PLANT FOOD 67 



PLANT FOOD FROM THE AIR. The term plant food is com- 

 monly used to designate all of the crude materials which are taken 

 into the plant, and which are utilized by it. Strictly speaking, the 

 term "plant food" is not analogous to the same term used in connec- 

 tion with animals. Plant foods are rather the raw materials used in 

 the manufacture of food. These materials are built into carbo- 

 hydrates, fat, and proteids, and in this form are used as food by the 

 plant. However, as we are here concerned with the source rather 

 than with the finished product, the term plant food will be used in its 

 more commonly accepted sense — that is, as meaning the crude mater- 

 ials. 



Disregarding the other constituents, which are present only in 

 very limited amounts, the atmosphere contains in one hundred parts :* 



By Weight. By Volume. 



Oxygen 23.17 20.95 



Nitrogen 76.83 79-OS 



(1) Oxygen. 



Free oxygen is utilized by plants in exactly the same way as in the 

 body of an. animal. Foods are required for the purpose of building 

 up new tissues and to furnish energy by their decomposition for the 

 growth and movement of a plant and its parts. Oxygen is necessary 

 for the latter process, the evolution of energy from the food material 

 being a process of oxidation. Carbon dioxide is given ofif as a result 

 and may again be utilized in photosynthesis, which is discussed below. 

 The liberation of energy from the food or tissue substance is known 

 as respiration. 



(2) Nitrogen. 



Free nitrogen as such cannot be assimilated by any green plant. 

 Small quantities of ammonia and nitric acid are washed down by rains 

 into the soil and are taken up by the roots. Certain bacteria, how- 

 ever, some living free in the soil and others in nodules of legumes fix 

 the free nitrogen of the atmosphere and convert it into a form which 

 can be utilized directly or indirectly by the plant. 



(3) Carbon. 



Just what is the source of the large amount of carbon used by the 

 plants was at one time a subject of extensive investigation. Experi- 

 ments show that plants flourish and increase in carbon content when 

 their roots feed in a nutrient solution containing no carbon, rhis 

 carbon must then, in such cases, be drawn from the air. But carbon, 



*Air also contains between .03 and .05 per cent of carbon dioxide. 



