4.2 Sexual Reproduction in Plants

• Flower


  The organ responsible for sexual reproduction in a plant. The gametes are made, released and combine in the flower. The female gamete is the ovule and the male gamete is 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
• Self-pollination


  The only variation that occurs in the offspring comes from mutation, independent assortment and crossing over in meiosis of gamete formation
• Cross-pollination


  Variation is 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 genetic variation in offspring
• Ways plants prevent self-pollination

  • Chemical self-incompatibility, gametes from the same plant cannot combine
  • Irregular flower structure for example different positioning of anthers and stigmas or different ripening times of anthers or stigmas; some plants like holly have male and female plants
• 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 the insects' bodies
  • Stamen (male) made of the anther and filament is tucked inside the flower, so the insect rubs past picking up pollen
  • Receptacle and the calyx, the 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 is large and feathery and hangs outside the flower to collect pollen blown past
  • Anther is large and hangs outside the flower, so the small light pollen gets carried away
• Pollen grain formation


  1. Pollen mother cells develop by mitosis

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

  3. In each haploid, pollen grain mitotic division of the nucleus forms a generative nucleus (that mitotically divides to produce 2 male nuclei) and a tube nucleus
• Tapetum


  A layer of cells that provide nutrients to developing pollen grains
• Pollen release


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

  2. Dehiscence occurs when 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. Pollen tube growth is negatively aerotrophic (away from air), and postively chemotrophic (towards the embryo sac)

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

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


  • 3 antipodal cells
  • 2 polar nuclei
  • Female gamete with 2 synergids either side
  • Micropyle
  • Integuments
  • Funicle
• 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 8 cells labelled in red on the diagram
• 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, radical 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 is 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


  • Temperature (optimum between 5°C and 30°C)
  • Water (required to make cells turgid, to transport substances and to mobilise enzymes)
  • Oxygen (required for aerobic respiration)
• Broad bean seed germination

  1. Germination begins with water entering the micropyle

  2. Water enters the seed and swells the cotyledons, splitting the testa

  3. The split testa allows more oxygen in for aerobic respiration

  4. Starch and proteins stored in the cotyledons are mobilised through hydrolysis

  5. The starch and proteins are used as sources of energy for use in respiration and the growth of radicle and plumule
• Maize seed germination


  1. Water enters the seed

  2. The embryo releases the plant hormone gibberellin

  3. Gibberellin diffuses to the protein rich aleurone layer and amylase enzymes are made to break down stored starch in the endosperm

  4. Glucose diffuses to the embryo and used for aerobic respiration and growth
• Broad bean seed


  • testa
  • cotyledon (two present, dicotyledon)
  • funicle
  • radicle
  • micropyle
  • hilum (scar of the funicle)
  • plumule
  • endosperm stores starch which provides a food source for the growing embryo
• Maize seed


  • testa and pericarp fused
  • cotyledon one present (monocotyledon)