Sexual Reproduction in Plants (CIE IGCSE Biology)

• Flowers are the reproductive organ of the plant
• 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 to the stigma
• This process is known as pollination and there are two main mechanisms by which it occurs: transferred by insects (or other animals like birds) or transferred by wind
• The structure of insect and wind-pollinated flowers are slightly different as each is adapted for their specific function

Parts of the flower

General flower structure

Structure of a flower

Features of an insect-pollinated flower 

Features of a wind-pollinated flower 

• The pollen produced by insect and wind-pollinated flowers is also different:
  • Insect pollinated flowers produce smaller amounts of larger, heavier pollen grains that often contain spikes or hooks on the outside so they are better able to stick to insects
  • Wind pollinated flowers produce large amounts of small, lightweight pollen grains that are usually smooth 

Here is an example of a multiple-choice question asking students to use their knowledge to identify types of pollen grain:

A multiple-choice question asking students to use their knowledge to identify types of pollen grain

Pollination

• Pollination is discussed above as the ways in which pollen grains can be transferred from an anther to a stigma

Fertilisation

• Fertilisation occurs when a pollen nucleus fuses with an ovum nucleus in the ovule
• As the pollen has no ‘tail’ to swim to the ovary of a plant, in order to reach the ‘female’ nucleus in the ovary it has to grow 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 slips down the tube as it grows down the style towards the ovary
• The ovary contains one or more ovules which each contain an ovum with a female nucleus that a male pollen nucleus can fuse with
• Once the nuclei (pl) have joined together, that ovule has been fertilised and a zygote has been formed
• The zygote will start to divide and eventually form a seed within the ovule
• As different plants have different numbers of ovules, this explains why different fruits (which develop from the ovary) have different numbers of seeds (which develop from the ovules)

Growth of a pollen tube

Fertilisation in a flowering plant

• Germination is the start of growth in the seed
• Three factors are required for successful germination:
  • Water - allows the seed to swell up and the enzymes in the embryo to start working so that growth can occur
  • Oxygen - so that energy can be released for germination
  • Warmth - germination improves as temperature rises (up to a maximum) as the reactions which take place are controlled by enzymes

• As carbon dioxide is not necessary for germination but also does not inhibit it, it makes no difference whether it is present or not

Investigating Germination

• Set up 4 boiling tubes each containing 10 cress seeds on cotton wool
• Set each test tube as shown in diagram below
• Leave tubes in set environment for a period of time: A, B and C incubated at 20°C; D placed in a fridge at 4°C
• Compare results and see which tube has the greatest number of germinated seeds

Conditions required for germination

 Conditions required for germination - results:

Self- & Cross-Pollination: Extended

• 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 improves genetic variation
• 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 genetic variety of 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
• On the other hand, cross-pollination relies completely on the presence of pollinators and this can be a problem if those pollinators are missing (eg the reduction in bee numbers is of great importance to humans as bees pollinate a large number of food crops) - this doesn’t apply to wind-pollinated plants

Growth of the pollen tube and its entry into the ovule followed by fertilisation