LIFE CYCLES OF PLANTS

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 sporophyte generation that produces spores (n) by meiosis.

6. 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.

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

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

11. SEED PLANTS GYMNOSPERMS ANGIOSPERMS

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

14. Flowering plants

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