RELATION TO ENERGY 25 



Only we must not expect to draw sharp lines. The 

 forms included in any groups we make will always 

 tend to shade off into forms belonging to other groups. 



Thus we can say that the essential animal character 

 depends upon the habit of consuming solid organic 

 food ; that the essential plant character depends upon 

 the habit of absorbing liquid and gaseous inorganic 

 food ; that a man is indubitably an animal, while 

 an oak tree is as indubitably a plant ; that with the 

 man we can group a host of other different kinds of 

 organism as animals, and with the oak tree a host of 

 other organisms as plants ; but that when we come 

 to minute microscopic organisms we find the differ- 

 ences which are so clear and sharp among the higher 

 forms becoming blurred, till finally we arrive at forms 

 of which it is impossible to assert that they are definitely 

 animals or definitely plants. 



Relation of Animals and Plants to Energy. — The 

 essential difference between animals and plants in 

 their relation to food determines also their character- 

 istic difference in relation to energy. All living organisms 

 may be regarded as machines transforming energy 

 from one form into another, for instance, from the 

 potential energy locked up in the molecules of organic 

 food to the kinetic energy seen in motion of the body 

 and in the production of heat (animal), or from the 

 " radiant " (kinetic) energy of sunlight to the potential 

 energy of organic substances formed in the body 

 (green plant). This subject will be dealt with - more 

 in detail in later chapters. 



Meanwhile we note the broad difference between 

 animals and plants in their relation to energy is that 

 animals consume organic food and spend the energy it 

 contains in heat and motion, while plants build organic 



