Sexual reproduction of plants requires pollen to land on the stigma, which then germinates and grows pollen tubes through the style to deliver sperm cells to the ovule. Pollen tubes are the fastest-growing plant cells known. Studies have shown that the growth rate of maize pollen tubes can reach up to 2.8 microns per second, while the growth rate of lily pollen tubes can also reach 0.2 to 0.3 microns per second.