You can imagine it as a vast filing cabinet, one that's large enough to walk around inside. These are big, high rooms, one on top of the other and filled with corridors of shelves stacked ceiling-high with red plastic boxes. It's dim in here, and quiet, and the air is soft with the leftover smell of naphthalene. This is the National Herbarium of New South Wales at the Royal Botanic Gardens, Sydney, repository for more than a million plant specimens – not just from every corner of this continent but from all over the world.

A three-dimensional reference work, herbaria aren't just for carefully worded definitions of everything from trees to lichen; they hold actual pieces of them. The Latin name for these collections, hortus siccus, is literally a dried garden, and each pressed specimen that's mounted on a sheet of paper is inscribed with what the plant is, who collected it, when, and where.

The ideal garden in many ways, a herbarium needs no water, no pruning, no fertiliser. It defies seasons, climate, geography and even time itself, accommodating the newly found spiky green leaves of Australia's famous dinosaur-tree, the 40–metre Wollemi Pine, as easily as a tiny starburst of whortleberry cactus picked in Mexico in the nineteenth century, or a banksia taken during European botany's first glimpse of eastern Australia in 1770. It's a place where buds from Australia's south coast can sit with buds from its north, alpine flora with algae, plants from this country with those from anywhere else.