Collections of pressed plants are called a herbarium
. They can provide a record of the past and present distribution of plant
species, are a basis for the identification of species and used for botanical and ecological research. Pressing plants properly involves more work than you might first think. The plants can dry too quickly and turn a brown or black, certain paper must be used and has to be changed regularly and there are tips to speeding up the process and the affects this has on the plant.
Plants that are pressed properly can keep for a very long time. Specimens collected by Carolus Linnaeus
in the eighteenth century are still excellently preserved in herbaria. You can do this too!
How to collect specimens
Collection of plants should be taken with care, picking plants in national parks, nature reserves, state forests or those plants listed under acts of parliament is prohibited, unless a permit has been obtained. Permits are generally only given for scientific study.
Wet or spirit collections
Some specimens should be preserved a liquid preservative, rather than by drying. These kinds of specimens include very fleshy or delicate structures- like complex flowers or fruit, some aquatic species or small algae. In these instances, glass jars are preferable to plastic, as they seal more tightly and are more easier seen through. Small glass bottles used for scientific specimens are called Macartney bottles. To preserve specimens satisfactorily, a solution of 7 parts ethanol with 3 parts water, usually referred to as 70% alcohol should be used. Methylated spirits may be substituted and made into a 70% solution. Unfortunately, colours fade in this liquid, so notes and photographs should be kept on the colours.
Pressed and dried collections
This is how most herbarium collections are held, preserved as pressed and dried specimens, since this is the easiest way of storing and handling. Occasionally, specimens are allowed to dry without being pressed, usually because the specimens are too bulky to press without being destroyed.
A typical portion of the plant would be 20-30cm in length, showing the leaves in position, both the upper and undersides, and flowers. If flowers are absent, buds may be included, as they can be dissected to read the floral formula. Developing fruits can also be dissected to show the arrangement in the ovary. Fruit and/or seeds may also be helpful in identification and can be stored in plastic bags.
Herbarium sheets are usually 43 by 28 cm’s wide. But anything between this and an A4 size sheet of paper is acceptable. Anything smaller encourages inadequate specimens, and should only be used as quick reference cards. Even for plants with large leaves do not limit the size of the specimen for convenience. There are different plant groups where there are specific requirements for pressing, these include:
- Bryophytes- mosses and liverworts, small, non-vascular plants that are slow growing. Many grow on the barks of trees, rocks or soil surface. These plants are allowed to dry without pressing, placed in a paper bag to dry.
- Lichens- like bryophytes, usually small, slow growing and sensitive to disturbance. These should be treated the same as bryophytes, but also removing part of what they are growing on, and fruiting bodies should also be included.
- Ferns- should include the spore-bearing fronds as well as the rhizome or stalk.
- Herbs- In the case of small herbs, the whole plant should be dug up.
- Grasses- collected as a whole, to show the rootstock. Best collected after the flowers have opened if the specimen is longer than the herbarium sheet, it should be bent one, twice or more, so as to form as V, N or M shape and pressed into this position.
- Trees- For most species it is recommended to collect flowers, fruit and leaves. Notes should be made on the form of the tree, sap colour and colour of the bark. This applies especially for eucalypt specimens, also including juvenile leaves, and a collection of wood or bark.
- Plants with large inflorescences or other large parts- the lengths of these heights should be noted. One way of pressing these leaves is to cut them up into numbered pieces and fit them onto separate pages. Photographs should also be included. The alternative to this is to cut the leaf whole, and make a special separate storage area for it.
- Small algae- using the wet or spirit collection techniques. Another option is to fix the algae in formalin and mount on a glass microscope.
- Macro-algae- larger, visible algae. If the body of the plant is attached to a substrate, then that should be collected too. Dried specimens can be made by covering them with a non-adhesive layer (like cheesecloth or wax paper) during the pressing.
Field notes and observations
To identify the plant at a later time, field notes must be taken. This includes the details of the plant- if it looks healthy or sick, the habitat and frequency of the species and associated species. This information should be later copied onto the label attached to the herbarium. It is very helpful, while collecting many specimens to attach a tag and number to each so that nothing gets mixed up. A very precise location should be noted, too. This could be the distance and direction from a well known landmark or if possible the longitude and latitude of the site. Flowers and leaves colour, soil type and altitude are also desirable. The collectors' name and the date collected must also be noted.
Pressing and drying specimens
Plants should be pressed as soon as possible after collection, before wilting and shriveling occurs. If this is not possible right away, most plants can be kept in sealed plastic bags for up to one day. If it needs to be any longer, than can be placed in an esky or refrigerator.
