216 A YEAR IN SCIENCE 



To a less extent oxygen is also taken in through the 

 stems. The oxygen, after entering the plant, passes 

 by diffusion 'to all parts of the plant, where it is used 

 to decompose the complex protoplasm, or perhaps food 

 materials, into simple substances. Just what chemical 

 processes this may involve we do not know, but oxygen 

 is consumed, complex plant substances are broken 

 down, and, as a result of this decomposition, energy is 

 released. By means of this energy plants are enabled 

 to perform their work. This process results in the 

 formation of various simple compounds, chief of which 

 are carbon-dioxide (C0 2 ) and water (H 2 0). These 

 are waste products, and they are passed off through the 

 same organs that take in oxygen. 



This process in plants (also animals) in which oxygen 

 combines with the living tissues and releases energy is 

 known as respiration. It should be kept distinctly in 

 mind that the exchange of gases in respiration in plants 

 is exactly the same as that in animals. It was once 

 customary to contrast plants and animals by stating 

 that the former took in C0 2 and gave off 0, Avhile the 

 latter took in and gave off C0 2 . This confusion arose 

 because the processes of food-making and respiration 

 in plants were not fully understood. In bright sun- 

 light the process of food-making takes place so rapidly 

 that it masks the process of respiration. As a result, 

 plants in breathing were supposed to use C0 2 and to 

 give off 0. 



Air storage. In most plants there is an abundant 



