• Flowers are the?reproductive organs?of plants
• They usually contain both male and female reproductive parts
• Plants produce?pollen, which contains a nucleus inside that is the?male gamete
• Unlike the male gamete in humans (sperm), pollen is?not capable of locomotion?(moving from one place to another)
• This means plants have to have mechanisms in place to?transfer?pollen from the?anther?(male part of the flower) to the?stigma?(female part of the flower)
• This process is known as?pollination?and there are two main mechanisms by which it occurs:
  • Pollination where the pollen is transferred by?insects?(or sometimes other animals like birds or bats)
  • Pollination where the pollen is transferred by the?wind

   

• The?structure?of insect and wind-pollinated flowers are?slightly different?as each is?adapted?for their?specific function

The general structure of a typical flower

Parts of the Flower Table

Insect-pollinated flowers

• For the flowers of many plant species, the?pollinating agents?are?insects?(e.g. bees)
• Insects often visit these flowers to collect?nectar?(a sugary substance produced by insect-pollinated flowers and the base of their petals, which provides the insects with?energy)
• As an insect enters the flowers in search of nectar, it often brushes against the?anthers, which deposit?sticky pollen?onto the insect's?body
• When the insect visits another flower, it may brush against the?stigma?of this second flower and in the process, may?deposit?some of the pollen from the?first flower, resulting in?pollination
• The structures of an insect-pollinated flower ensure that the flower is well-adapted for pollination by insects

Features of an Insect-pollinated Flower Table

Wind-pollinated flowers

• For wind-pollinated flowers, the process of pollination is more?random?than it is for insect-pollinated flowers
• When ripe, the?anthers open?and?shed their pollen?into the open air
• The pollen is then either blown by the wind or carried by air currents until it (by chance) lands on the?stigma?of a plant of the same species, resulting in?pollination
• The structures of a wind-pollinated flower ensure that the flower is well-adapted for pollination by the wind

Features of a Wind-pollinated Flower Table

Cross-pollination and self-pollination

• In insect or wind-pollinated plants, pollination can either occur via?cross-pollination?or?self-pollination
• Cross-pollination occurs when the?pollen from one plant?is transferred to the stigma of?another plant of the same species
• This is the way most plants carry out pollination as it?increases the genetic variation in the offspring
• Occasionally, the pollen from a flower can?land on its own stigma?or on the stigma of another flower on the?same plant?- this is known as self-pollination
• Self-pollination?reduces the genetic variation in the offspring?as all the gametes come from the?same parent?(and are therefore?genetically identical)
• Lack of variation in the offspring is a disadvantage if environmental conditions change, as it is?less likely that any offspring will have adaptations that suit the new conditions?well

• In plants, the?ovary?contains one or more?ovules
• The ovules are the structures that eventually develop into?seeds
• Each ovule contains an?ovum
• An ovum is an?egg cell?that contains the?female nucleus?that a?male pollen nucleus?can?fuse with
• In plants,?fertilisation?occurs when the pollen grain nucleus fuses with the ovum nucleus

The growth of the pollen tube

• Unlike the male gametes in animals (sperm), the pollen grain has no ‘tail’ to swim to the ovary of the flower
• Instead, in order to reach the ovum nucleus, the pollen grain grows a?pollen tube
  • This only happens if the pollen grain has landed on the right kind of?stigma?(i.e. of the?same species?as the flower the pollen came from)

   

• The?nucleus?inside the pollen grain?moves down the tube?as the tube grows down the?style?towards the?ovary?(which contains the ovule that, in turn, contains the ovum)
• Once the nucleus of the pollen grain and the nucleus of the ovum have?fused?(joined together), that particular ovule has been?fertilised?and a?zygote?has been formed
  • The zygote will then start to?divide?(it is the structure that eventually develops into an?embryo plant)

   

The process of fertilisation in a typical plant

Seed and fruit formation

• After fertilisation, the?ovule?(that contains the zygote) develops into the?seed
• The?wall of the ovule?develops into the?seed coat, known as the?testa
• The parts of the flower surrounding the ovule (mainly the?ovary walls) develop into the?fruit, which contains the seeds
  • The fruit provides a mechanism for?seed dispersal?(getting the seeds?away from the parent plant)
  • Some fruits are eaten by animals, which then disperse the seeds in their droppings (the tough outer coat of seeds stops them from being digested)
  • Some other fruits have sticky hooks that get caught in the fur of passing animals

   

• As different plants have different numbers of ovules, this explains why different fruits have different numbers of seeds (which develop from the ovules)

• Germination?is the?start of growth?in the seed
• The seed contains the?zygote?(the fertilised egg cell), which?divides?into cells that then develop into the?embryo plant
• When the seed?germinates, this embryo begins to grow into the young?seedling
• Structures known as?cotyledons?surround the embryo
  • Some plants have?one?cotyledon, whereas others have?two
  • The cotyledons contain?food reserves?that supply the young seedling with?food?(and, therefore,?energy for growth) when the seed starts to germinate
  • The cotyledons fulfil this role?until the young plant grows its own leaves?and becomes capable of?making its own food via photosynthesis

   

• After taking in water, the?seed coat?(testa)?splits
• This leads to the production of the?plumule?(the?first emerging shoot) and?radicle?(the?first emerging root)

The structures of a germinating seed

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