CHEMICAL PROCESSES IN CELLS. 137 



again rendered inorganic, or become combustion products. The}' are, 

 therefore, restored to the mineral world by the animal in the same form 

 in which they were originally absorbed by the vegetable. 



Vegetables and animals are the depositories and agents of life on the 

 surface of the earth. The essential characteristic of the vital operations 

 of the former is their power of elaborating organic from inorganic ma- 

 terial. Animals are charged to destroy, after assimilation, the results of 

 the vital operations of the vegetable. The animal kingdom is thus subor- 

 dinate to the vegetable, and organic life represents a closed circle of 

 metamorphosis of matter. Plants appropriate inorganic matter out of 

 the surrounding inorganic nature, out of the ground and air, and convert 

 it into the constituents of their own tissues. They then become food for 

 animals, are converted into animal tissues, and are again returned to the 

 ground and air as inorganic compounds. Thus, the carbon of the carbon 

 dioxide of the air becomes the carbon of cellulose or starch, of sugar, of 

 fat, of gum, and of albumen in the plant ; as food of animals it then 

 becomes the carbon of various animal tissues. In the vital processes of 

 the animal the carbon of the tissues undergoes oxidation, and is returned 

 to the atmosphere through the expelled air as carbon dioxide, or, in 

 other words, in the form in which it originally left the atmosphere. An 

 analogous circle might also be traced for the other constituents of the 

 animal tissues. 



We can thus understand how the constituents of animal and vege- 

 table cells may in all essential points be analogous; but, while this is so, 

 the chemical processes in each are very different. Green plants, in their 

 capability of deoxidizing inorganic food elements, are dependent upon 

 power from without — the heat and light from the sun. They therefore 

 store up energy in their tissues. Animal cells, in oxidizing the materials 

 derived from the vegetable world, liberate a force, as in all other forms of 

 oxidation, which in this case represents an equivalent of mechanical 

 energy precisely equal to the force rendered latent in the nutritive proc- 

 esses in the vegetable. In the animal cell this energy may take on the 

 form of heat, electricity, or light, as in certain organisms, or mechanical 

 movement. 



1. The Vegetable Cell. — The assimilative processes in the vegetable 

 cell are dependent upon the presence of protoplasm, which in its modi- 

 fied form as chlorophyll has the power of making use of the sunlight for 

 purposes of organic deoxidation, and constitutes the most powerful re- 

 ducing body known. The properties of chlorophyll are not exactly 

 known, as it has probably never been prepared in a perfectly pure state. 

 In the chlorophyll granules are often to be found, the results of its organic 

 activity, such as starch-granules; but their precise mode of formation, or 

 the precise share which chlorophyll has in producing their formation, is 



