110 THE NUTRITIVE FUNCTIONS. 



sented as consisting in the absorption of the carbonic acid 

 contained in the air, and its decomposition under the 

 direct influence of the solar light. That such a pro- 

 cess does go on in the leaves is undeniable. The carbon 

 is fixed in the plant, and the greater part of the oxygen is 

 exhaled through the leaf-pores into the atmosphere ; at the 

 same time it is evident, from the nature of the nutritive 

 processes going on in the interior, and the intimate connec- 

 tion subsisting between the duct-cells and the leaves, that 

 some of the oxygen will be necessarily absorbed from the 

 leaves into the interior of the plant. A small quantity is, 

 however, amply sufficient to supply the wants of vegetation, 

 and the rest is poured into the atmosphere to meet the 

 wants of animality. It is therefore true that carbonic acid, 

 which is given off from the lungs of animals as waste matter, 

 is taken in by the leaves of plants ; that vegetables purify 

 the air which animals contaminate ; that what is poison to 

 us is food to plants. It is also true that the leaves are 

 to a certain extent the pulmonary organs of plants, because 

 it is through the pores on their surface that the air gains 

 admission by means of which the fluids, not only in the 

 leaves but in the interior of the organism, are rendered nu- 

 tritious. Now the predominating function exercised by the 

 leaves is unquestionably that of de-oxydation or diges- 

 tion, which is antagonistic in its results to animal respira- 

 tion. When we speak of respiration in plants, assuredly 

 those processes in which oxygen is consumed deserve the 

 name, far more than the exhalation of oxygen by their 

 leaves and other green parts. 



The matters contained in the tissues of plants are very 

 various, and are either solid, liquid, or gaseous. All solid 

 organic matters, such as starch and chlorophyl, are formed 



