Reproduction in Plants and Animals

Why this topic matters

WAEC and NECO often ask for a labelled drawing of a flower, and the differences between insect- and wind-pollinated flowers. JAMB tests the human reproductive organs, the menstrual cycle and the stages of germination.

Asexual and sexual reproduction

Asexual reproduction involves one parent and no gametes, and the offspring are identical to the parent (a clone).

• Binary fission: Amoeba and bacteria.
• Budding: yeast and Hydra.
• Spore formation: Rhizopus (bread mould) and ferns.
• Vegetative propagation: cassava stem cuttings, yam tubers, onion bulbs, ginger rhizomes, and banana suckers.

Sexual reproduction involves two parents. Male and female gametes (sex cells) fuse at fertilisation to form a zygote. The offspring show variation, which helps a species survive changes in its environment.

The flower

• Sepals (the calyx) protect the flower in bud.
• Petals (the corolla) are brightly coloured and scented to attract insects.
• Stamen (male part) = anther, which makes pollen grains, + filament, which holds up the anther.
• Carpel (female part) = stigma, which receives pollen + style + ovary, which contains the ovules.
• Nectaries make sugary nectar, which rewards visiting insects.

Pollination

Pollination is the transfer of pollen from an anther to a stigma.

• Self-pollination: pollen goes to the stigma of the same flower or another flower on the same plant.
• Cross-pollination: pollen goes to a flower on another plant of the same species. This gives more variation.

Insect-pollinated flowers:

• large, brightly coloured, scented petals, with nectar;
• a small amount of sticky, spiky pollen;
• anthers and stigma inside the flower;
• a sticky, flat stigma.
• Examples: hibiscus, flamboyant.

Wind-pollinated flowers:

• small, green petals, or none, with no scent or nectar;
• a large amount of light, smooth pollen;
• anthers and stigma hanging outside the flower;
• a feathery stigma to catch pollen.
• Examples: maize, grasses.

Fertilisation, seeds and fruits

1. A pollen grain lands on the stigma and grows a pollen tube down the style to the ovule.
2. The male nucleus fuses with the egg nucleus: this is fertilisation, which forms a zygote.
3. After fertilisation:

• the ovule becomes the seed;
• the ovary becomes the fruit;
• the ovary wall becomes the fruit wall (pericarp);
• the integuments become the testa (seed coat).

Dispersal carries seeds away from the parent, which reduces competition:

• Wind: light seeds, or seeds with wings or hairs (e.g. cotton).
• Animals: succulent fruits that are eaten (mango, guava), or hooked fruits that cling to fur.
• Water: fibrous, buoyant fruits (coconut).
• Explosive mechanism: pods split open (pride of Barbados, and bean pods).

Germination

Seeds need water, oxygen and a suitable temperature to germinate. They do not need light at first. Water activates the enzymes, which digest the stored food for the growing embryo.

• Epigeal germination: the cotyledons are carried above the ground (bean).
• Hypogeal germination: the cotyledons stay below the ground (maize).

Human reproduction

Male reproductive system:

• The testes make sperm and the hormone testosterone. They sit in the scrotum, which keeps them cooler than body temperature.
• Sperm passes along the sperm duct. The seminal vesicles and prostate gland add fluid to make semen, which leaves through the urethra in the penis.

Female reproductive system:

• The ovaries release ova (eggs) and make oestrogen and progesterone.
• An egg passes into the oviduct (fallopian tube), where fertilisation takes place.
• The embryo implants in the uterus. The cervix and vagina lead to the outside.

The menstrual cycle (about 28 days)

1. Days 1–5: menstruation. The uterus lining breaks down and is shed.
2. Days 6–13: the lining rebuilds, under oestrogen.
3. About day 14: ovulation, when an egg is released from the ovary.
4. Days 15–28: progesterone keeps the lining thick. If there is no fertilisation, the progesterone falls and the cycle starts again.

Fertilisation and pregnancy

• Fertilisation happens in the oviduct. The zygote divides by mitosis as it moves to the uterus, then implants in the lining.
• The placenta exchanges food and oxygen from the mother for carbon dioxide and urea from the fetus. The two bloods do not mix.
• The umbilical cord joins the fetus to the placenta. The amniotic fluid cushions the fetus.
• Pregnancy (gestation) lasts about 9 months (40 weeks).

Top exam traps

• The ovule becomes the seed and the ovary becomes the fruit. Don't swap them.
• Pollination is the transfer of pollen. Fertilisation is the fusion of the gametes.
• Human fertilisation happens in the oviduct, not the uterus.
• Mother's and fetus's blood do not mix in the placenta.