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Pressed Specimens

While Horatio Fenn was studying at the Yale College Medical Institute in 1822, he created a book containing over seven hundred pressed plant specimens he found in and around New Haven. Fenn’s book is now in the Yale Herbarium at 21 Sachem Street, where the university’s plant specimens are kept. The pages are yellowing, but the specimens themselves look surprisingly well preserved.

Patrick Sweeney, collections manager at the Yale Herbarium, is plucking pages from the spine of Fenn’s book and carefully turning them to prevent the pressed plants from cracking. He explains that the book contains only some of the 350,000 plant specimens in the herbarium. Sweeney replaces Fenn’s book in a covered box, which he then places in a metal cupboard on the side of the room. There are rows and rows of these metal cupboards—the Yale Herbarium looks more like a bank vault than a museum. There is a surgical feel to the room—its humidity and temperature levels are closely regulated and recorded.

In the years since Fenn assembled his book, botanists have improved the ways they prepare plant pressings. They freeze specimens at -20 degrees Celsius for a week before storage to kill insects and use archival-quality papers and glues that won’t easily deteriorate or give off harmful gases.

The next advancement is digitization. Botanists are currently engaged in the colossal task of preserving fragile, sometimes centuries-old leaves like Fenn’s in online databases where scientists worldwide can access and analyze them. They take photographs of a specimen and store them with its taxonomic information and other data such as the time and location it was collected. Digitization saves time for plant scientists and helps them efficiently analyze differences over time in traits such as leaf size, the presence of glands on leaves, and flowering time.

“With a high resolution image of a specimen, I don’t have to go to the herbarium,” said Michael Donoghue, the herbarium’s curator.
Although herbariums have existed since the mid-sixteenth century, experts estimate that at least seventy thousand plant species have yet to be named. Digital herbariums will allow researchers to identify these new species more efficiently, study plant evolution, and strengthen relationships among botanists in different countries.

The Yale Herbarium recently finished digitizing the Peabody Museum’s collection of Connecticut plant specimens. From 2008 to 2011, nearly fifty thousand were catalogued. Some of the specimens are more than two hundred years old.

Outside the herbarium is the Herb Scan, which resembles a washing machine with an inverted scanner on top of it. This machine takes high-resolution pictures of pressed plant specimens with speed and uniformity. A grant from the Andrew Mellon Foundation’s Global Plants Initiative, the world’s largest coordinated effort to digitize plant type specimens and scholarly resources from herbaria, paid for it.

Melissa Tulig, an associate director at the New York Botanical Garden, called the Global Plants Initiative a success because of the valuable funding it provided. “We needed that start-up,” she said.

A similar project, the National Science Foundation’s Advancing Digitization of Biological Collection, began in 2011 and aims to create digital records of the nearly one billion plant specimens in American museums within ten years. Together these projects preserve our world’s wealth of natural resources and create a global network of information for plant scientists.

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