A herbarium is a collection of preserved plant specimens maintained for scientific purposes. The specimens are collected, mounted on rigid paper, and accessioned into the herbarium by faculty, students, amateur botanists, or professionals, including agency biologists and environmental consultants. Herbaria are like libraries, but differ in that the information is stored in a biological form, as pressed, dried, and annotated plant specimens. Herbarium specimens are vouchers documenting a species growing at a given site at a certain time, and as such, herbarium holdings worldwide collectively provide the raw data underpinning our scientific knowledge of what species exist, what their diagnostic features are, what their geographic ranges are, and how they change over time. Herbaria are used by the scientific community, particularly plant systematists, for conservation efforts, morphological or molecular studies, and the creation of floras of a particular region or a monograph of a particular genus. Herbarium specimens also provide materials for research on variation at the DNA level, genome structure, and gene expression. Herbaria are resources used in research, education, and conservation, and they play the dual role of documenting specimens associated with formal research so they may be used later, and providing access to previously collected specimens as researchers request them. Herbarium specimens are also useful for educational purposes, because live material is not always available.