16.1.3 Sexual Reproduction in Plants

• Flowers are thereproductive organof the plant
• They usually contain both male and female reproductive parts
• Plants producepollenwhich contains a nucleus inside that is themale gamete
• Unlike the male gamete in humans (sperm), pollen isnot capable of locomotion(moving from one place to another)
• This means plants have to have mechanisms in place totransfer pollen from the anther to the stigma
• This process is known aspollinationand there are two main mechanisms by which it occurs: transferred byinsects(or other animals like birds) or transferred bywind
• 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 producesmaller amountsoflarger, heavierpollen grains that often containspikes or hookson the outside so they are better able to stick to insects
  • Wind pollinated flowers producelarge amountsofsmall, lightweightpollen grains that are usuallysmooth

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

Amultiple-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 apollen nucleus fuses with an ovum nucleusin 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 apollen 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 grainslips down the tubeas it grows down the style towards the ovary
• The ovary contains one or moreovules每个包含一个卵子菲male nucleus that a male pollen nucleus can fuse with
• Once the nuclei (pl) have joined together, that ovule has been fertilised and azygotehas been formed
• The zygote will start to divide and eventually form aseedwithin the ovule
• 不同的植物有不同数量的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

• Germinationis thestart of growth in the seed
• Three factors are required for successful germination:
  • 水- 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 thepollen from one plantis transferred to the stigma ofanother plant of the same species
• This is the way most plants carry out pollination as itimproves genetic variation
• Occasionally, the pollen from a flower canland on its own stigmaor on thestigma of another flower on the same plant- this is known as self-pollination
• Self-pollinationreduces genetic variety of the offspringas 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 isless likely that any offspring will have adaptations that suit the new conditionswell
• On the other hand, cross-pollination relies completely on the presence ofpollinatorsand this can be a problem if those pollinators aremissing(eg the reduction inbeenumbers 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