The good news is that a rare and previously unknown daguerreotype of Sophia Thoreau has turned up. The even better news is that she doesn’t look much like her famous brother Henry David Thoreau, who had a nest of unruly hair and a chin-strap beard.

The photo, discovered on the bicentennial of H.D. Thoreau’s birth, has been given to the Concord Museum by the Geneva Frost Estate in Maine, and will be incorporated into “This Ever New Self: Thoreau and His Journal,’’ a major new exhibition dedicated to the celebrated botanist that opens at the Concord Museum Sept. 29.

“It is especially remarkable that her image should come to the Concord Museum, since all the great Thoreau objects in our collection came through her hands,’’ Concord Museum curator David Wood said in a statement. “Sophia gave her brother’s desk to Cummings Davis, the founder of the collection that became the Concord Museum. And, of course, Thoreau’s journal went through Sophia’s hands to H.G.O. Blake, who first edited substantial portions of it for publication.’’

The Concord Museum’s Thoreau collection, which includes some 250 artifacts, is the largest repository of objects related to Concord’s native son.

Sophia Thoreau, who was 57 when she died in 1876, was a brilliant botanist in her own right. Her brother had noted several times that his sister found rare specimens that he’d never seen. She also prepared plant specimens in an interesting way. On the backing sheet for a shagbark hickory leaf, for example, Sophia included a line from Shakespeare’s 79th sonnet: “Thou by the dial’s shady stealth may’st know/ Time’s thievish progress to eternity.’’

In the just discovered photo, Thoreau’s sister looks rather serious, with her dark hair parted severely in the middle, as was the style of the day.

Mark Shanahan can be reached at mark.shanahan@globe.com.