Specimens are pressed flat and usually dried between sheets of semi-absorbent paper, such as newspaper. Papers with glossy surfaces should be avoided. The plants have to be laid out very carefully because it is at this stage that their final appearance depends on. The flowers should be spread out with petals carefully arranged, wilted leaves straightened out and excess twigs cut away. Nectar must be shaken out as much as possible, since copious amounts cause mould. Between the sheets of paper, thick, centre-corrugated cardboard should be placed. This assists with air circulation between the specimens. If plants are of uneven thickness, sheets of spongy, plastic foam, about 1cm thick should be placed between the newspapers to help distribute pressure evenly. If foam sheets are not available, several sheets of newspaper should suffice.
To make the press, strap this pile of newspapers and cardboard together with heavy wood on each end. The best is usually a lattice of wood, the same size or a little larger than the drying papers. Placing books or weights on the pile of specimens is also another way to do this, but is not recommended, as specimens will quickly go mouldy without proper air circulation.
The papers should be checked for dampness every day, and changed when necessary. Most plants should dry in less than ten days. If your conditions are wet or tropical, you will have to change the paper every day. In warmer conditions, every few days is fine. To aid the drying process, a hot-air fan directing air around the press can be used. Small numbers of specimens can be dried using a microwave oven. While this is a quick was of drying, the microwave can damage seeds and the cellular structure of the plants, and may reduce the long-term value of the specimens. But to do this, it is best to place the plants on butchers paper, since newspaper may catch alight. The specimens should be placed in a special press of microwavable material. If this is not available, then sheets of cardboard can be used instead. In most cases maximum power for 1-2 minutes is enough drying time. It is best to only process no more than 10-12 specimens at once. If the specimen is damp when taken out of the oven, allow it to dry. This is a very delicate procedure and care must be taken not to disturb the plants too much.
In general, if a specimen turns black or brown while drying, or there is mould, then the drying was too slow- the papers were not changes regularly enough.
Mounting and labeling specimens
Specimens should be mounted on cardboard (like manila folder thickness) and should be attached with quality tape or string. Dental floss can be used to tie down bulky or woody specimens, but attaching specimens by tape is the quickest and easiest method. The tape that Australian herbarium gatherers hve been using for 15 years is Long-lasting 3M tape (Y8440) available from ‘Scotch’ brand. This tape is archival quality, and the adhesive doesn’t break down as quickly as most ordinary tape.
Some points must be kept in mind when mounting specimens:
- Do not tape over flowers of fruits
- Remove loose soil from roots
- Leave sufficient space in the bottom right hand corner for labels
- Arrange to display as many features as possible
- For small plants, you can mount more than one on a sheet, with the largest toward to base.
The plant name and notes should be written on a permanent label fixed to the bottom right hand conrner. Sometimes annotations can be written directly onto the card. The information used here should be under headings, such as: Location, Collector, Notes, etc.
Storage and filing
Specimens should be stored at a humidity below 50% and constant temperatures around 21 degrees Celsius to discourage growth of fungi and insect pests. Storage in plastic sheets is recommended. When wet or spirit collections are being stored, the solution tends to evaporate over time, so it will require topping up.
Pests and their control
The most effective means of pest control is through regular inspection and maintenance of the collection. The most common causes of deterioration are insects (such as book lice and tobacco beetles) and fungi. The plant collection should be treated before storage to avoid the introduction of pests.
- Freezing- the specimens must be frozen to -18 degrees Celsius or colder, and kept that way for 72 hours. The specimens must be in sealed plastic bags before freezing.
- Microwave- This is the fastest method. No absolute guidelines can be given, since it is a trial and error test. But times of 1-2 minutes per plant should be adequate. Note that mounted specimens should not be placed in ovens, as the adhesive will melt.
- Poisoning- This is a traditional method to make the specimens deadly to pests. This treatment is not recommended, because of health risks. Sprays can be used on the specimens to kill insects, but they don’t always kill insects living right inside the plant. If you are going to use this method, make sure to note the poison used on the label.
- Insect deterrents- This method is not recommended either, because of the effects on health, but when it was used, naphthalene (commonly found as mothballs) was the most frequently used chemical.
- As mentioned above, freezing specimens in sealed plastic bags for 74 hours helps get ride of pests before mounting.
Fungal attack is only really a problem for damp specimens, either from incomplete drying, or collections later becoming wet through floods or high humidity levels. If fungus grows, it can be brushed off with alcohol or methylated spirits. This does not correct the problem that allowed the fungus to develop, but there is not much else that can be done.
And that concludes our lesson in pressing plants. Goodluck!
’How to collect plants’ by G.J. Harden and D.C. Gooden. 2000