Kew’s herbarium houses approximately 7.5 million specimens of plants, which are used by Kew scientists and visitors to the herbarium on a daily basis – and are occasionally sent to other institutions for study. There are specimens in the collection that are 250 years old, and include material collected by the likes of Charles Darwin and some of Kew’s previous directors including W.J. Hooker and J.D. Hooker. In this blog post we explain how we mount (display on paper) specimens to ensure their availability for future scientists.
The purpose of mounting specimens for a herbarium is to preserve plants, to prevent them from being damaged and to prepare them for scientific research. Each specimen must be attached to paper by applying glue or stitching. Since the process of preserving plants in herbaria began hundreds of years ago, the type of paper and glue used to secure specimens has changed. In the past, the paper was poor quality and would eventually crumble, causing damage to the plants that were placed upon it. Today, we use archival quality, acid free paper which ensures herbarium specimens will survive for years to come.
The development of modern adhesive began in 1690 with the development of the first commercial glue in Holland. From 1750, the first British patent was issued for the production of a glue made from fish: the main adhesive used for years in mounting dry plants. Today we use a pH neutral glue which is water soluble and acid free which minimizes damage to the plants.
In order to protect herbarium specimens from being damaged by pests, pesticides were in past years applied to the mounted sheets. This method was replaced by deep freezing (-40°C), and today we have two dedicated walk-in freezers for all specimens coming through Kew’s herbarium.
Herbarium specimens are frequently loaned out to other specialist institutes, so the plants must be securely glued to herbarium sheets. Our current specimens are mounted using museum standards, but as we have very old specimens, we often need to restore them using our modern methods.
Each specimen is attached to a non-acidic, cotton-based sheet of paper, using a water-based PVA glue. All relevant plant parts are displayed, such as both sides of leaves and flowers. A capsule containing any loose plant material is attached with labels to the bottom right corner of the sheet. Any excess glue is removed with a sponge & warm water. The glued specimens are then covered with a wax sheet and a drying sheet. Sand bags are used to weigh down piles of specimens while they are drying for two to three days. Once dried, some stems that need extra support are stitched with linen thread. The specimens are then frozen for four days.
Kew Botanists travel to remote and far flung places around the world on plant collecting expeditions. The collected plants are dried and pressed in the field in specially made plant presses the size of a Kew Herbarium sheet.
The herbarium sheet needs to include both sides of leaves, flowers (both sexes), fruit, and entire compound leaves (a leaf divided into two or more parts), which can involve lots of sections if the specimen is large like a tree fern! The plants are dried with heat, but in very damp (tropical) areas they are dipped in alcohol to prevent mould growing on the drying specimen.
On return to Kew, the plants are frozen to kill off any hitch-hiking insects, recorded, sorted, named and prepared by curators for mounting to become part of the collection. After mounting, the specimens are frozen again in case pests such as the biscuit beetle (which likes to eat dried plants) have got in, then incorporated by the teams into the collection in order to be studied by botanists. Key specimens will be given a barcode and saved as a digital file so that they can be added to Kew’s botany databases and shared within Kew as well as with other institutions.
Join us again soon on Kew’s Science Blog for part 2 of how Kew uses its herbarium collection to further plant science.
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