THE ATMOSPHERE 201 



poor in oxygen, and possessing a high affinity for oxygen, store 

 up chemical potential energy. 



As a generalisation, plants build and animals destroy organic 

 substances. The life of the plant is synthetic, a reduction process ; 

 that of the animal analytic, an oxidation process. The plant 

 converts kinetic into potential energy ; the animal converts the 

 potential energy of its food-stuff the plant into kinetic energy. 

 But there is no jump in nature. There are unicellular chloro- 

 phyll-free organisms moulds and bacteria which cannot as- 

 similate carbon out of C0 2 . They must be fed on organic 

 combinations on sugar, tartrates, &c. On the other hand, these 

 organisms assimilate nitrogen from inorganic combinations salts 

 of ammonia, nitrites and synthesise proteid. By producing 

 fermentation they decompose organic compounds, and like animals 

 use up potential and develop kinetic energy warmth and move- 

 ment. Certain highly organised parasitic plants, belonging to the 

 phanerogams, are also chlorophyll-free and feed on organic matter 

 synthesised by other plants. 



On the animal side there are certain worms and Ccelenterates, 

 such as Hydra viridis, which contain chloroplasts, and seek the 

 sunlight and die when kept in the dark. The chloroplasts belong 

 to monocellular algse, which live symbiotically in these organisms. 

 Within the stiff cellulose membrane of each cell of the higher 

 plants there lies a contractile protoplasmic body which respires 

 and shows active streaming movements, using up oxygen and 

 giving out carbonic acid. Only in the chloroplast-holding cells is 

 this process of oxidation covered by an intenser process of reduc- 

 tion which obtains during the incidence of sunlight. Similarly 

 in every cell of the higher animals synthetic as well as analytic 

 processes take place. 



The combustion that takes place in the living organism is no 

 such simple process as the combustion of coal in a steam-engine. 

 The food, before it reaches its final end products, undergoes a 

 whole row of different chemical changes, changes of varying 

 energetic significance. In the steam-engine the heat, derived 

 from the chemical energy of the fuel, is the motive power, and 

 it matters little from what fuel the heat comes. In the organism 

 the food is not only heat-producer, but tissue builder and repairer, 

 and generator of new organisms of a similar type. 



Regarded as a source of energy, the food must be converted 



