NATURE OF PLANTS 21 



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 CO 2 and H 2 O are formed which we have seen are 

 utilized in photosynthesis in the manufacture of foods. These 

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

 cordingly respiration is usually indicated by the absorption of 

 oxygen and the giving off of CO 2 . 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 CO 2 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 

 added power to build up the complex food compounds from 

 crude material. By the decomposition of these foods it gains 

 energy to grow and carry on its other vital functions. The work 

 of respiration is carried on more economically by green plants 

 than by animals since in the animal the CO 2 escapes in the breath 

 as a waste product, while the plant uses this CO 2 during the 

 day time for the construction of foods. Consequently the escape 

 of CO 2 cannot be detected in the light unless an examination of 

 very rapidly growing organs be made. If a jar be nearly rilled 

 with opening buds of dandelions or rapidly growing shoots and 

 then closed air tight, sufficient CO 2 will be respired in a few 

 hours to extinguish a light that is lowered into the jar. In such 

 instances as these very rapid respiration is necessary to furnish 

 the required energy for growth and the volumes of CO 2 expired 



