PHENOMENA OF LIFE 17 



3. PLANTS AND ANIMALS COMPARED 



It is easy to choose characteristics that will serve to distinguish 

 a tree from a man, but the separation of the simplest animals 

 from the simplest plants is a more difficult problem. In fact, 

 there are at the present time a number of organisms that are 

 claimed by both botanists and zoologists. There is no single 

 peculiarity which can be used in all cases to discriminate between 

 these groups of organisms. The view now generally accepted 

 is that plants and animals originated together but have developed 

 along divergent lines. However, certain general features can 

 be indicated in which the two kingdoms differ. These are given 

 in the following table; but the reader should bear in mind that 

 there are exceptions to every one of these criteria. 



TABLE II 



CHIEF CHARACTERISTICS OF PLANTS AND ANIMALS CONTRASTED 

 PLANTS ANIMALS 



1. Structure Form of body rather vari- Form of body usually inva- 



able ; new organs added riable ; organs compact 

 externally. and mostly internal. 



2. Locomotion Usually none in adult con- Usually well developed. 



dition. 



3. Irritability Respond to stimuli slowly ; Respond to stimuli quick- 



no nervous system. ly ; nervous system pres- 



ent in higher forms. 



4 Metabolism Possess chlorophyll ; manu- No chlorophyll ; require 

 facture organic food from organic food. 

 CO-2 and H 2 O in the pres- 

 ence of light. 



5. Waste products Oxygen, carbon dioxide, Carbon dioxide, water, urea, 

 water. faeces. 



The presence of chlorophyll in green plants has an important 

 bearing on metabolism, for it enables plants to manufacture 

 starch when light is available. This simple organic compound is 



