Pollen grains. Coloured scanning electron micro- graph (SEM) of three types of pollen grain: Scot's pine (large yellow, Pinus sylvestris), horse chestnut (brown, Aesculus hippocastanum) and ash tree (small yellow, at lower left, Fraxinus excelsior). The pine pollen has two air-filled, rounded and wing-like bladders which are extensions of the pollen grain's wall. These wings help this wind-dispersed pollen to float on air. The insect- borne horse chestnut pollen has tiny projections to help them adhere to insects. Pollen contains the male gametes of the plant which fertilize the female egg (or ovule) to form the plant's seed. Magnification: x1600 at 6x6cm size.