Matt Collins, Head Gardener at the Garden Museum and regular writer for TOAST, goes behind the scenes at Kew Herbarium.

I'm standing in the centre of the herbarium at Royal Botanic Gardens Kew, surrounded by grand Victorian architecture. Three-storey staircases spiral beside tall columns of iron, and vast wooden cupboards handcrafted and heavily-set file in rows like lockers in an old schoolhouse.

A remarkably precious artefact, extracted from one of these cupboards, lies on a large oak table in front of me. It is a solanum flower a wild tomato pressed, dried and mounted on paper, collected during Charles Darwin's early 19th century expedition to the Galapagos Islands. The cutting shows every bit its age, all moisture long departed from its browned and brittle stems. However, new annotations decorate the paper mounting. There are catalogue numbers, location notes and sticker- printed barcodes: indications of historical but also ongoing use. As with the seven million other specimens stored in the cupboards of Kew's herbarium, this is not a museum piece; it is a resource with which fundamental questions of plant identity, diversity, conservation and usefulness continue to be answered.

Nina Davies, Senior Curator-Botanist at the herbarium, is showing me round its five wings and two cavernous basements. We've begun in Wing C, the first extension to be added to Hunter House, where the collections began. Hunter House faces Kew Green in front and a sweep of the Thames behind, an 18th Century, Baroque-style house occupied by Kew since 1851. Botanist Sir William Hooker was director of the gardens at that time, Davies tells me, and initially the ground floor of Hunter House accommodated his own personal herbaria. With new specimens flooding in from all across the British Empire, however, and eminent botanists soon donating their own collections to Kew, expansion became imperative.

Wing C was constructed in 1877 and, bar a few alterations (a fire-proof concrete floor, for example), it remains today much as it was conceived. The building is a relic from an age of imperial discovery, necessitated by the same scientific endeavours that occupy the herbarium today. Between 25,000 and 30,000 specimens arrive here each year, says Davies, that's Kew collecting its own plant samples in places like Madagascar and Cameroon, as well as exchanges with other herbaria around the world. A staggering 2,000 new species are named and described annually, though that's only a fraction of the material waiting patiently in the wings to be classified.

So what constitutes a Kew herbarium specimen? Ideally samples are taken from a plant in flower or fruit, as this makes it easier to later identify, Davies replies, but, failing that, whatever material is available at the time. Samples are typically pressed in the field, using somewhat surprisingly a flower press no less rudimentary than the kind familiar to children, before being packed and shipped on to Kew. I picture Darwin standing amidst a tropical idyll, squashing his Galapagos tomatoes between slips of newspaper and thumb- screwed wood.

From Wing C we move into Wing B, added in 1902. The galleried interior matches C in design: the wood and ironwork remain consistent, only the cupboards are fractionally newer. Here Davies shows me pressings donated by the East India Company. Neatly stacked throughout this section of the herbarium are the company's original tea species collection, including Camellia sinensis, the shrub from which all tea is derived. The camellia pressings, Davies tells me, form what is referred to as a type; a physical representative definitive of its species. Retaining types is one of the key remits of the herbarium as a whole: a resource for verifying species authentication. In other words, when progressing botany in the name of tea, for example, scientists still compare findings against the samples in front of me.

As my tour continues along wings A (1932), D (1969) and E (2009) I'm transported through a series of time capsules, each modified by the advancements of their age. By Wing E compactors (wheel operated shelves) have replaced cupboards, and windows have been swapped for a stable climate control system (we stand in a pleasant 18 degrees celsius). Moving down to the basement, Davies shows me immaculately preserved cacti, pulling custom-made boxes from shelves that disappear off into a low-lit vanishing point like a never-ending library. As the walk comes full circle I'm beginning to comprehend how seven million specimens might actually fit within these buildings, and what a precious resource the herbarium provides. In recent years Kew has begun digitalising their collection (scanning and barcoding), making specimens available to researchers online.

The obvious benefit to the accumulation of so much plant data is a better understanding of how plant species are connected, and how they might then be used. The more we know about plants the more species we find, says Davies. These species might, for example, provide new or more sustainable choices of food and medicine; new kinds of hemp to make clothes. Analysing samples collected from diverse environments also tells us about plant populations and how they've changed over the years; we can make assessments from this data that will help conserve endangered species. Global warming can be explored here too, and Davies gives the example of the coffee plant, Coffea arabica, as a species particularly sensitive to environmental changes. By studying and comparing herbarium specimens scientists are gaining an insight into the future effects of our increasingly volatile climate. And that's quite a legacy for the humble pressed flower.

Words by Matt Collins. Images by Kendal Noctor.

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