An outgrowth of a pollen grain, which transports the male gametes to the ovule. It will only grow if the pollen grain is compatible with the female tissue. In angiosperms, the pollen grain is deposited on the stigma and the pollen tube grows down through the style and into the ovule. Germination of the pollen grain is initiated by imbibition of water from the stigma. Development of the pollen tube through the tissue of the style is guided by signal molecules secreted by the synergids in the embryo sac. In some conifers, e.g. Pinus (pines), the pollen tube penetrates the nucellus but does not develop further until the following year, when the female part of the plant is mature. See also embryo sac; fertilization; tube nucleus.