234 BIOLOGY 



NATURE'S LIFE CYCLE 



Construction and Destruction. From a general survey of 

 the facts which have thus been explained, it will be seen that 

 there is a grand cycle in nature, in which the life of animals 

 and plants is concerned. All organisms need food, and the only 

 explanation of the fact that the food supply has not long since 

 been exhausted is the fact that the same materials have been 

 used over and over again, passing from plants to animals and 

 from animals to plants. The chemical processes going on 

 in the living world are of two types: those of construction 

 (synthetical), by which complex substances are built out of 

 simple ones; and those of destruction (analytical), by which 

 the complex materials are reduced to simpler ones. Green 

 plants growing in sunlight manufacture starch out of the 

 simple ingredients which they extract from the soil and the 

 air, utilizing sunlight as a- source of energy for this purpose. 

 Though they are building up these materials primarily for their 

 own life, they build more than they need, so that there is a 

 large surplus. This surplus is utilized by animals and by the 

 colorless plants. It is taken into their bodies as food, and serves 

 them as a source of energy, as well as material out of which 

 they can manufacture new substances, and grow. Eventually 

 the material is broken to pieces in the animal body and reduced 

 once more to a simpler condition. In this way animals utilize 

 as food a part of the surplus manufactured by green plants, 

 consuming the surplus of proteids, starches, etc. But other 

 materials made by the plants, like wood and leaves, do not 

 so readily serve as food for animals. These materials must 

 usually be broken down into simpler compounds, or the sub- 

 stance of which they are made would not get into a condition 

 where it could again be utilized. This seems to be the special 

 function of the Fungi. 



The Significance of the Fungi in Nature. Special emphasis 

 must be given to the significance of the Fungi in these de- 

 structive processes. In order that nature's processes may 



