The substance known as sporopollenin is found in the outermost layer of pollen grains. Sporopollenin is a strong, highly cross-linked, and remarkably stable polymer made of carbon, hydrogen, and oxygen. It is virtually completely resistant to environmental deterioration, which safeguards plants in a hard terrestrial environment. It cannot be broken down by any biological or physical process and is resistant to all enzymes. It is discovered in spore and pollen walls, from where it derives its name. It aids the fossilisation of pollen grains.