Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. While processes such as insemination or pollination which happen before the fusion of gametes are also sometimes informally referred to as fertilisation, these are technically separate processes. The cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. History In Antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging gradually, in a mode called by him as epigenetic. In 1784, Spallanzani established the need of interaction between the female's ovum and male's sperm to form a zygote in frogs. In 1827, von Baer observed a therian mammalian egg for the first time. Oscar Hertwig (1876), in Germany, described the fusion of nuclei of spermatozoa and of ova from sea urchin. Evolution The evolution of fertilisation is related to the origin of meiosis, as both are part of sexual reproduction, originated in eukaryotes. There are two conflicting theories on how the couple meiosis–fertilisation arose. One is that it evolved from prokaryotic sex (bacterial recombination) as eukaryotes evolved from prokaryotes. The other is that mitosis originated meiosis. Fertilisation in plants The gametes that participate in fertilisation of plants are the sperm (male) and the egg (female) cell. Various families of plants have differing methods by which the gametes produced by the male and female gametophytes come together and are fertilised. In Bryophyte land plants, fertilisation of the sperm and egg takes place within the archegonium. In seed plants, the male gametophyte is called a pollen grain. After pollination, the pollen grain germinates, and a pollen tube grows and penetrates the ovule through a tiny pore called a micropyle. The sperm are transferred from the pollen through the pollen tube to the ovule where the egg is fertilised. In flowering plants, two sperm cells are released from the pollen grain, and a second fertilisation event involving the second sperm cell and the central cell of the ovule, which is a second female gamete. Pollen tube growth Unlike animal sperm which is motile, plant sperm is immotile and relies on