PART I 

 PHYSIOLOGY OF NUTRITION 



CHAPTER I 



ASSIMILATION OF CARBON AND OF THE RADIANT 



ENERGY OF THE SUN BY GREEN PLANTS 



§i. Importance of the Assimilation of Carbon by Green Plants.— Plants 

 may be classified according to their color into two groups, those that are 

 green and those that are not. The green color forms such a conspicuous char- 

 acteristic of many plants that certain ones are sometimes spoken of as "greens." 

 The general distribution of the green coloring would itself suggest that some 

 important property must be connected with it, and such is indeed the fact; upon 

 this green coloring depends one of the main cosmic functions of plants, the 

 building up of organic compounds from inorganic substances. A simple ex- 

 periment will show this. A seed is placed in quartz sand and is watered from 

 time to time with a solution of mineral salts. A plant grows from the seed, 

 blooms and bears fruit. Comparison of the amount of organic material origi- 

 nally present in the seed, with the corresponding amount found in the grown 

 plant, shows that the latter amount is very much greater. If follows that green 

 plants are able to form organic compounds from inorganic ones. Animals, and 

 plants without green pigment, generally lack this power; they obtain organic 

 compounds only after these have been already manufactured by green plants. 

 The formation of organic substances by green plants is thus not only important 

 from the standpoint of plant physiology, but it acquires a much broader interest, 

 since the whole animal kingdom, including even mankind, is dependent upon 

 green plants. In a physiological sense, green plants form the connecting link 

 between the animal and mineral kingdoms. 



Since all organic compounds are characterized by their carbon content and 

 by their combustibility — the latter property implying that energy was stored 

 up in their formation— the study of plant physiology may begin with an in- 

 quiry as to the sources of the carbon and the energy necessary for the formation 

 of organic compounds in the organism. The answer is derived mainly from the 

 study of the assimilation of carbon dioxide. This process consists, essentially, in 

 the absorption of carbon dioxide by the green parts of plants and in the elimination 

 of oxygen, in sunlight. Since the volumes of the two gases involved in this proc- 

 ess are found to be about equal, it follows that for each molecule of carbon 

 dioxide absorbed a molecule of oxygen is eliminated; C0 2 = 2 + C (principle 

 of Avogadro). The carbon remains in the plant and thus produces an increase 

 in its weight, this process being a part of what is called nutrition. 



