Fertilisation
Fertilisation is the fusion of haploid male and female gametes; the resulting cell is the zygote, in which the diploid set is restored: one of each homologous pair from the father and one from the mother. Fish and amphibians living in water often have external fertilisation (gametes are released into water), while land animals have internal fertilisation. The spermatozoon dissolves the egg covering with enzymes of its acrosome, and its nucleus enters the egg and fuses with the egg nucleus; usually only one spermatozoon fertilises. Flowering plants have double fertilisation (S. G. Navashin, 1898): of two sperm cells arriving through the pollen tube, one fuses with the egg cell to form the zygote (embryo, 2n) and the other with the central cell to form triploid (3n) endosperm, the reserve food of the embryo. Parthenogenesis is development from an unfertilised egg cell: in nature it occurs in Daphnia and in honeybees (the drone).
“Double fertilisation scheme”. Groups draw a flower diagram: anther, pollen tube, two sperm cells, egg cell, central cell. Show with arrows who fuses with whom and write the chromosome set (n, 2n or 3n).