Saturday, August 25, 2007

Compass Plant

Compass Plant (Silphium laciniatum) in Carleton's Cowling Arboretum. Notice that the basal leaves are all oriented in the same direction.

The first settlers on the prairies in the 1830s found among the unusual plants growing there a tall, broad-leafed plant known as rosin weed, from its gum-like sap, or compass plant. One settler, writing in 1833, described the plant like this: "It is one of the most extraordinary plants in nature. The name compass plant proceeds from the fact of its leaves, which are very large, rising from the root to the height of from one to two feet, presenting their edges almost invariably to the north and south—and consequently their broadsides to the east and west. This circumstance renders this plant almost an unerring guide to the traveller in cloudy days, who may be caught in those large plains... Such is the uniformity of surface in those extensive prairies, that the most skillful woodsman would be unable to steer his course on a cloudy day, but for the aid of this plant. And the fact of its being found (so far as I have heard) only in those places, where the polarity of its leaves, so to speak, renders it eminently useful to man, seems to present it as one of the strongest proofs of special providence that has been presented to my mind."

In 1839, the Secretary of War (under President Martin Van Buren) was Joel R. Poinsett. Poinsett was not just a politician and military man, he was also a naturalist and founder of the National Institute for the Promotion of Science and Useful Arts (which later became the Smithsonian Institution). Earlier in his career, as United States Minister to Mexico, he discovered a plant that now famously bears his name, the poinsettia. As Secretary of War, Poinsett instructed officers in the army to be on the lookout for other possible contributions to the field of natural history. One of the officers who heeded his call was a solider and former West Point mathematics professor named Benjamin Alvord.

While he was stationed near the Arkansas border, fighting the Seminoles and Cherokees, Alvord collected specimens of the compass plant (Silphium laciniatum), and in 1840 delivered the first scientific paper describing the plant. At first, Alvord thought that the plant might have absorbed iron from the soil into its resinous sap and become magnetic, but he later realized that the leaves of the compass plant were photosensitive, and aligned themselves to receive an even exposure to the sun's rays. What looked like "special providence" to the early settlers, lost in the prairies, was in fact an instance of biological adaptation.

Compass plants are common to the tallgrass prairie, and around Northfield can be found in McKnight Prairie and, as in the photograph above, in the Hillside Prairie in the Lower Arboretum.

For Alvord's description of the compass plant, see B. Alvord, "On the Compass Plant," The American Naturalist 16 (August 1882), 625-635.

The opening quotation is from an account by Thomas Speed (1768-1842), former U.S. Representative from Kentucky, writing in the Southern Agriculturalist and Register of Rural Affairs (Charleston), May 1833.

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