In botany, a herbarium (plural: herbaria) – sometimes known by the Anglicized term herbar – is a collection of preserved plant specimens. These specimens may be whole plants or plant parts: these will usually be in a dried form mounted on a sheet but, depending upon the material, may also be kept in alcohol or other preservative. The same term is often used in mycology to describe an equivalent collection of preserved fungi, otherwise known as a fungarium.
The term can also refer to the building where the specimens are stored or to the scientific institute that not only stores but researches these specimens. The specimens in a herbarium are often used as reference material in describing plant taxa; some specimens may be types.
A xylarium is a herbarium specialising in specimens of wood. A hortorium (as in the Liberty Hyde Bailey Hortorium) is one specialising in preserved specimens of cultivated plant.
Specimen preservation 
To preserve their form and color, plants collected in the field are spread flat on sheets of newsprint and dried, usually in a plant press, between blotters or absorbent paper. The specimens, which are then mounted on sheets of stiff white paper, are labeled with all essential data, such as date and place found, description of the plant, altitude, and special habitat conditions. The sheet is then placed in a protective case. As a precaution against insect attack, the pressed plant is frozen or poisoned, and the case disinfected.
Certain groups of plants are soft, bulky, or otherwise not amenable to drying and mounting on sheets. For these plants, other methods of preparation and storage may be used. For example, conifer cones and palm fronds may be stored in labeled boxes. Representative flowers or fruits may be pickled in formaldehyde to preserve their three-dimensional structure. Small specimens, such as mosses and lichens, are often air-dried and packaged in small paper envelopes.
No matter the method of preservation, detailed information on where and when the plant was collected, habitat, color (since it may fade over time), and the name of the collector is usually included.
Collections management 
Most herbaria utilize a standard system of organizing their specimens into herbarium cases. Specimen sheets are stacked in groups by the species to which they belong and placed into a large lightweight folder that is labelled on the bottom edge. Groups of species folders are then placed together into larger, heavier folders by genus. The genus folders are then sorted by taxonomic family according to the standard system selected for use by the herbarium and placed into pigeonholes in herbarium cabinets.
Locating a specimen filed in the herbarium requires knowing the nomenclature and classification used by the herbarium. It also requires familiarity with possible name changes that have occurred since the specimen was collected, since the specimen may be filed under an older name.
Modern herbaria often maintain electronic databases of their collections. Many herbaria have initiatives to digitize specimens to produce a virtual herbarium. These records and images are made publicly accessible via the Internet when possible.
Herbaria are essential for the study of plant taxonomy, the study of geographic distributions, and the stabilizing of nomenclature. Thus, it is desirable to include in a specimen as much of the plant as possible (e.g., flowers, stems, leaves, seed, and fruit). Linnaeus's herbarium now belongs to the Linnean Society in England.
Specimens housed in herbaria may be used to catalogue or identify the flora of an area. A large collection from a single area is used in writing a field guide or manual to aid in the identification of plants that grow there. With more specimens available, the author of the guide will better understand the variability of form in the plants and the natural distribution over which the plants grow.
Herbaria also preserve a historical record of change in vegetation over time. In some cases, plants become extinct in one area or may become extinct altogether. In such cases, specimens preserved in an herbarium can represent the only record of the plant's original distribution. Environmental scientists make use of such data to track changes in climate and human impact.
Many kinds of scientists use herbaria to preserve voucher specimens, representative samples of plants used in a particular study to demonstrate precisely the source of their data.
They may also be a repository of viable seeds for rare species.
Largest herbaria 
Many universities, museums, and botanical gardens maintain herbaria. Herbaria have also proven very useful as sources of plant DNA for use in taxonomy and molecular systematics. The largest herbaria in the world, in approximate order of decreasing size, are:
- Muséum National d'Histoire Naturelle (P) (Paris, France)
- New York Botanical Garden (NY) (Bronx, New York, USA)
- Komarov Botanical Institute (LE) (St. Petersburg, Russia)
- Royal Botanic Gardens (K) (Kew, England, UK)
- Conservatoire et Jardin botaniques de la Ville de Genève (G) (Geneva, Switzerland)
- Missouri Botanical Garden (MO) (St. Louis, Missouri, USA)
- British Museum of Natural History (BM) (London, England, UK)
- Harvard University (HUH) (Cambridge, Massachusetts, USA)
- Swedish Museum of Natural History (S) (Stockholm, Sweden)
- United States National Herbarium (Smithsonian Institution) (US) (Washington, DC, USA)
- Nationaal Herbarium Nederland (L) (Leiden, Netherlands)
- Université Montpellier (MPU) (Montpellier, France)
- Université Claude Bernard (LY) (Villeurbane Cedex, France)
- Herbarium Universitatis Florentinae (FI) (Florence, Italy)
- National Botanic Garden of Belgium (BR) (Meise, Belgium)
- University of Helsinki (H) (Helsinki, Finland)
- Botanischer Garten und Botanisches Museum Berlin-Dahlem, Zentraleinrichtung der Freien Universität Berlin (B) (Berlin, Germany)
- The Field Museum (F) (Chicago, Illinois, USA)
- University of Copenhagen (C) (Copenhagen, Denmark)
- Chinese National Herbarium, (Chinese Academy of Sciences) (PE) (Beijing, People's Republic of China)
- University and Jepson Herbaria (UC/JEPS) (Berkeley, California, USA)
- Herbarium Bogoriense (BO) (Bogor, West Java, Indonesia)
- Royal Botanic Garden, Edinburgh (E) (Edinburgh, Scotland, UK)
See also 
- List of herbaria
- Plant collecting
- Plant taxonomy
- Virtual herbarium
- Pressed flower craft
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- For links to a specific herbarium or institution, see the List of herbaria.
- Index Herbariorum
- Linnean Herbarium
- Lamarck's Herbarium (online database with 20,000 sheets in HD)
- French Herbaria Network
- Montpellier Herbarium