Pollen grains of other species are not compatible and do not germinate on the stigma of a flower of different species. A number of pollen tube of the same species can grow into a style. A pollen grain germinates and the pollen tube grows down the style toward the ovary. The pollen tube passes through the micropyle and discharges a pair of sperm into the female gametophyte within an ovule. The style is the elongated part of the pistil, i.e. the female reproductive organ of a flower which connects the stigma of a plant to the ovary of that plant, whereas a pollen tube is a tube which is formed by the pollen grains after the landing on the stigma. Pollen grains are male gametes. These are formed in another sac-like structure called anther by meiosis. Each microspore mother cell produces four microspores or pollen grains. Pollen is the powder-like substance that consists of pollen grains which are male microgametophytes that produce male gametes. Pollen in plants is used for transferring haploid male genetic material from the anther of a single flower to that of the stigma of another in cross-pollination. Pollen grains of one species can't germinate on a stigma of some other species. The stigma of a flower may receive much pollen even from other species also, but they do not get germinated because they do not get the required conditions.