(hortus siccus) A collection of dried pressed plants, mounted on sheets of thin card, accompanied by data labels and stored in pest-proof wooden or metal cabinets. Smaller organs, including pollen grains, are perfectly preserved in this way, and many other features of the plant (e.g. anatomy, morphology, and chemistry) may be retained virtually unaltered. Details of floral structure can be observed by boiling or soaking in a wetting agent. The data labels, besides giving the plant's name, usually also include the data and place of collection and the collector's name. Notes on habitat, local names, and local uses of the plant (for food, medicine, etc.) may be invaluable in later searches for new sources of drugs, etc.
Specimens are normally arranged according to a particular taxonomic system (e.g. that of Bentham and Hooker in many UK herbaria) but occasionally material is filed in alphabetical order of family, genus, and species. Often specimens are further segregated into geographical regions within their family or generic groups. Herbaria may be small local collections, containing, for example, a county flora, or large international assemblages as at the Royal Botanic Gardens, Kew, where approximately five million specimens are housed. The larger herbaria are centres for taxonomic research and usually provide a plant identification service to other institutions and to the public.