312 RESPIRATION OF PLANTS. 



similar offices to the lungs of animals. They are the 

 breathing organs. In the animal economy "a certain 

 quantity of carbon is necessarily retained, in combination 

 with nitrogen and other elements, to form muscle ; but 

 this is constantly undergoing change ; the entire system 

 being renewed within a comparatively limited period. 

 The conditions with plants are somewhat different. For 

 instance, the carbon is fixed in a tree, and remains 

 as woody fibre until it decays, even though the life of 

 the plant may extend over centuries. 



Animals, then, are constantly supplying carbonic 

 acid ; plants are as constantly feeding on it ; thus is the 

 balance for ever maintained between the two kingdoms. 

 Another condition is, however, required to maintain for 

 the uses of men and animals the necessary supply of 

 oxygen gas. This is effected by one of those wonderful 

 operations of nature's chemistry which must strike 

 every reflecting mind with admiration. During the 

 night plants absorb carbonic acid ; but there is a con- 

 dition of repose prevailing then in their functions, and 

 consequently their powers of effecting the decomposition 

 of this gas are reduced to their minimum. The plant 

 sleeps, and vital power reposes; its repose being as 

 necessary to the plant as to the animal. With the first 

 gleam of the morning sun the dormant energies of the 

 plant are awakened into full action ; it decomposes this 

 carbonic acid, secretes the carbon, to form the rings of 

 wood which constitute so large a part of its struc- 

 ture, and pour out oxygen gas to the air. The plant 

 is, therefore, an essential element in the conditions 

 necessary for the support of animal life. 



The animal produces carbonic acid in an exact pro- 

 portion to the quantity of carbonaceous matter which 

 it consumes. Fruit and herbage contain a small quan- 

 tity of carbon in comparison with muscle and fat. But 

 let us confine our attention to the human race. Man 

 within the Tropics, where the natural temperature is 



