442 CHARACTER OF VEGETABLE VITALITY. CHAP. XI. 



short time, and again to present its upper surface 

 to the sun or light. This it was sometimes found 

 to do even in the night ; but always the most ex^ 

 peditiously in young subjects. If the experiment 

 is often repeated, the leaf resumes its original posi- 

 tion with more difficulty, and exhibits evident 

 marks of being injured by the exertion, in the 

 appearance of several black spots about the veins 

 of the under surface, and in the scaling off of the 

 cuticle. 



But all leaves are not equally susceptible to the 

 action of the stimulus of light. The leaves of the 

 Mallow are said to exhibit but slight indications of 

 this susceptibility, as also sword shaped leaves ; the 

 leaves of the Missletoe, which have never been 

 known to resume a former position in consequence 

 of any change in the position of the branch, be- 

 cause perhaps they are equally susceptible on both 

 sides.* But succulent leaves are said to be parti- 

 cularly susceptible, notwithstanding their thick and 

 firm texture ; and if the leaf of a Vine is even se- 

 parated from the branch and suspended by a fine 

 thread, so as that the upper surface shall be turned 

 from the light, it will yet gradually alter its position 

 till it comes round again to it.-f- This experiment 

 requires to be made with great care and delicacy 

 lest the leaf should be made to turn by means of 

 the effect of the atmosphere upon the thread ; 

 though in this case it may perhaps be said that the 



* Smith's Introduction, p. 208. t Ibid. 



