develops into the megaspore mother cell (2n).
  2. Megaspore mother cell undergoes meiosis and produces 4 haploid megaspores (n).
  3. 3 of the megaspores degenerate and 1 megaspore develops into an embryo sac by mitosis.
  4. 3 mitotic divisions occur forming 8 haploid nuclei.
  5. 2 nuclei move to the centre of the embryo sac forming polar nuclei, the remaining nuclei develop cytoplasm and become separated by cell walls.
  6. 3 of these nuclei become antipodal cells, 1 female gamete closest to the micropyle, and 2 synergids either side of the female gamete.
  7. The synergies will degenerate after fertilisation
• Pollination

  Movement of male gametes to fertilise the female gamete
• Self pollination

  • Transfer of pollen from the anther to the stigma of the same flower or different flower of the same plant.
  • It leads to self fertilisation or inbreeding
  • Less genetic variation
  • There is a greater chance of 2 harmful recessive alleles combining
  • Successful genomes are preserved
• Cross pollination

  • Transfer of pollen from the anther to the stigma of another flower on a different plant of the same species.
  • It leads to cross fertilisation or outbreeding
  • More genetic variation
  • There is a reduced chance of harmful recessive alleles combining
  • Advantageous for evolution
• Adaptations to promote cross pollination

  • Chemical self incompatibility
  • Irregular flower structure
  • Dichogamy
  • monoecious plants
  • dioecious plants
• Chemical self incompatibility

  Gametes from the same plant are unable to fuse and form a zygote, if a zygote forms then it is unable to develop.
• Irregular flower structure

  Stigma is located above the anthers so there is no chance of self pollination
• Dichogamy

  Anthers and stigmas mature at different times
• Monoecious plants

  Separate male and female flowers on the same plant
• Dioecious plants

  Separate male and female plants
• Double fertilisation

  Involves 2 male gametes in 2 separate fertilisation events:

  • Fertilisation of the female gamete to form a diploid zygote
  • Fertilisation of the polar nuclei to form a triploid endosperm nucleus
• Double fertilisation

  1. Pollen grain lands on the stigma and absorbs water. Pollen tube now germinates.
  2. Pollen tube grows down the style under the control of the tube nucleus which codes for the enzyme hydrolase.
  3. During the germination and growth of the pollen tube the generative nucleus divides by mitosis to form 2 male nuclei (gametes).
  4. The growth of the pollen tube is a positive chemotropic response, causing it to grow towards the chemicals secreted by the embryo sac.
  5. Pollen tube grows through the micropyle, a gap between the 2 integuments, and pass into the embryo sac.
  6. Once the pollen tube has entered the pollen sac the tube nucleus disintegrates.
  7. Double fertilisation then occurs, 1 male gamete fuses with the female gamete forming a diploid zygote and the other male gamete fuses with the 2 polar nuclei forming a triploid endosperm nucleus.
  8. After fertilisation, the antipodal cells and synergies play no further role.
• After fertilisation:

  1. Diploid zygotę divides by mitosis to form the diploid embryo which then becomes the young shoot (plumule) and the young root (the radical) and one or two seed leaves (the cotyledons)
  2. Triploid endosperm tissue divides by mitosis to form the endosperm tissue which forms the food source for the growing embryo
• In monocotyledons endosperm remains and only 1 cotyledon remains
• In dicotyledons the endosperm is quickly absorbed and stored in 2 cotyledons
• After fertilisation

  Ovule --> Seed

  Integuments --> seed coat (testa)

  Micropyle --> pore

  Ovary wall --> Fruit wall (pericarp)

  Content of ovary --> Fruit

  Attachment point of funicle to ovule --> Hilum
• Germination

  Involves the rapid onset of biochemical activity and growth until the plant can carry out photosynthesis and become independent of the food stores contained in the cotyledons or endosperm.
• Conditions for germination

  Water - to mobilise the enzymes; for transport; and vacuole cells making them turgid

  Oxygen - for aerobic respiration

  Temperature - suitable temperature for the enzymes to operate.