CHAPTER I 



ABSORPTION OF CARBON AND ACCUMULATION OF 



POTENTIAL ENERGY 



1. Sources of Carbon for the Plant. Method of Artificial 

 Cultures. — Plant life is characterized by continuous accumulation 

 of organic compounds. These compounds are not absorbed from 

 the environment in an elaborated condition. They are synthe- 

 sized in the body of the plant itself, from simpler substances taken 

 by the plant from its surroundings. The absorbing organs of 

 the plant are the roots and leaves. Roots absorb from the soil, 

 water and the substances dissolved in it, while leaves take in 

 gaseous substances from the atmosphere. 



The organic compounds constituting the body of a plant are 

 rather complex in their composition. We may subject plants to 

 analyses of two kinds — an organic analysis and an elementary 

 one. In organic analysis we endeavor to single out, in a state as 

 unaltered as possible, the chemical compounds whose complex 

 mixture constitutes the body of the plant. These are in the first 

 place the carbohydrates, such as cellulose, starch, and the different 

 sugars. Cellulose forms the framework of the plant, the cell walls. 

 The other carbohydrates serve as reserve substances. Fats and 

 proteins are present in smaller amounts serving as constituents of 

 protoplasm and as reserve nutrient compounds. Still less in amount 

 are the pigments which produce the color of plants. The presence of 

 organic acids, glucosides and alkaloids frequently impart to the 

 plant an acrid flavor and poisonous properties. The detailed exami- 

 nation of these and numerous other substances, such as the 

 tannins, resins, ethereal oils, etc., constitutes organic or biological 

 chemistry. Essential components of the plant body also are the 

 different inorganic salts which are partly dissolved in the cell sap, 

 and are found in the protoplasm or in the cell wall. Most easily 

 detected in the plant are the salts of calcium, the carbonates, and 

 especially oxalates. These are deposited in the cells in crystal- 

 line formations, described in plant anatomy. By means of sensi- 



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