Pollen is thought of as the diamond of the plant world as it is practically indestructible. This image is of pollen grains from various plants including the sunflower, pine and dandelion. Pollen or flower sperm comes in a variety of shapes and sizes, such as round, oval, disc-or bean-shaped. It may be smooth or spiky, and ranges from about 10 micrometres (one-tenth the width of a strand of human hair) to 200 micrometres. It is naturally white, cream, yellow or orange. Pollen grains transport the male sex cell of a plant from its male to female parts, protecting it from the sun's ultraviolet rays and water loss. Scientists at the Nanyang Technological University's Engineering in Translational Science Group have found a way to remove the materials within pollen grains, leaving behind empty shells which are then used to encapsulate drug compounds. The pollen grains are hardy enough to withstand corrosion by gastric acid in the human stomach.