33 



THE POPULAR SCIENCE MONTHLY. 



a % 



Fig. 4. Leaf of Baithinia 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 com- 

 pass-plant, from the property its radical leaves bave of pointing north 

 and south. The phenomenon has long been known to hunters and 

 frequenters of the prairies, and has been scientifically verified by Gen- 

 eral Benjamin Alford and other American and European observers 

 since 1839. The secret of the property lies in the fact that the num- 

 ber of stomata is equal on both sides of the leaf, and both sides, there- 

 fore, 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 foi'ces, by 

 turning one side toward the morning, the other side toward the even- 

 ing, sun. This would throw its breadth in a north-and-south direc- 

 tion. Since attention has been turned to this subject, the leaves of 

 several other plants have been found to possess similar properties. 



