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LIFE CYCLES OF PLANTS

LIFE CYCLES OF PLANTS. ALTERNATION OF GENERATIONS. DEFINITIONS OF THE FOLLOWING:. 1. Alternation of generations : haploid (n) stage that produces gametes followed by diploid (2n) stage producing spores.

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LIFE CYCLES OF PLANTS

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  1. LIFE CYCLES OF PLANTS ALTERNATION OF GENERATIONS

  2. DEFINITIONS OF THE FOLLOWING: 1. Alternation of generations: haploid (n) stage that produces gametes followed by diploid (2n) stage producing spores. 2. Haploid: gametes; half the number of chromosomes in somatic cell, indicated by n, result of meiosis. 3. Diploid: somatic cells; double number of chromosomes in gametes, indicated by 2n; result of fertilisation.

  3. More terminology ... 1. Gametophyte: generation producing gametes in gametangium/ gametangia sexual stage. 2. Sporophyte: generation producing spores in sporangium/ sporangia – asexual stage. NB: In higher plants the generations alternate in the life cycle of a plant; in lower plants reproduction is either through gametes (sexual) or spores etc. (asexual).

  4. All terrestrial plants have two phases in their life cycles ... 1. A haploid gametophyte generation that produces gametes (n) by mitosis ... the gametes (n) fuse to form zygote (2n), the start of ... 2. A diploid sporophytegeneration that produces spores (n) by meiosis.

  5. Three groups of plants with life cycles showing alternation of generations: • Mosses (Bryophyta) • Ferns (Pterophyta/ Pteridophyta) • Seed plants (Spermatophyta) • Gymnospermae (cone-bearing plants producing seeds) • Angiospermae (flowering plants producing seeds) Exam guidelines require study of mosses and flowering plants to highlight alternation of generations.

  6. MOSSES Leaf-like structures of moss gametophyte Sporangium of moss sporophyte

  7. FERNS Fern leaf (sporophyte) with sori (groups of sporangia

  8. SEED PLANTS GYMNOSPERMS ANGIOSPERMS

  9. Gymnosperms have two types of sporangia – female and male Female cones at different ages Male cones

  10. Flowering plants

  11. Which generation is dominant in which plant group? Mosses Gametophyte dominant Ferns, gymnosperms and flowering plants Sporophyte dominant

  12. LYFE CYCLES IN ANIMALS INSECT METAMORPHOSIS

  13. METAMORPHOSIS • Physical changes from one stage in life cycle to another. • TWO TYPES OF METAMORPHOSIS: • 1. Complete metamorphosis • 2. Incomplete metamorphosis

  14. Comparison of metamorphosis • COMPLETE • Four stages: • 1. egg • 2. larva • 3. pupa • 4. adult • E.g. Butterflies, bees, housefly • INCOMPLETE • Three stages: • 1. egg • 2. nymph • 3. adult • E.g. Locust, cricket, cockroach 88 % of all insects 12 % of insects

  15. Complete metamorphosis

  16. Incomplete metamorphosis

  17. Advantages and disadvantages of complete metamorphosis • Advantages: • Larva and adult occupy different habitats and niches – not in competition. • Carrying capacity of ecosystem increased. • Egg and pupa stages allow insect to survive unfavourable conditions. • Disadvantages: • Larval forms vulnerable to predators. • Suitable habitat for laying of eggs needed. • Change from pupa to adult requires high energy-input.

  18. Advantages and disadvantages of incomplete metamorphosis • Advantages: • Vulnerable pupa stage avoided. • Less energy is needed – no drastic changes from pupa to adult. • All stages can eat same food. • Disadvantages: • Nymph and adult often compete for same food. • Insect vulnerable to dehydration and predators when moulting. • Entire insect population could be threatened by environmental changes.

  19. Amphibian metamorphosis

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