Germinating horse chestnut pollen grains. Coloured scanning electron micrograph (SEM) of germinating pollen grains from a flowering horse chestnut tree (Aesculus hippocastanum). The grains here are swollen with water. When dry, the orange exine (outer wall) tightly encloses the male reproductive cells. Here, the exine has been split by the expanding cells within (brown) due to germination. Inside are male reproductive cells. When the pollen lands on the female parts of a flower, a pollen tube grows from the pollen grain long enough to carry the male nuclei to the ovaries, where they would fertilise an ovum. Magnification: x1,350 at 6x7cm size. x2,100 at 3.5x4.5"~LANDSCAPE"