18 AN INTRODUCTION TO ZOOLOGY 



built from the inorganic substances, carbon dioxide and water. 

 With starch as a basis more complex organic compounds are 

 elaborated by the living substances. Now the living substance 

 of plants is practically the same as that of animals, and it seems 

 strange at first thought that plants should give off oxygen as a 

 waste product, for this gas is one of the constant needs of living 

 substance. If, however, we keep in mind the fact that chloro- 

 phyll is carrying on one set of processes (Fig. 3) which require 



CARBON DIOXIDE 



FIG. 3. Diagram showing the activities of chlorophyll. 



carbon dioxide and liberate oxygen as a waste product, while 

 the living substance is carrying on another set which use oxygen 

 and release carbon dioxide (Fig. 2), the matter becomes one of 

 simple addition and subtraction; one group of processes uses the 

 waste products of the other. 



The qualities that are usually cited as being peculiarly charac- 

 teristic of animals are locomotion and nervous activity. With the 

 exception of a few extremely sensitive species of which the com- 

 mon sensitive plant, Mimosa pudica, is the most familiar ex- 

 ample, plants respond very slowly to external stimuli and their 

 power of transmitting impulses is poorly developed. Locomotion 

 is absent except in a few simple forms and free swimming repro- 

 ductive cells. 



