Flowering plants fertilise twice, and a burst of oxygen chemistry releases the sperm
Most plant sperm cannot swim. Instead, a pollen grain grows a tube that tunnels through the flower to the ovule, where a chemical signal from the female tissue makes the tube burst open. In flowering plants two sperm are released: one fertilises the egg and the second fuses with another cell entirely.
Fertilisation is the fusion of two gametes into a zygote, the first cell of a new individual. Steps that come before it, such as insemination or pollination, are sometimes loosely called fertilisation but are technically separate. The process is tied to the origin of meiosis; both evolved in eukaryotes as parts of sexual reproduction, and one hypothesis holds that meiosis grew out of ordinary cell division.
Understanding came in stages. Aristotle imagined new life arising from mingled male and female fluids, with form emerging gradually. In 1784 Spallanzani showed with frogs that egg and sperm must meet to form a zygote. Karl Ernst von Baer first saw a mammalian egg in 1827, and in 1876 Oscar Hertwig described the nuclei of sperm and egg merging in sea urchins.
Plants handle the meeting differently from animals. In mosses and ferns it happens inside a structure called the archegonium. In seed plants the pollen tube pushes through the stigma and style, enters the ovule by a tiny pore called the micropyle and releases its cargo. How the tube finds its way was poorly understood until 1995, when work on tobacco identified TTS proteins that tripled the growth rate of pollen tubes in the lab. Tubes grew straight towards patches of the protein, and plants unable to make it were less fertile.
The final release is controlled by the female side. In the plant Arabidopsis, FER protein kinases in the ovule regulate reactive oxygen species, which peak when the ovule is most receptive. These molecules open calcium channels in the pollen tube; the flood of calcium makes it rupture. Suppressing them with a chemical blocker stopped the tubes from bursting. In angiosperms the second sperm joins the central cell to make a triploid tissue called endosperm.
Source: Fertilisation
