Pollination (DP IB Biology)

Promoting Cross-Pollination

• 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

Insect-pollinated flower features

• 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

Wind-pollinated flower features

• 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

• 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

• Cross-pollination relies completely on the presence of pollinators and this can be a problem if those pollinators are missing (e.g. 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

• In addition to the mechanisms described above for insect and wind pollinated plants, plants also have a variety of other methods to ensure successful cross-pollination

  • Different maturation times for the pollen and ovules of the same flower. This prevents self-pollination from occurring

  • Self-incompatibility mechanisms are used in some species that ensure if pollen lands on the stigma from the same plant the plant produces chemicals that ensure a pollen tube does not grow

  • Plants can produce flowers that only have either male or female parts or the whole plant is either male or female

  • Wind-pollinated plants are less likely to self-pollinate due to the wind carrying the pollen far from the parent plant

Preventing Self-Pollination

• 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 results in less genetic variation in the offspring in comparison to cross-pollination

  • No new alleles are introduced into the offspring from other individuals

• 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

• Genetic mechanisms in many plant species ensure male and female gametes fusing during fertilisation are from different plants

  • Each plant has a set of genes that controls the growth of a pollen tube so that when pollen lands on the stigma of a flower of the same plant protein interactions occur that prevent the growth of a pollen tube

  • This is an example of a self-incompatibility mechanism

  • The mechanism may include

    • A pollen grain fails to germinate into a pollen tube

    • A pollen grain germinates but does not enter the style

    • The pollen nuclei enters the ovule but it degenerates before fertilisation can occur

    • Fertilisation occurs but the embryo degenerates before growth is established