454 CHARACTER OF VEGETABLE VITALITY. CHAP. XI. 



Under similar circumstances the leaves of the 

 Indian Mallow become concave ; and it seems as if 

 the effect were produced merely, or at least, chiefly 

 hy means of heat ; because the same effect may be 

 produced even by means of the application of a hot 

 iron ; and yet the leafits of many such plants fold 

 themselves back at night so as to meet under the 

 leaf-stalk, a phenomenon equally wonderful with 

 that of nutation, and not attributable to heat. 

 But several species of Mimosa exhibit a singular 

 phenomenon even in the common foot-stalk, which is 

 found to have a sort of natural movement dependant 

 upon temperature also, so that it is elevated in the 

 course of the day, and depressed in the course of the 

 night, according to the observation of Du Hamel. At 

 nine o'clock in the morning of a day in the month of 

 September, the weather being moderately fine, the 

 foot-stalk of a leaf of the Mimosa pudica formed 

 by its position an angle of 100 with the lower part 

 of the stem : at noon it formed an angle of 112: 

 at three o'clock in the afternoon it had fallen to an 

 angle of 100* : and during the night it fell to an 

 angle of QO ;* thus indicating an evident suscepti- 

 bility to the stimulus of the action of heat. 

 Jfce vital As the elevation of temperature induced by the 

 exerts P its heat of summer is essential to the full exertion of 

 even g iT tne ener gi es f tne vital principle, so the depression 

 winter. of temperature consequent upon the colds of winter 

 has been thought to suspend the exertion of the 



* Phys, des Arb. liv. iv. chap, vi. 



