She and her husband were waiting when the gates opened. Interested in both horticulture and architecture, they had come to see what science lay inside the herbarium.

“It’s a chance to see what you generally cannot,” she said.

Admiring the beautifully precise sketches made by John Kirk on the Livingstone expedition, Petra Broddle, an assistant botanist who helped lead the tours, urged visitors to imagine their journey more than 150 years ago from Central Africa to Kew.

“Canoes overturned, things were lost, Kirk almost drowned, but this is here,” she said, sweeping her hand above the table. “That is hard-core traveling these specimens have done to get here.”

Catherine Maher, a writer and teacher from Shropshire, marveled at the scope of the collection, much of it assembled when the British Empire stretched around the world.

“The idea that they’ve got all this stuff is incredible,” she said. “The Victorians were just mad. They just went round the world nicking all they could. But in a way, thank goodness they did.”

One wooden table held two boxes of giant curled palms collected in the Amazon by Alfred Russel Wallace, the Welsh naturalist and a contemporary of Darwin. Next to them was a field guide Wallace wrote about palms after the expedition, placed there by David Goyder, the herbarium botanist who chose the items for the open house.