ORGANS OF AERATION. 103 



same time exhaled ; none of which take place in the 

 dark. Heat may cause a trifling evaporation, but 

 nothing in proportion to that caused by light. It is on 

 this account that plants exposed to much light are 

 greatly harder and tougher than when grown in more 

 shady places : a mountain oak, for example, more than 

 a forest oak ; or a wild carrot on an exposed bank, 

 more than a garden one shaded by the leaves of its 

 fellows. 



The green colour of leaves, as well as the varied 

 colours of flowers, though very imperfectly understood, 

 may be plausibly explained from the same principles. 



Sennebier thinks that the real colour of carbon is 

 dark blue rather than black ; while the tissue of the 

 cells and vessels of which the body of plants is com- 

 posed, is yellow ; consequently, when the blue carbon 

 is lodged in these yellow translucent cells, a green 

 colour is the result. Hence, in the spring, the newly 

 expanded leaves, before they have had time to prepare 

 much carbon, are yellowish ; and when plants are kept 

 from the light, so that no carbon can be prepared by 

 their leaves, they become white, and also crisp and 

 succulent from the same cause, as is seen in blanched 

 celery and endive. 



In autumn, when the leaves assume various tints, 

 it was found by Macaire to arise from their taking in 

 oxygen during the night, and being too feeble to open 

 their pores for its escape during the day. The oxygen 

 thus confined unites with the materials of the pulp, 

 producing various acids, whose known action is to 

 change blues to reds; and consequently, when the 



