2 68 READINGS IN BIOLOGICAL SCIENCE 



the algae 'burns' in respiration. Minute fish, however, happen to feed on 

 algae and the stored sugars; also vitamins pass into the systems of the 

 fish where they are utilized. These small fish are eaten, in turn by larger 

 fish and so on until they find their M^ay to the table of man or die in some 

 other way. Other organisms may enter into this chain and change its di- 

 rection. Animal life is dependent upon the sugars made by green plants 

 and the latter could not exist without light, heat, minerals, gases and water. 

 Thus we see a chain of dependence stretching throughout the realm of the 

 living. This is the chain of life. 



INTERDEPENDENCE OF PLANTS AND ANIMALS * 



A . S . PE ARS E 



Since life began plants and animals have developed together. A plant 

 and an animal are more or less interdependent. The activities of one result 

 in waste products which are necessary for the syntheses of the othej*. 

 Photosynthesis, the process which gives rise to nearly all the organic foods 

 which are used by plants and animals, requires carbon dioxide which is, 

 in turn, liberated by the breaking down of the substance of living or dead 

 animals or plants. Decay is commonly brought about by bacteria and other 

 fungi. All living substance forms carbon dioxide as a result of its activities. 

 With a few exceptions, both plants and animals require a continual supply 

 of oxygen, and oxygen is an end product of photosynthesis. Carbon dioxide 

 in large amounts is injurious to living substance, and its presence may make 

 a particular locality unfit for life. On the other hand, the fact that it is 

 necessary for photosynthesis makes it a continual necessity for green plants. 



Chlorophyll is the remarkable, life-supporting green substance which, 

 when given a supply of water and carbon-dioxide and energy in the form 

 of light, can manufacture simple starches and sugars. These in turn can 

 be built up by living things into simple protein substances if nitrogen and 

 a few other chemical compounds are available. Although nitrogen makes 

 up a large proportion of the atmosphere it cannot be used by living things 

 in its simple, gaseous state, but must combined with hydrogen, oxygen, 

 and carbon before it can be made a part of living substance. The only 



Water J 2 hydrogen 



contains \i oxygen These in the 



presence of - 

 Carbon f chlorophyll 



dioxide -i i carbon and light 



contains U oxygen 



The Manufacture by Plants of Organic Food from Water and Carbon Dioxide. 



simple fi carbon 

 Will form — sugar •{ 2 hydrogen 



•-I oxygen 

 Will se^f'^ree {' ^^^S^" 



* Reprinted from Envirorifnent and Life by A. S. Pearse, by permission of Charles 

 C. Thomas, publisher. Copyright 1930. 



