HERBARIUM or HORTUS SICCUS, a collection of plants so dried and preserved as to illustrate as far as possible their characters. Since the same plant, owing to peculiarities of climate, soil and situation, degree of exposure to light and other influences may vary greatly according to the locality in which it occurs, it is only by gathering together for comparison and study a large series of examples of each species that the flora of different regions can be satisfactorily represented. Even in the best equipped botanical garden it is impossible to have, at one and the same time, more than a very small percentage of the repre sentatives of the flora of any given region or of any large group of plants. Hence a good herbarium forms an indispensable part of a botanical museum or institution. There are large herbaria at the British Museum and at the Royal Gardens, Kew, and smaller collections at the botanical institutions of the principal British universities. The original herbarium of Linnaeus is in the possession of the Linnean Society of London. It was purchased from the widow of Linnaeus by Dr. (afterwards Sir) J. E. Smith, one of the founders of the Linnean Society, and after his death was purchased by the society. Herbaria are also associated with the more important botanic gardens and museums in other coun tries. The value of a herbarium is much enhanced by the pos session of "types," that is, the original specimens on the study of which a species was founded. Thus the herbarium at the British Museum, which is especially rich in the earlier collections made in the i8th and early i9th centuries, contains the types of many species founded by the earlier workers in botany. It is also rich in the types of Australian plants in the collections of Sir Joseph Banks and Robert Brown, and contains in addition many valuable modern collections. The Kew herbarium, founded by Sir William Hooker and greatly increased by his son Sir Joseph Hooker, is also very rich in types, especially those of plants described in the Flora of British India and various colonial floras. The collection of Dillenius is deposited at Oxford, and that of Professor W. H. Harvey at Trinity College, Dublin. The col lections of Antoine Laurent de Jussieu, his son Adrien, and of Auguste de St. Hilaire, are included in the large herbarium of the Jardin des Plantes at Paris, and in the same city is the ex tensive private collection of Dr. Ernest Cosson. At Geneva are three large collections—Augustin Pyrame de Candolle's, contain ing the typical specimens of the Prodromus, a large series of monographs of the families of flowering plants, Benjamin De lessert's fine series at the Botanic Garden, and the Boissier Her barium, which is rich in Mediterranean and Oriental plants. The university of Gottingen had bequeathed to it the largest collec tion (exceeding 40,00o specimens) ever made by a single in dividual—that of Professor Grisebach. At the herbarium in Brussels are the specimens obtained by the traveller Karl Fried rich Philipp von Martius, the majority of which formed the groundwork of his Flora Brasiliensis. The Berlin herbarium is especially rich in more recent collections ; and other national her baria sufficiently extensive to subserve the requirements of the systematic botanist exist at Leningrad, Vienna, Leyden, Stock holm, Uppsala, Copenhagen and Florence. Of those in the United States of America, the chief, formed by Asa Gray, is the property of Harvard university; there is also a large one at the New York Botanical Garden. The herbarium at Melbourne, Australia, under Baron Muller, attained large proportions ; and that of the Botanical Garden of Calcutta is noteworthy as the repository of numerous specimens described by writers on Indian botany.
Specimens of flowering plants and vascular cryptogams are generally mounted on sheets of stout smooth paper, of uniform quality ; the size adopted at Kew is 17 in. long by I I in. broad, that at the British Museum is slightly larger; the palms and their allies, however, and some ferns, require a larger size. The tough but flexible coarse grey paper, upon which on the Continent speci mens are commonly fixed by gummed strips of the same, is less hygroscopic than ordinary cartridge paper, but has the disad vantage of affording harbourage in the inequalities of its surface to a minute insect, Atropos pulsatoria, which commits great havoc in damp specimens, and which, even if noticed, cannot be dis lodged without difficulty. The majority of plant specimens are most suitably fastened on paper by a mixture of equal parts of gum tragacanth and gum arabic made into a thick paste with water. Rigid leathery leaves are fixed by means of glue, or, if they present too smooth a surface, by stitching at their edges. Where, as in private herbaria, the specimens are not liable to be handled with great frequency, a stitch here and there round the stem, tied at the back of the sheet, or slips of paper passed over the stem through two slits in the sheet and attached with gum to its back, or simply strips of gummed paper laid across the stem, may be resorted to.
To preserve them from insects, the plants, after mounting, are often brushed over with a liquid formed by the solution of lb. each of corrosive sublimate and carbolic acid in I gallon of methyl ated spirits. They are then laid out to dry on shelves made of a network of stout galvanized iron wire. The use of corrosive sub limate is not, however, recommended, as it forms on drying a fine powder which when the plants are handled will rub off and, being carried into the air, may prove injurious to workers. If the plants are subjected to some process, before mounting, by which injurious organisms are destroyed, such as exposure in a closed chamber to vapour of carbon bisulphide for some hours, the presence of pieces of camphor or naphthalene in the cabinet will be found a sufficient preservative. After mounting are writ ten—usually in the right-hand corner of the sheet, or on a label there affixed—the designation of each species, the date and place of gathering, and the name of the collector. Particulars as to habit, local abundance, soil and claim to be indigenous may be written on the back of the sheet. A careful dissection of a flower greatly increases the value of the specimen. The species of each genus are then arranged either systematically or alphabetically in separate covers of stout, usually light brown paper, or, if the genus be large, in several covers with the name of the genus clearly indi cated in the lower left-hand corner of each, and opposite it the names or reference numbers of the species. Undetermined species are relegated to the end of the genus. Thus prepared, the speci mens are placed on shelves or movable trays, at intervals of about 6 in., in an air-tight cupboard, on the inner side of the door of which, as a special protection against insects, is suspended a muslin bag containing a piece of camphor.
The systematic arrangement varies in different herbaria. In the great British herbaria the orders and genera of flowering plants are usually arranged according to Bentham and Hooker's Genera plantarum ; the species generally follow the arrangement of the most recent complete monograph of the family. In non flowering plants the works usually followed are for ferns, Hooker and Baker's Synopsis filicum; for mosses, Muller's Synopsis mus corum frondosorum, Jaeger & Sauerbeck's Genera et species muscorum, and Engler & Prantl's P flanzen f amilien; for algae, de Toni's Sylloge algarurn; for hepaticae, Gottsche, Lindenberg and Nees ab Esenbeck's Synopsis hepaticarum, supplemented by Ste phani's Species hepaticarum; for fungi, Saccardo's Sylloge fun gorum, and for mycetozoa Lister's monograph of the group. For the members of large genera, e.g., Piper and Ficus, since the number of cosmopolitan or very widely distributed species is com paratively few, a geographical grouping is found specially con venient by those who are constantly receiving parcels of plants from known foreign sources. The ordinary systematic arrange ment possesses the great advantage, in the case of large genera, of readily indicating the affinities of any particular specimen with the forms most nearly allied to it.