LIFE CYCLES OF PLANTS

ALTERNATION OF GENERATIONS

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.

DEFINITIONS OF THE FOLLOWING:

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).

More terminology ...

1. A haploid gametophyte generation that produces gametes (n) by mitosis ...

the gametes (n) fuse toform zygote (2n), the start of ...

2. A diploid sporophyte generation that

produces spores (n) by meiosis.

All terrestrial plants have two phases in their life cycles ...

Mosses (Bryophyta) Ferns (Pterophyta/ Pteridophyta) Seed plants (Spermatophyta)1. Gymnospermae (cone-bearing plants producing

seeds)2. Angiospermae (flowering plants producing seeds)

Exam guidelines require study of mosses and flowering plants to highlight alternation of generations.

Three groups of plants with life cycles showing alternation of generations:

MOSSES Leaf-like structures of moss gametophyte

Sporangium of moss sporophyte

FERNS

Fern leaf (sporophyte) with sori (groups of sporangia

SEED PLANTS

GYMNOSPERMS

ANGIOSPERMS

Female cones at different ages Male cones

Gymnosperms have two types of sporangia – female and male

Flowering plants

Which generation is dominant in which plant group?Mosses

Gametophyte dominant

Ferns, gymnosperms and flowering plants

Sporophyte dominant

LYFE CYCLES IN ANIMALS

INSECT METAMORPHOSIS

Physical changes from one stage in life cycle to another.

TWO TYPES OF METAMORPHOSIS:

1. Complete metamorphosis

2. Incomplete metamorphosis

METAMORPHOSIS

Comparison of metamorphosis

88 % of all insects 12 % of insects

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

Complete metamorphosis

Incomplete metamorphosis

Advantages and disadvantages of complete metamorphosis Advantages:1. Larva and adult

occupy different habitats and niches – not in competition.

2. Carrying capacity of ecosystem increased.

3. Egg and pupa stages allow insect to survive unfavourable conditions.

Disadvantages:1. Larval forms

vulnerable to predators.

2. Suitable habitat for laying of eggs needed.

3. Change from pupa to adult requires high energy-input.

Advantages and disadvantages of incomplete metamorphosis Advantages:1. Vulnerable pupa

stage avoided.2. Less energy is

needed – no drastic changes from pupa to adult.

3. All stages can eat same food.

Disadvantages:1. Nymph and adult often

compete for same food.2. Insect vulnerable to

dehydration and predators when moulting.

3. Entire insect population could be threatened by environmental changes.

Amphibian metamorphosis