Henry David Thoreau came to Maine to see a kind of nature he could not see in Concord, Mass., a ''mossy and moosey'' forest that might look as it did before the Europeans arrived.

The Canada Lily, or Lilium Canadense. Photo used by permission from Paul Christian's Rare Plants site.

And he wanted to see it up close - with a magnifying glass.

Thoreau had developed an interest in botany - the science of plants - at an early age. He briefly worked as a teacher after college and once told his students he knew the blossoming times of local flowers well enough to determine the month of the year.

In truth, Thoreau's abilities as a botanist improved as he grew older. By the time of his second trip to Maine in 1853, he was among the most knowledgeable botanists in the country, able to identify most of the common species of plants, grasses and sedges in New England.

In Concord, Thoreau walked daily through Walden woods, inspecting, measuring and collecting - in his old Kossuth hat - specimens of plants. He was so focused that he sometimes complained that observing nature gave him a headache.

''I have the habit of attention to such excess that my senses get no rest, but suffer from constant strain,'' Thoreau wrote in his journal. ''When I have found myself ever looking down and confining my gaze to the flowers, I have thought it might be well to get into the habit of observing the clouds as a corrective; but no! that would be just as bad.''

Thoreau's actual collection, while interesting and a good size, is of little scientific value because of the way he recorded the data.

''He didn't take a lot of notes, sometimes only the name,'' said Ray Angelo, the curator of the New England Botanical Club and the author of an index of Thoreau's botany. ''Typically, you record the place - the town and state, the date and something about the habitat. . . . His purpose in collecting was not to document, but for his personal use.''

The Bearberry, or arctostaphylos uva-ursi. Photo by Peter Nelson used by permission from the photographer and the Brooklyn Botanic Garden Web site.

Of the estimated 3,000 species of vascular plants in New England - trees, ferns, shrubs, grasses and sedges - Thoreau collected specimens of 900.

Today, the collection is locked away at Harvard University's Gray Herbarium in giant paper folios.

In Maine, Thoreau saw several plants and even a few trees he rarely if ever saw in Concord.

On the northern shore of Telos Lake, he came upon a stand of red pines, which are uncommon in Concord. And on an island in Grand Lake Matagamon, he discovered some jack pines, which do not grow at all in Concord, because the conifer favors a harsher climate.

''A peculiar evergreen overhung our fire which at first glance looked like a pitch pine with leaves little more than an inch long,'' Thoreau wrote. ''But we found it to be the Banks' or Labrador Pine, also called a Scrub Pine, Grey Pine, a new tree to us.''

Listed in the appendix of ''The Maine Woods'' are all of the trees, flowers, shrubs and plants Thoreau noted on his three trips. The most common trees, he writes, were fir, spruce and cedar; the most interesting flower was the purple orchis.

In the end, Thoreau's most important scientific work was not his botany but the 1860 essay ''The Succession of Forest Trees'' and the book ''The Dispersion of Seeds,'' published after his death.

''From a scientific point of view, it can be said he documented for the first time how ecological succession works,'' said Richard T. Forman, professor of landscape ecology at Harvard University. ''The mechanism was animals and weather. Squirrels carry acorns so oak trees replace pine when the pines are cut down. And pine seeds blow over to replace the oak.

''Thoreau saw things no one else did,'' Forman said. ''He saw nature as things happening, patterns emerging.''

It was while doing research for ''The Dispersion of Seeds'' - counting tree rings - that Thoreau caught the cold that ultimately led to the tuberculosis that killed him in 1862.

Botanical highlights of Thoreau's journey

• Botanical Index of Thoreau's Journal, from Ray Angelo at Harvard University.
• Viburnum: Thorea calls this bright red fruit a "tree-cranberry" but it is not actually related to the cranberry. He picked a couple of quarts for use in a sauce, and tried two other varieties, which he found "rather insipid and seedy."
• Creeping snowberry (gaultheria hispidula): Thoreau's Penobscot Indian guide Joe Polis says this plant, with its small evergreen leaves and white fruit, makes "the best tea of anything in the woods." Thoreau liked the wintergreen flavor, calling this his favorite of the many teas he and his companions made in the woods.
• canada lily (lilium canadense): Thoreau sees one of these - 6 feet tall, with 12 yellow flowers - on the West Branch of the Penobscot River. Polis says the roots can be used to thicken soups, taking the place of flour.
• White pine (pinus strobus): The tallest tree on the East Coast, though by the time Threau arrived many of the oldest and tallest already had been cut down. In the 18th century, pine trees measuringmore than two feet in diameter and within three minles of water were branded with the "King's Arrow" - a mark that reserved the trees for the British Navy, which used them for masts on ships. "The pine is no more lumber than man is," Thoreau wrote.
• mountain cinquefoil (potentilla tridentata) and bearberry (arctostaphylos uva-ursi): Even on Mount Kineo, a 700-foot-high mass of hornstone that Throeau calls "a dangerous place to steady your nerves," the author wanders about looking at plants. These two, found near the water, "chiefly attracted our attention," he writes.
• White spruce, black spruce and balsam fir (abies basamea): These are the most common conifers Thoreau sees, though he sometimes confused the two types of spruce. The White spruce is taller and has longer, smoother needles and lighter bark. The balsam fir has the triangular shape of a Christmas tree.
• Labrador Tea
• Teaberry
• Clinton's Lily (Clintonia borealis)