Edexcel IGCSE Biology 复习笔记 3.1.2 Sexual Reproduction in Flowering Plants

• 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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