Geranium sp. pollen grain germination, coloured scanning electron micrograph (SEM). Geranium is a genus of 422 species of annual, biennial, and perennial plants that are commonly known as geraniums or cranesbills. Pollen tubes are produced when a pollen grain lands on the pistil (female part) of a flower. The tube burrows down into the pistil to reach the ovaries, where it fertilises the ovum (egg cell). Pollen grains are the male sex cells of a flowering plant. Their characteristic surface is used by botanists to recognise and classify plants. Pollen in plants is used for transferring male genetic material from the anther of a single flower to the stigma of another in cross-pollination. In a case of self-pollination, this process takes place from the anther of a flower to the stigma of the same flower. Magnification: x800 when printed at 10cm wide.