The specimens may be whole plants or plant parts; these will usually be in dried form mounted on a sheet of paper but, depending upon the material, may also be stored in boxes or kept in alcohol or other preservative. The specimens in a herbarium are often used as reference material in describing plant taxa; some specimens may be types.
The same term is often used in mycology to describe an equivalent collection of preserved fungi, otherwise known as a fungarium. A xylarium is a herbarium specialising in specimens of wood. The term hortorium (as in the Liberty Hyde Bailey Hortorium) has occasionally been applied to a herbarium specialising in preserving material of horticultural origin.
The oldest traditions of making herbarium collection or Hortus sicci have been traced to Italy. Luca Ghini and his students created herbaria of which the oldest extant one is that of Gherardo Cibo from around 1532. While most of the early herbaria were prepared with sheets bound into books, Carolus Linnaeus came up with the idea of maintaining them on free sheets that allowed their easy re-ordering within cabinets.
Commensurate with the need of wildlife conservation, it is often desirable to include in a herbarium sheet as much of the plant as possible (e.g., flowers, stems, leaves, seed, and fruit), or at least representative parts of them in the case of large specimens. To preserve their form and colour, plants collected in the field are carefully arranged and spread flat between thin sheets, known as 'flimsies', (equivalent to sheets of newsprint) and dried, usually in a plant press, between blotters or absorbent paper. During the drying process the specimens are retained within their flimsies at all times to minimise damage, and only the thicker, absorbent drying sheets are replaced. For some plants it may prove helpful to allow the fresh specimen to wilt slightly before being arranged for the press. An opportunity to check, rearrange and further lay out the specimen to best reveal the required features of the plant occurs when the damp absorbent sheets are changed during the drying/pressing process.
The specimens, which are then mounted on sheets of stiff white paper, are labelled 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 labelled 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.