Fig. 4.—Leaf of Bauhinia Braziliensis. a, folded; b, expanded.
An American prairie plant, commonly known as the rosin-weed or turpentine-plant (Silphium laciniatum), bas also been named the compass-plant, from the property its radical leaves have of pointing north and south. The phenomenon has long been known to hunters and frequenters of the prairies, and has been scientifically verified by General Benjamin Alford and other American and European observers since 1839. The secret of the property lies in the fact that the number of stomata is equal on both sides of the leaf, and both sides, therefore, are equally acted upon by light. Hence, if the leaf is equally exposed to the morning sun and the afternoon sun, it will naturally tend to assume a position of equilibrium between the two forces, by turning one side toward the morning, the other side toward the evening, sun. This would throw its breadth in a north-and-south direction. Since attention has been turned to this subject, the leaves of several other plants have been found to possess similar properties.
