NATURE OF PLANTS 21 



plants are constantly inhaling oxygen and exhaling CO2. This 

 interchange of gases is the reverse of that occurring in photo- 

 synthesis. Furthermore respiration goes on all the time when 

 conditions are suitable for life while photosynthesis only is pos- 

 sible in the light. Let us consider the significance of this inter- 

 change of gases. Why do animals and plants breathe? The 

 construction of the foods to which attention has been called may 

 well be termed the storing up of the energy of the sunlight. 

 When the chlorophyll absorbs certain rays of light it does not 

 destroy any of the energy in those rays. The energy is changed 

 to another form and it still exists in the sugars and other com- 

 pounds that are built up through its power. The energy is 

 locked up, so to speak, in the food compounds. If these com- 

 pounds are decomposed into their simple elements again the 

 energy will be set free unimpaired. Oxygen is the principal 

 agent in the decomposition of foods. It has a great attraction 

 for nearly all the elements that compose the foods. It pulls 

 them away from each other and we say the foods are decomposed. 

 The changes occurring in the foods as the result of the action 

 of oxygen vary in different plants according to the nature of 

 the plant foods. In the majority of cases the decomposition of 

 the complex food compounds continues until such simple com- 

 pounds as CO2 and H2O are formed which we have seen are 

 utilized in photosynthesis in the manufacture of foods. These 

 products escape from the stomata as gas and water vapor. Ac- 

 cordingly respiration is usually indicated by the absorption of 

 oxygen and the giving off of CO2. Some plants, however, have 

 sufficient oxygen in their tissues to enable them to breathe for 

 considerable periods without absorbing oxygen from the air and 

 on the other hand in some plants CO2 is not always exhaled 

 because the decomposition of foods is not carried so far as to 

 result in the formation of so simple a product. We eat in order 

 to gain possession of the energy locked up in the foods. We 

 breathe in order that the oxygen may decompose these com- 

 pounds and set free this energy which gives us power to work 

 and move and keep our bodies warm. The plant lives in the 

 same way. It only differs from the animal in that it has the 



