This section is from the book "Sub-Alpine Plants Or Flowers Of The Swiss Woods And Meadows", by H. Stuart Thompson. Also available from Amazon: Sub-Alpine Plants: Or, Flowers of the Swiss Woods and Meadows.
In hot countries it is desirable to poison collections of dried plants by painting them over with a solution of mercuric chloride or corrosive sublimate, to protect them against insects. This is done at Kew Herbarium, and also by a few amateur Botanists in this country; but in England it is not really necessary, if camphor or napthaline be freely used, as is the case in the National Herbarium at South Kensington. In addition to spoiling some specimens, and to the subsequent peculiar blackening of the mounting paper in many cases, and to the offensive fumes which in hot weather sometimes rise from specimens treated with corrosive sublimate, the solution is, of course, a most deadly poison, and must be handled with great care.
The preservative solution used at Kew is as follows:
1/2 oz. corrosive sublimate,
1/2 oz. carbolic acid,
1 pint methylated spirit.
It is better that the specimens should be quite dry before they are poisoned. It is usually done with a large camel-hair brush, but there should be no metal mountings about it, and all steel instruments such as knives, scissors, or forceps must be kept away from the solution or it will quickly corrode them. If the solution contains too much chloride of mercury a white crystalline deposit will be left on the specimens. But we say again emphatically that in this country ' the game is not worth the candle.' If further proof be needed it may be mentioned that the writer has in his own herbarium many hundreds of perfect specimens collected eighty or more years ago which were never 'poisoned,' but which have suffered nothing from the attacks of insects, and are to-day as complete and in as good condition as ever.
It has been customary in this country to mount dried plants on paper by means of paste, good gum, or liquid glue. When frequently handled this may have its advantages, and especially if little envelopes containing loose portions of the flower and fruit are attached for careful examination or dissection; but many amateur botanists attach their specimens to the paper with narrow strips of gummed paper, so that they can be examined on both sides, and altogether removed if desired. The little rolls of transparent adhesive paper sold by stationers for repairing torn music, books, etc., cannot be improved upon for this purpose. Another method sometimes adopted on the Continent is to attach the thin portions of stem, etc., to the paper by means of ordinary pins, of course placed horizontally. If gum be used it is best made of a mixture of gum Acacia (gum Arabic) and gum Tragacanth, it being both clean to use and very adhesive. In rare instances collections of plants are not mounted at all, but simply left loose in folded sheets of paper.
However, they are better more or less mounted, and the paper should be a thick, white cartridge or some similar paper, which will remain rigid and flat when one end is held in the hand.
After the plants are mounted they should be labelled. The labels should be about 3 1/2 x 2 inches in size, of rather thin but good white paper so that they can easily be gummed or pasted in a corner of the mount. In British collections it is usual to have the name of the owner neatly printed at the head of the label after the contracted word 'Herb.' (before which 'Ex.' can be written when specimens are exchanged or given away). A broad space is then left for the name of the plant, and usually there are lines for the habitat and locality, and half-lines for the Vice-County, collector's name, date and number in the last edition of the London Catalogue of British Plants. But for European herbaria a simpler label is usually adopted, with the same simple line border, and either with the heading 'Herbarium Europaeum, A.B.C.------' or 'Flora of Switzerland, ' Plants of Norway,' or something of that sort. It saves time when many specimens have been collected by the same person to have the collector's name, preceded by 'Coll.' or 'Legit,' printed in small type at the base of the label.
It should have been mentioned that in mounting many specimens which do not fill a sheet, it is important not to place them always in the centre, but rather at one side if narrow, or in one corner if very small. This will not only tend to keep the bundles of sheets fairly level, but allow several examples of the same species from other districts or from other countries to be added later. The label should, of course, be placed near the plant, and it is sometimes well to rule off with a pencil line one specimen from another from a different district. In this way it is quite easy to have four or five gatherings of the smaller Alpine plants with different labels mounted on the same sheet. In starting a continental collection young botanists are tempted to economise in paper and space by mounting different species on the same sheet. This is greatly to be discouraged, for, apart from the want of systematic order, the space may be needed on future occasions for plants of the same species or variety.
As previously suggested, it is an excellent plan to have a series of very small envelopes, which can be home-made, in which to keep seeds, fruits, and sometimes individual specimens of the flowers or even some leaves, so that they can be easily examined either with an ordinary pocket-lens or under the microscope. Such envelopes should be gummed at the back to the sheet of mounting-paper, preferably with the name of the plant and its collection number, if any. These field numbers are quoted, and save much trouble and needless explanation in the event of any subsequent correspondence on the specimens they refer to. In collecting obscure forms and little known varieties and all plants such as Hawkweeds, Willows, Roses, Sedges, etc., in any quantity, all of one gathering should bear the same numeral. This is particularly the custom with collectors of sets of rare plants in new or little-known countries, and these numbers are referred to and quoted afterwards in books written on the flora of those countries.
In working at any special genus or at the plants of one country, whether it be at Petersburg, Vienna, London, or New York, it is a great help to find a collector's specimens all uniformly numbered thus.
The arrangement of the specimens in genus covers, and of the herbarium generally in a cabinet, must depend upon the individual taste of the botanist and upon the size of his collection. But except in very small collections when several genera can be placed together in one cover, with its name and that of the various genera neatly written outside, it is better to place all the species of one genus only in a cover. These genus covers should be made of stout brown paper folded to a slightly larger size than the mounting paper. The name of the genus should be written on the end of the cover so that it can be readily found when packed in the cabinet. As the collection grows it may be necessary to have more than one cover for many of the larger genera.
Cabinets should be made of well-seasoned wood - what is called American whitewood is a very good and inexpensive material. The usual form is a tall, upright cupboard, divided perpendicularly into two equal parts, and with two closely fitting doors opening in the middle (two doors are very much better than one). The shelves should be made very carefully of thin wood which will not warp, and they should slide easily in shallow grooves cut in the framework of the cupboard. They are better supported in this way than on narrow strips of wood nailed to the sides, for such strips interfere with the papers when the shelves are very full.