CHAP. II. FUNCTION OF NUTRITION. 189 



than the atmosphere. At present so little has been 

 ascertained of the conditions under which this air has 

 been introduced into the vessels, or of the peculiar office 

 which it is destined to perform, that we can do no more 

 than just mention the fact, and state the opinion of 

 some botanists, who have considered it probable that in 

 these situations also it is subservient to the process of 

 respiration, and who conclude that it is not impossible 

 there may exist a strong analogy between the manner 

 in which this function is performed by plants and by 

 some of the inferior tribes of animals. Insects for 

 example breathe by introducing air through several 

 spiracles ranged along each side of their abdomen, 

 and which open into certain ducts or pipes, singularly 

 resembling in their general appearance the tracheae or 

 spiral vessels of plants. 



(1?5.) Fixation of Carbon. When all those parts 

 of plants which are capable of assuming a green tint, but 

 more especially the leaves, receive the stimulus of light, 

 they immediately decompose the carbonic acid contained 

 in the sap. The result of this action is the retention of 

 the carbon, and the expiration of the greater part of the 

 oxygen into the surrounding atmosphere. The most 

 obvious effect produced by this fixation of carbon is the 

 appearance of that green colour which we find in nearly 

 all leaves, and in some other organs. In the few 

 cases which militate against this rule, we may reason- 

 ably imagine the existence of some other cause in 

 operation which speedily modifies the initial result. 

 Thus for instance, the peculiar tinge assumed by the 

 leaves of the red-beech, may possibly be owing to the 

 presence of an acid secreted simultaneously with the 

 fixation of the carbon, which converts the green to 

 red. The fixation of the carbon by plants appears 

 to be the first step in that elaborate process by which 

 brute matter is converted into an organisable compound ; 

 that is to say, into a material capable of being afterwards 

 assimilated into the substance of an organised body. 



