plant reproduction

• Flower


  The organ responsible for sexual reproduction in a plant. The gametes are created, released and combine in the flower.
• Female gamete


  The ovule
• Male gamete


  The pollen
• Pollination


  The transfer of pollen to the stigma
• Types of pollination


  • Self-pollination
  • Cross-pollination
• Self-pollination


  Pollen from the plant is transferred to the stigma of the same plant
• Cross-pollination


  Pollen from one plant is transferred to the stigma of a different plant of the same species
• Variation in self-pollination


  • Only from mutation, independent assortment and crossing over in meiosis of gamete creation
• Variation in cross-pollination


  • From meiosis, mutation and having genetic material from 2 different parent plants
• Self-pollination


  Good for successful plants in a stable environment, but species will be unable to adapt to change
• Cross-pollination


  Species more likely to survive a changing environment due to varied offspring
• Ways plants prevent self-pollination

  • Chemical self-incompatibility - gametes from the same plant cannot combine
  • Irregular flower structure
• Dicotyledonous, insect pollinated flower structure

  • Corolla of petals - colourful and scented to attract insects
  • Carpel (female) - made of a sticky stigma to collect pollen from insects' bodies
  • Stamen (male) - made of the anther and filament, tucked inside the flower so insect rubs past picking up pollen
  • Receptacle and calyx - outer ring of sepals that cover the flower in bud
• Wind pollinated flower structure

  • Petals are often lacking or if present small and green
  • Carpel - large and feathery, hanging outside the flower to collect pollen blown past
  • Anther - large and hanging outside the flower, so the small light pollen gets carried away
• Pollen grain development


  1. Pollen mother cells develop by mitosis

  2. Meiosis occurs to produce a tetrad of 4 haploid cells

  3. In each haploid, pollen grain meiotic division of the nucleus forms a generative nucleus and a tube nucleus
• Tapetum


  A layer of cells that provide nutrients to developing pollen grains
• Anther dehiscence


  1. Tension in lateral grooves increases as the anther dries out

  2. Walls of the pollen sac curl away exposing pollen grains to wind or insects
• Fertilisation


  1. Pollen grain on a compatible stigma produces hydrolase enzymes, forming a pollen tube leading to the micropyle of the embryo sac

  2. One male gamete enters the embryo sac and fuses with the female gamete to produce a diploid zygote

  3. The second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus
• Embryo sac


  • Contains 3 antipodal cells, 2 polar nuclei, and the female gamete with 2 synergids either side
• Ovule formation


  1. A megaspore mother cell carries out meiosis and 4 haploid nuclei are formed

  2. 3 nuclei degenerate and 1 divides by mitosis three times to produce the cells in the embryo sac
• Seeds


  An adaptation to terrestrial life. They can survive very dry conditions, have enough of a food store to provide food until the new plant can photosynthesise and plants have developed different mechanisms for dispersal of the seeds to reduce competition with parent plants
• Seed development after fertilisation

  1. Ovule develops into a seed

  2. Diploid zygote divides by mitosis to form the diploid embryo, made of the plumule, radicle and 1 or 2 cotyledons

  3. Triploid endosperm nucleus divides by mitosis to form endosperm tissue, an important food storage tissue

  4. Integuments develop into the testa

  5. Micropyle - a pore in the testa

  6. Ovary develops into a fruit wall, enclosing seeds
• Germination


  The series of biochemical and physiological processes through which a seed becomes a photosynthesising plant, independent of the food stores in the cotyledons
• Conditions for germination


  • Suitable temperature (between 5°C and 30°C)
  • Water
  • Oxygen
• Broad bean seed germination

  • Begins with water entering the micropyle
  • Water enters the seed and swells the cotyledons, splitting the testa
  • The split testa allows more oxygen in for aerobic respiration
  • Starch and proteins stored in the cotyledons are mobilised through hydrolysis
  • The starch and proteins are used as sources of energy for use in respiration and the growth of radicle and plumule
• Maize seed germination


  • Water enters the seed
  • The embryo releases the plant hormone gibberellin
  • Gibberellin diffuses to the protein rich aleurone layer and amylase enzymes are made to break down stored starch in the endosperm
  • Glucose diffuses to the embryo and used for aerobic respiration and growth
• Broad bean seed structure

  • Testa
  • Cotyledons (two present, dicotyledon)
  • Funicle
  • Radicle
  • Micropyle
  • Hilum (scar of the funicle)
  • Plumule
• Maize seed structure


  • Testa and pericarp fused
  • Cotyledon (one present, monocotyledon)
  • Endosperm stores starch which provides a food source for the growing embryo