A heterogeneous collection of dried plants, however well preserved, is of very little service, and can scarcely be termed an herbarium. The specimens must be named and arranged in a systematic manner so as to admit of easy reference, in order to economize time and labour. The herbarium may comprise the plants of a small district only, as of the Port or Provincial District of Nelson, or it may comprise the plants of an entire continent. It should be kept in an air-tight and dust-proof cabinet.
The first point to be determined is the size of the herbarium-sheets. Attention has been drawn to the desirability of the drying-papers being of the same size as the herbarium-papers, but the dimensions have not been considered. Much will depend upon the taste of the collector, but practically the choice lies between foolscap, demy, and folio. The first—13in. by 8in.—although often used for private collections, is rather too small to allow of characteristic specimens of many New Zealand plants. The first cost of the third—20in. by 12in. or 14in.—is needlessly large, while it is very cumbrous in the field, and involves an unnecessary expenditure of time and labour, without securing any corresponding advantage. Sheets of demy size, 18in. by 10in. or 11in., afford the maximum amount of convenience. In some herbaria the sheets are equal in length and breadth. The Linnean Herbarium is mounted on sheets of foolscap size; Bentham's Herbarium on small demy, 16 ½in. by 11in.; Cunningham's New Zealand Herbarium, on folio.
The sheets should be non-absorbent, hard, firm, and of even texture. A pale-brown tint similar to that of light cartridge-paper is to be preferred to white. All papers used in the herbarium should have perfectly even margins; even labels should not be exempted from this rule, as ragged margins invite the attacks of certain insects.
For all public herbaria, at least, the specimens should be poisoned before mounting. In private collections mounting may be dispensed with, and the specimens simply laid on the sheets. If the specimens are properly prepared, and constantly inspected, poisoning is not absolutely necessary for private collections. There is, however, great danger of the introduction of insects through badly-prepared specimens received in exchange. I have never poisoned a specimen for my own herbarium, although my experience has extended over fifty years. Sometimes insects have been introduced in specimens received from other collectors; but it has always been found possible to get rid of the invaders without poisoning, although not before a small amount of injury has been done.
The herbarium of Wellington College was formed in 1874–75: the specimens were mounted on sheets of white paper, but were not poisoned, and, although but little used of late years, the collection is scarcely touched by insects, having been kept in an excellent air-tight cabinet.
The most troublesome insects in the colony are Anobium paniceum, L., which has become almost cosmopolitan; two small moths; and one or two Acari. Another small beetle, apparently a species of Ptinus, is only occasionally met with. Camphor freely distributed through the herbarium tends to deter insects, but is not an infallible preventive. Biological specimens attacked by insects of any kind may be thoroughly cleaned by exposure to the vapour of carbondioxide, which destroys even the eggs; but this remedy is a dangerous one, and can only be applied under proper precautions, while its effects are transitory. The best poison for plant-specimens is common methylated spirit charged to a little below the point of saturation with corrosive sublimate: if fully saturated, an efflorescence will be left on such specimens as Aciphylla, Ligusticum, Gentiana, & c., which absorb it freely.* Large Compositæ, Ranunculaceæ, Umbelliferæ, Cruciferæ, Leguminosæ, Liliaceæ, & c., are specially attractive to insects. Myrtaceæ, Pittosporeæ, arborescent Saxifrageæ, Myoporineæ, Glumiferæ, and Filicales are rarely attacked. The poison should be applied with a soft brush, such as is used by house-painters: brushes with metal attachments should be avoided: thick specimens may be dipped in the solution. Where numerous specimens have to be treated, guttapercha or vulcanite trays, or photographic dipping-dishes, will be found useful.
Specimens are occasionally fastened to the herbarium-sheets by sewing, a process not to be recommended. Strips of adhesive paper may be employed with advantage, or they may be attached by washing the undersurface with a solution of gum-arabic and tragacanth or with weak glue: common paste should be avoided. In no case should more than a single species be placed on the sheet, although the specimens may be numerous. The name of the plant should be written at the lower right-hand corner of the sheet, together with the locality in which it was collected, the date of collection, and the collector's name. If specimens from more than one locality are placed on the sheet the name of each locality should be stated under the proper specimen. A neat form of ticket may be adopted if thought desirable.
The different sheets of the same genus should be placed
[Footnote] * The mixture used at the Kew Herbarium is composed of corrosive sublimate, 1oz.; carbolic acid, 1oz.; methylated spirit, 1qt.
in a generic cover, which when folded ought to be ¼in. longer and broader than the sheets on which the specimens are mounted, and of rather stouter quality: the name of the genus, and, if the genus contain numerous species, the name of the sub-genus or section, should be written at the right-hand corner.
The genera must now be arranged under their respective natural orders in systematic sequence and placed in the herbarium cabinet, which, as already stated, should be airtight and dust-tight. A convenient size for the cabinet when demy paper is used is 36in. by 40in. and 21in. deep, outside measurement. This allows of twelve pigeon-holes 12in. by 8in. and 19in. deep. If sliding trays are required the width must be slightly increased. The outer edges of the cabinet should be bevelled inwards so as to allow of the doors being fitted as tightly as possible, and the hinges should fold backwards when required, so as to avoid projecting edges.
Mosses, Hepaticæ, and many Algæ are usually mounted on small sheets, which can be arranged in generic covers of the same size as that adopted for vascular plants: most lichens and hard Fungi should be mounted on small cards placed in shallow trays or drawers on account of the greater thickness of the specimens.