Why Would a Museum Keep Four Hundred Million Dried Plants? Evidence, Not Decoration

A specimen is the plant itself rather than a picture of it, pressed flat, dried and fastened to a standard sheet alongside a label giving the species, the exact place it was collected, the date, the habitat and the collector's name. That combination makes it a piece of evidence, since anybody can re-examine the actual material and check the identification, which no photograph or description permits. It also fixes a species to a moment and a place, so the sheet proves that this plant grew here on this day. Type specimens, the individual sheets designated as the permanent reference for a species name, carry that function further, since the name is defined by the physical object.

The procedure has changed remarkably little in four centuries:

• •Collect with flowers or fruit present, since identification usually depends on them
• •Arrange between sheets of absorbent paper, with some leaves turned to show both sides
• •Press under even weight, changing the paper as it takes up moisture
• •Dry with gentle warmth and airflow to prevent moulding
• •Mount on acid-free card with glue or stitching, with loose parts in a packet
• •Label fully, since a specimen without data is almost worthless

Collections assembled for classification are supplying answers to questions that did not exist when they were made. Flowering dates recorded on centuries of labels are being compared against temperature records, producing direct measurements of how far seasonal timing has shifted, and the shifts are substantial. Distribution data shows where species used to grow, documenting range changes and local extinctions. Leaf pores counted on old specimens record atmospheric carbon dioxide levels through the period of collection. Pollutants including heavy metals and persistent chemicals are measured in the tissue, giving a history of contamination. DNA is now extracted routinely from specimens collected long before anybody knew what DNA was, including from type specimens of extinct species.

The technique appeared in Italy in the sixteenth century and spread with remarkable speed. Luca Ghini, a professor at Bologna and later Pisa, is credited with the idea of drying plants flat and binding them into volumes around the 1540s, and his students carried the method across Europe within a generation. The motivation was medical, since physicians needed to identify plants correctly and a dried specimen settled arguments that a description or a woodcut could not. Early collections were bound as books, which is why the volumes survive intact, and the shift to loose sheets came later because sheets can be reordered as classification changes. Collections assembled on voyages of exploration from the seventeenth century onwards brought back material that reshaped European understanding of how many plants exist.

These collections are more fragile institutionally than physically. Insects that eat dried plant material can destroy a collection, which is why specimens are frozen on arrival and why cases are sealed and monitored. Fire and water are catastrophic and have destroyed major collections, including the loss of a substantial part of the Berlin herbarium in 1943. The more common threat now is funding, since a collection generates no income, requires skilled staff and permanent controlled space, and has repeatedly been closed or dispersed by institutions under financial pressure, with several such decisions provoking international protest in recent years. Digitisation photographs the sheets and captures the labels, which improves access enormously and does not preserve the material that future techniques will need.

A dried plant mounted with full collection data is physical evidence that can be re-examined, and type specimens define species names by being the actual object. The method has barely changed in four centuries. Collections made for classification now supply measurements of shifting flowering dates, range changes, historical carbon dioxide levels, pollution and DNA, and the main threat to them is institutional funding rather than decay.