Sexual reproduction in flowering plants

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Sexual reproduction in flowering plants
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Flower structure
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• The calyx consists of sepals, which are usually
  green and small. They enclose and protect the
  rest of the flower while it is in the bud.
• The corolla consists of petals, which are often
  coloured and scented. They attract insects that
  visit the flowers and collect nectar and pollen,
  pollinating the flowers as they do so. Small
  grooves or darker lines in the petals called
  'nectar guides' are thought to direct the insect
  to the nectaries within the flower. The calyx and
  corolla are collectively known as the perianth.
  The term is important in describing many
  monocotyledons, where there is no obvious
  distinction between petals and sepals.
• The androecium is the male part of the flower and
  consists of stamens. The stalk of the stamen is
  the filament. At the end of the filament is an
  anther that contains pollen grains in four pollen
  sacs. The pollen grains contain the male
  reproductive cells or gametes.
• The gynoecium (pistil) is the female part of the
  flower. It consists of carpels, which may be
  either single and solitary, many and separate
  from each other, or few and joined together. In
  all of them, the ovules, which contain the female
  gametes, are enclosed in the ovary. Extending
  from the ovary is a style, expanded or divided at
  one end into a stigma, which will receive pollen
  from another flower. The ovules when fertilised
  will become seeds, while the whole ovary will
  become the fruit. The wall of the ovary develops
  into the pericarp of the fruit.
• Nectaries are glandular swellings, often at the
  base of the ovary or on the receptacle, which
  produce a sugary solution called nectar. Insects
  visit the flower and drink or collect this
  nectar.

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Pollination

• The transfer of pollen from anthers to stigma is
  called pollination. Cross-pollination is the
  transfer of pollen from the anthers of one flower
  to the stigma of another flower of the same
  species. In some species self-pollination occurs.
  In cross-pollination pollen is usually
  transferred on the bodies of insects entering the
  flowers, or by chance air-currents carrying the
  pollen from one flower to the next. The
  structures of many flowers are closely adapted to
  the method of insect or wind pollination.
• Insect-pollinated flowers are brightly coloured
  and produce nectar and scent to attract insect
  visitors.
• Wind-pollinated flowers are often a dull colour
  and are adapted to distribute large quantities of
  pollen far and wide.

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Fertilisation in plants
Fertilisation follows pollination, but the
interval of time between the two events varies in
different species from sixteen hours to twelve
months. The pollen grain absorbs substances
secreted by the stigma, and the cytoplasm in the
grain grows out as a tube. This tube grows down
through the style between the cells. On reaching
the ovary it grows to one of the ovules and
enters it through a hole, the micropyle. The tip
of the pollen tube breaks open in the ovule, and
the male nucleus, which has been passing down the
tube, enters the ovule and fuses with the female
nucleus there. Each egg cell of an ovule can only
be fertilised by a male nucleus from a separate
pollen grain.
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Result of fertilisation
Fruit and seed formation. After fertilisation the
petals, stamens, style and stigma wither and
usually fall off. The sepals may persist in a
dried and shrivelled form. Food made in the
leaves reaches the fertilised ovules and the
ovary, which grow rapidly. Inside the ovule, cell
division and growth produce a seed containing a
potential plant or embryo. The integuments of the
ovules become thicker and harder, forming the
testa of the seeds, and finally, water is
withdrawn from the seeds, making them dry and
hard. The ovary wall may become dry and hard,
forming a capsule, or pod, or it may become
succulent and fleshy forming a fruit. Fruits. The
term fruit means the fertilized ovary of a
flower. The products of flowering plants that are
grown, eaten and sold as fruits have an ovary
wall that is fleshy and edible. A dehiscent fruit
is one that opens to release its seeds.
Indehiscent fruit does not open it falls from
the plant as a whole and may have to decay before
the seeds are able to germinate. The part of the
fruit or ovary to which the seeds are attached is
called the placenta.
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Dispersal of fruits and seeds

• When flowering is over and the seeds are mature
  the whole ovary, or the individual seeds, fall
  from the parent plant to the ground, where if
  conditions are suitable germination will
  subsequently take place. In many plants, the
  fruits or seeds are adapted in such a way that
  they are distributed away from the parent plant,
  this helps to reduce overcrowding among and
  competition between members of the same species
  for light, air, water and mineral salts, and
  results in the colonisation of new areas.
• Wind dispersal
• Censer mechanism. The ovary becomes a dry capsule
  that partially opens at the top. The capsule is
  at the end of a long stalk that is shaken to and
  fro by the wind. The seeds that have become
  detached from the placenta are shaken out and
  scattered. e.g. Mexican poppy (Argemone).
• 'Parachute' fruits and seeds. Feathery hairs
  projecting from the fruit or seed increase its
  surface area so much that air resistance to its
  movement is great. As a result it sinks to the
  ground slowly and is likely to be carried great
  distances from the parent plant by slight
  air-currents, e.g. Sonchus.
• Winged seeds. Some seeds have papery extensions
  formed from the placenta or testa making
  wing-like structures. The extra surface area of
  these wings offers increased air resistance, so
  delaying the fall of the seed and increasing its
  chances of being blown away from its parent plant
  by the wind.

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Dispersal of fruits and seeds

• Animal dispersal
• Mammals hooked fruits. The hooks that develop on
  some seeds catch in the fur of passing mammals or
  in the clothing of people. The seeds may fall
  from the fruit during the animals movements or
  the whole fruit may be brushed off or scratched
  off some distance from the parent plant. If the
  seeds fall in a situation where there is adequate
  soil, moisture and light, they will germinate to
  new plants.
• Mammals and birds succulent fruits. The
  succulent texture and in some cases the bright
  colour of these fruits attract birds and mammals.
  Sometimes, as in the guava (Psidium), the fleshy
  part of the fruit is eaten and the seeds with
  their resistant seed coats pass undigested
  through the animals alimentary canal, to be
  dropped with the faeces some distance from the
  parent plant. In the case of the mango and
  avocado, the fruit may be carried away from the
  parent tree, the flesh is eaten and the seed
  discharged.
• Self-dispersal
• The pods formed by the flowers of the bean
  (Phaseolus) dry and shrivel. The tough fibres in
  the pericarp shrink and set up a tension. When
  the pod splits in half, down the two lines of
  weakness, the two halves twist up trapping the
  seeds between the coils. As the coils tighten the
  seeds are suddenly squeezed out and projected
  away from the parent.
• Note Move your mouse over the images.

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The End
5A Ngan Chi Chuen