FOOD MANUFACTURE 39 



process of breaking down the protoplasmic body is respira- 

 tion, and one of the superficial indications that respiration 

 is going on is that oxygen is taken in and carbon dioxide is 

 given off. This gas exchange, therefore, is not respiration, 

 but is merely the external evidence that the process is going 

 on. 



32. Summary. The peculiar work of green plants is to 

 manufacture carbohydrates. The raw materials used are 

 carbon dioxide and water, which the chloroplasts, with energy 

 obtained from sunlight, use in the manufacture, a certain 

 amount of oxygen being given off as a by-product. The car- 

 bohydrates thus manufactured are the basis of other foods (as 

 proteins). Water and carbon dioxide, therefore, are not foods, 

 but materials from which foods are manufactured. The food 

 of all plants and animals is the same, and when used it is 

 digested (if necessary) and assimilated (built up into proto- 

 plasm) ; and the evidence that the living protoplasm is 

 working is that respiration is going on, an external indication 

 of which is the entrance of oxygen and the escape of carbon 

 dioxide. All plants and animals, therefore, use the same 

 food and " breathe " in the same way, but only green plants 

 can manufacture food from material that is not food. 



