The Kinds of Work That Are Done by Plants 95 



under the suitable machinery, the carbon of the food is allowed 

 to unite with ox^^gen, and the energy is released to do the need- 

 ful work. And that is the way in which plants and animals ac- 

 complish their work; and the power to do this, — to absorb stored 

 energy, transfer it to all of their parts, hold it ready for use, and 

 release it when needed, — is the most distinctive feature of Uving 

 beings. 



The reason is now evident also for the reciprocal character 

 of the photosynthetic and respiratory equations. In photosyn- 

 thesis carbon dioxide and water are made into sugar and oxygen 

 with storage of energy; the sugar is transported by plants or by 

 animals to places of need, undergoing chemical changes on the 

 way but ever retaining the store of unsatisfied carbon; then in 

 respiration oxygen is allowed to come into chemical contact with 

 the sugar, and the two are changed back to carbon dioxide and 

 water with release of energy. It is because substances exist which 

 thus permit of such storage and transportation of energy that 

 organisms as we know them are possible. 



It may aid still more to a clear understanding of these two most 

 fundamental and important of all physiological processes if we 

 set their chief features in contrast in form of a table ; — 



Photosynthesis Respiration 



Occurs only in plants Occurs equally in plants and animals 



Occurs only in chlorophyll grains Occurs in all living protoplasm 



Occurs only in light Occurs equally in light and darkness 



Manufactures food Destroys food 



Increases weight Lessens weight 



Absorbs carbon dioxide Releases carbon dioxide 



Releases oxygen Absorbs oxygen 



Forms CeHiiOe from CO2 and H2O Reduces CcHisOg to CO2 and H2O 



Stores energy Releases energy 



We can now gather up these latter facts in another of our 

 verities thus, — the energy released in respiration ivas previously 

 latent in the unsatisfied affinity of the carbon in the food for the 



