Pollen grains of flowering plants are protected because of a stable, rot-resistant outer rind.
Scanning electron microscope image of pollen grains from Helianthus annuus (common sunflower).
Pollen from a variety of common plants: sunflower (Helianthus annuus), morning glory Ipomoea purpurea, hollyhock (Sildalcea malviflora), lily (Lilium auratum), primrose (Oenothera fruticosa) and castor bean (Ricinus communis). The image is magnified some x500, so the bean shaped grain in the bottom left corner is about 50 ?m long.
"The outer rind that carries [pollen grains] is composed of a substance so stable and so resistant to rotting that it may survive for tens of thousands of years and still be recognisable." (Attenborough 1995:95)
Book The Private Life of PlantsBBC BooksFebruary 26, 2017 David Attenborough