WASHINGTON — A few weeks ago, visiting a rare book library, I came across a volume of pressed plants and was particularly struck by a specimen of primrose. It was mounted by a German botanist in the 1590s.

Later I visited the office of Laurence J. Dorr, who heads the Department of Botany at the National Museum of Natural History, and he was looking at another dried, pressed specimen, this one collected in Brazil just four years ago. It was a mallowlike weed named Urena lobata, or Caesarweed. “Native to Asia, but it shows up in the tropics,” he said.

The world has changed beyond comprehension in the past four-and-a-quarter centuries, but we are still sticking flattened plants to pages to satisfy our deep need to understand them. I find this comforting.

Since that primrose was collected and preserved, our understanding of the plant kingdom has expanded exponentially while the world’s vastness has shrunk.

In 1595, a knowledge of plants could save your life. In 2017, a knowledge of plants might save the planet, or at least allow scientists such as Dorr to take its pulse.

Botanists are still using these unprepossessing, desiccated branches and leaves to discover some pretty vital aspects of our world, primarily the evolutionary relationships among plants.

A collection of preserved, field-gathered plants is known as an herbarium. Dorr oversees the National Herbarium, which dates to the 19th century and is one of the hidden treasures of the Smithsonian Institution. Occupying three floors on the western side of the museum, it contains some 5 million specimens of vascular plants, lichens and marine algae. Most museum visitors have no idea of its existence — it’s open to researchers but not the general public — and yet it stands as one of only a dozen or so herbariums of this magnitude in the world. In the United States, that would include herbariums at Harvard University, the New York Botanical Garden and the Missouri Botanical Garden.

Established to identify and study new species of plants, they are used more commonly today by botanists to examine the evolutionary relationships among plants.

Most herbarium specimens come from the tropics, a focus of collectors because less is known about tropical flora and because life is simply more abundant nearer the equator. “There are about 20,000 plant species in the U.S., Canada and Greenland,” Dorr said. “There are 20,000 species in Colombia.” There might be more, considering we have logged maybe 80 percent of the world’s flora.

Many of the Smithsonian’s specimens come from Andean countries, Guyana and the Philippines. Dorr has been part of a team that has scoured a single mountain in Venezuela to record its flora. Earlier in his career, he collected specimens in Madagascar, for the herbarium at the Missouri Botanical Garden.

Gardeners at heart are botanists; they share a need to know what a plant is, how it grows, and its needs of soils and climate. But if gardeners observe plants, botanists scrutinize them.

And although bloom colors may be the primary interest of the gardener, this is a trait “of least importance to us,” Dorr said. This is just as well, because the petals lose most of their color (not to mention sculptural beauty) as herbarium specimens. If Dorr is drawn to flowers, it’s to see the arrangement of stamens to the pistil, the shape of the calyx, and the overall size and placement of blooms.

Experts glean more data from these seemingly inert specimens — the shape and position of the leaves, the branch structure, the architecture of the seed pods. Some identifying traits are discovered under the microscope — leaf hairs or the form of the pollen. Other characteristics are uncovered at the molecular level.

The fundamental object is to identify a plant, because you can’t distinguish a species and relate it to others of its ilk until you know definitively what it is.

One of the great values of an herbarium formed over many years is that you can also figure out whether species in a given region are evolving, disappearing, spreading or flowering at different times.

The herbarium itself is a dynamic creature: Some 300,000 specimens have yet to be mounted — a task aided by a team of volunteers — and the collection grows by as much as 40,000 fresh specimens annually. They are collected by scientists at the Smithsonian and other institutions around the world.

The age of the independent amateur collector, once a key source of specimens, is on the wane because wild collected plants need permits from their countries of origin “and the permitting has become such an issue,” Dorr said. “We aren’t seeing so many casual collectors.”

The herbarium’s biggest shift in the 21st century is the current multiyear digitization of the specimens, laboriously imaged sheet by sheet.

When it came time recently to photograph the 1 millionth specimen, Dorr turned to a historically significant plant that demonstrates how the collection connects science to history.

The Cinchona micrantha was collected in Peru in 1944 by botanist W.H. Hodge. He was part of a team searching for an alternative source of quinine, the antimalarial needed by Allied troops in the war with Japan. The Japanese had seized major Dutch plantations of cinchona on the Indonesian island of Java in 1942. (The cinchona has another useful relative, the coffee plant.)

The digital database will permit researchers to examine the collection remotely but also put the herbarium to use in ways inconceivable before. Linking the images to optical technology will allow the comparison of millions of specimens to find hidden patterns and relationships. “We can start to ask questions we haven’t asked before,” Dorr said.

Other aspects of the herbarium will never change, it is hoped. In an office overlooking 12th Street, botanical illustrator Alice Tangerini sits at a drafting table. Her job is to take the two-dimensional specimens and create line drawings that bring them back to voluminous life. Her drawings are assembled into plates for scientific journals.

“I’ve worked here since 1972. It’s the job you love,” she said. “People don’t really leave here.”

* Gardening tip: Resist the urge to cut back scraggly lavender plants. Lavenders react badly to hard pruning. Trim and groom plants as they produce new growth in early spring, but save any significant pruning until after flowering in June.

Laurence J. Dorr, chair of the National Museum of Natural History’s Department of Botany, studies an herbarium specimen.

Botanical illustrator Alice Tangerini sits at a drafting table in the National Herbarium, housed inside the National Museum of Natural History in Washington. 

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