Plant sexual reproduction

production of flower gametes

• anthers

  • produce pollen containing male gametes

  • diploid cells inside anther divide by meiosis to produce 4 haploid cells, each develops into pollen grain

  • nucleus inside pollen grain divides by mitosis to produce male gametes

• ovules

  • ovoid structures inside the ovary

  • one cell in center of ovule grows particularly large and divides by meiosis

  • 1 of the haploid nuclei produced divides 3 times by mitosis producing 8 haploid nuclei, one of which becomes female gamete

process of sexual reproduction in flower

• pollination

  • transfer of pollen from anther to stigma, either by wind or animal

• fertilisation

  • happens inside an ovule

  • each pollen grain that lands on stigma grows a tube from the grain down to the style to the ovary

  • male nuclei travel down the tube to the ovule and digest a route into it

  • male gametes are released fusing with female gamete, fertilisation occurs, producing a zygote

• embryo development

  • zygote divides repeatedly by mitosis and cells produced develop into a seed

• many flowers are hermaphrodite (have male and female parts) so can act as both male and female parent, so reproduction is sexual

diagram of flower

• sepal

  • protect developing flower

• petal

  • large brightly coloured to attract insects

• anther

  • part of stamen that produces male gametes

  • firmly attached to brush against insects

• pollen

  • contains male nuclei for fertilisation

  • large and spiky so stick to visiting insect

• filament

  • stalk of stamen that holds up anther

• stigma

  • top of carpel, female part of flower, pollen lands here

  • sticky so pollen grains stick to it when insects brushes past

• style

  • part of cartel that supports stigma

• ovary

  • contains ovules

• ovule

  • chamber within ovary where female gametes develop

increasing cross pollination

• cross pollination: transfer of pollen from anther of one flower to stigma of another flower. leads to fusion of male and female gametes from diff plants so promotes genetic variation & therefore evolution

• outside agent required to transfer pollen

• most plants can self pollinate bc they’re hermaphrodites, but inbreeding increases chance of rare recessive allele in one ancestor being inherited twice by an individual, causing genetic disorder

• natural selection therefore favours plants that reproduce by cross pollination, and mechanisms exist to promote it

  • separate male and female plants so anthers and carpels are on diff plants

  • separate male and female flowers on the same plant so anthers and carpels are on diff flowers

  • stigmas and pollen on anthers mature at diff times

self incompatibility mechanisms

• sometimes self pollination does occur despite adaptations to reduce the chances

• in many plants this pollen fails to germinate, or pollen tube stops growing before reaching ovary, this is self incompatibility and it has a genetic basis

• plants with the same self-incompatibility alleles cannot produce offspring together

dispersal and germination of seeds

• seeds often transported long distances from the parent plants

  • helps to spread the species and reduces competition between offspring and parent

  • the function of the fruit that develops from the ovary of the flower

• germination follows seed dispersal

  • happens when conditions are suitable: oxygen, water, warmth

  • food reserves inside seed are mobilised by being digested and transferred to growing embryo