70 GROWTH AND SECRETIONS. 



if, as has been shewn (13), they are articulated, 

 they fall off ; if not, they are destroyed by the 

 inclemency of the air. The leaves of what are 

 called evergreens form no exception, although 

 they endure longer than those of deciduous 

 plants, and instead of all falling off together in 

 the autumn, are renewed at various intervals ; 

 yet each individual leaf undergoes the ordinary 

 course of growth and decay. The change of 

 colour which withering leaves present is a very 

 curious subject, and one which the recent experi- 

 ments of Sir John Herschell and others have 

 tended to explain. Mr. Hunt thus expresses his 

 views of this phenomenon : " The change in the 

 colour of leaves appears to be entirely dependent 

 upon the absorption of oxygen, which all the 

 green parts of plants have the power of absorb- 

 ing, particularly in the dark. This true case of 

 chemical affinity, it would appear, goes on equally 

 with the spring or the summer leaves, but during 

 these periods the vital force, under the stimulus 

 of light, is exerted in producing the assimilation 

 of the oxygen for the formation of the volatile 

 oils, the resins, and the acids. In the autumn 

 the exciting power is weakened ; the summer sun 

 has brought the plant to a certain state, and it 

 has no longer the vital energy necessary for 



