Not all plants or parts of plants are suitable for pressing because their colour or structure might be significantly lost in the process or they are simply too thick or woody. These special cases are outlined in The Herbarium Handbook
and other similar works.
Flowers and fruits
Delicate flowers keep their colour and petals better if each is carefully wrapped in one or two folds of tissue paper. Tubular flowers should be sliced down the middle, one half being mounted facing upwards to show the interior. Stout fleshy organs are also best sliced and in extreme cases, scooped out to leave just the skin. Squashy fruits like blackberries are best collected unripe. Harder ones like hazelnuts padded around with extra drying papers to equalise pressure on the foliage.
Seeds and wood samples
Seeds are usually collected into linen or paper bags and dried and cleaned. They may be stored in paper envelopes or folded packets. The species of some genera, e.g. Rosmarinus
, etc., should not be dried and so plastic bags can be used. These species are also best kept in the dark and not stored, but sown at once. Wood samples should be dried if necessary.
Spirit collections are also sometimes called wet, pickled or alcohol collections. The main purpose of spirit collections is to maintain the three dimensional shape of parts of the plant. Spirit collections are mainly used for preserving succulent or delicate structures like petalous flowers or fleshy fruits that shrivel upon drying, or when the structure or shape of the specimens is required for botanical illustration or microscopy. Note, however, that plant pigments are usually dissolved out.
Until Formalin was discovered to be extremely toxic, spirit collections required a Formaldehyde solution. Now though, solutions such as 70% ethanol 30% water are preferred for wet collections. Occasionally 1% glycerol is added to stop the specimens becoming brittle. This is much safer but protective clothing should still be warn when using ethanol. Ethanol is also highly flammable so precautions need to be taken regarding storage and use.
For floating aquatics, place a sheet of white paper in a bowl of water with the plant on top. Gently lift out the paper so that the plant is spread over it in a natural way. If the whole sheet is then dried in the press in the normal way the plant will usually stick by itself to the paper which can then itself be gummed to a herbarium sheet. Some supplementary 'strapping' may be necessary.
Halophytes and hydrophytes
Halophytes and hydrophytes mostly blacken on drying, whatever precautions are taken. Succulent plants continue growing and some Compositae may ripen seed from flowers in the press. To kill them first, place in a polythene bag with a few drops of absolute alcohol, shake up well and leave for 24 hours. In the laboratory, immersion in boiling water is an alternative. We find that succulents are best killed and dried fastest by placing them in a Polish press in a deep freeze until well frozrn, before placing in a conventional press - a method recommended by the late Henk t'Hart.
Mosses, liverworts, lichens and other bryophytes
Mosses, liverworts, lichens and other bryophytes do not need pressing in a conventional plant press. They only require soil removal and drying in a temporary absorbant paper pocket before being stored in a new pocket of archival paper with the label on the front.
The larger fungal fruiting bodies do not require pressing. They should be dried as rapidly as possible in a constant flow of warm air (c. 40°C, not more; too low and the fungus will be eaten by insects). Multiple collections of these should include one specimen cut in half longitudinally and a spore print. A spore print is taken by taking the cap and placing it gill side down on paper overnight. The spores will be released in this time and this provides a useful identification aid. Spore prints are air dried and stored with the specimen. Microfungi are stored much like mosses and liverworts in small packets, most often as a part of a plant infected with the fungus.