Reproduction in Plants and Humans

From flower to seedling:

• Pollination: pollen moves from the anther to the stigma. Self-pollination uses the same flower or plant; cross-pollination uses another plant.
• Fertilisation: a male gamete fuses with the egg to form a diploid zygote.
• The zygote divides to form the embryo; the ovule becomes the seed and the ovary the fruit. The seed germinates into a seedling.

Inside the ovule:

• The embryo sac comes from a haploid megaspore, so its cells are haploid: the egg, two synergids and three antipodals (the central cell has two haploid nuclei).
• The nucellus around it is tissue of the parent plant, so it is diploid.

Selfing as a test: a pure-breeding tall plant, when self-pollinated, gives only tall plants; a hybrid tall plant gives about one short plant in four (Mendel's 3 : 1).

Apomixis: forming an embryo without fertilisation. Three ways it happens:

• Parthenogenesis: the embryo grows from an unfertilised egg.
• Apogamy: it grows from another cell of the embryo sac, not the egg.
• Adventive embryony (sporophytic budding): it grows from a cell of the nucellus or integument, as in citrus and mango.

The seed's embryo has the same genes as the mother. This lets a good hybrid keep its qualities, so farmers need not buy new hybrid seed every year.

Human reproduction and the newborn:

• Fertilisation takes place in the oviduct (fallopian tube); the embryo implants in the uterus.
• The placenta links the foetus to the mother for food, oxygen and wastes. Pregnancy (gestation) lasts about nine months.
• Colostrum: the first milk, made in the first few days after birth; rich in antibodies (IgA).
• Antibodies received ready-made, from the mother's milk or across the placenta, give passive immunity. It works at once but does not last, because the baby did not make them and has no memory cells for them.