A pollen tube is a hollow tube that forms from a pollen grain after pollination. Pollen tubes penetrate the ovule of a flower and release male gametes.
Pollen tubes only grow at their tips, and they do so at a very fast rate and reach extended lengths. Actin, a protein, is necessary for the polarized tip to grow. The pollen tubes of most plants contain two layers: an inner sheath of callose, which is a substance made out of sugars, and an outer coating of pectin, cellulose and hemicellulose. The pollen tube's directional growth through the pistil toward the flower’s ovary is essential for egg fertilization.