The herbarium plays a central role in the study of plant diversity. It allows comparison of plants from many parts of the world, from related groups, from different habitats and from a given area over time. It provides material for fundamental descriptions of plant form, microscopic study of surface features or pollen grains, chemical analyses of leaves or flowers, and genetic (DNA) studies. The specimens are also of general reference value to workers wishing to identify unknown plants or learn more about the distribution and variation of particular species.