CHAPTER EIGHT 



THE UTILIZATION OF FOODS 



IN the preceding chapters we have seen how food is manu- 

 factured by the plant, how it is made soluble and is trans- 

 ferred, and how the excess food is accumulated in various 

 organs of the plant. Food is finally utilized by the cells in 

 respiration, assimilation, and growth. In this chapter we shall 

 learn the meanings of these terms and study the changes that 

 the foods undergo in connection with the production of 

 energy, the making of protoplasm, and the growth of the 

 cells. 



Energy necessary to plant cells. In order to do work, 

 each machine in a manufacturing establishment must be 

 supplied with energy, and every living cell in a plant requires 

 energy for carrying on its work of repair, growth, and move- 

 ment. In manufacturing establishments the energy is usually 

 generated at one place and is then transmitted by means of 

 shafts and belts or wires and motors to all parts of the fac- 

 tory. The plant cannot transmit energy from one part 

 to another, but it can and does send food to all its living cells, 

 and from this food each cell generates within itself the energy 

 that it needs. 



Respiration. A steam engine is supplied with energy by 

 the oxidation of fuel beneath the boiler that is connected 

 with it. A cell is supplied with energy by the oxidation of 

 food within it. The process by which the cells obtain energy 

 through the oxidation of foods is called respiration. In the 

 process oxygen is absorbed and carbon dioxid is given off. 

 Respiration takes place in all living cells, and to carry on this 

 necessary process all parts of the plant must be supplied with 

 oxygen. The leaves and stems of land plants obtain their 



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