CHAPTER I. PROPERTIES OF PROTOPLASM. 223 



is suspended, while respiration, which goes on continuously, can 

 readily be discovered by appropriate experiment. 



The fact of respiration in plants has also been demonstrated 

 by experiments on fungi and other plants destitute of chloro- 

 phyll, and which, therefore, cannot utilize carbon dioxide as 

 food. Here the inhalation of oxygen and exhalation of carbon 

 dioxide is found to go on continuously, as in animals. 



Both classes of organisms are also in substantial agreement 

 as regards destructive metabolism. In both, the energy which 

 results in the various phenomena of life is derived from the 

 breaking down of complex into simpler matters by processes of 

 oxidation. In both, complex compounds with much potential 

 energy become simpler compounds with less potential energy, 

 and the difference becomes kinetic or actual energy in the form 

 of heat, electricity and mechanical motion, giving rise to the 

 various activities of the organism. Here lies the significance 

 of the respiratory process. In this transfer of matter from a 

 higher to a lower potential, oxygen is consumed, and gaseous 

 carbon dioxide escapes as one of the products of the change. 

 The organism is therefore in many respects comparable to an 

 engine in which the latent energy of the fuel is converted into 

 work, while during the process, the wood or coal passes into 

 carbon dioxide and water which are no longer available as 

 sources of energy. 



The products of metabolism are not always the same in the 

 plant as in the animal, but the differences are only such as can 

 readily be accounted for by differences of habit ; indeed, they are 

 scarcely greater than those existing between animals of widely 

 different habits. There are, in fact, few vegetable products 

 whose analogue is not somewhere found in the animal kingdom. 



The divergence between plants and animals is perhaps widest 

 in the matter of assimilation or constructive metabolism. Green 

 plants have a power not possessed by most animals, of raising 

 mineral matter into comparatively complex organic compounds. 

 Thus they derive their sustenance directly from the inorganic 

 world. From the elements of water and carbon dioxide they 

 form a carbohydrate, and then, by bringing this into other com- 

 binations, or causing it to pass through other chemical changes, 

 they use it to build up their tissues. This the animal cannot do; 



