30 TEXTBOOK OF ZOOLOGY 



cause carbon dioxide, a gas of the air, and water to unite in the 

 formation of a simple carbohydrate. This is the basic food material 

 of plants as well as of animals. During daylight hours while photo- 

 synthesis is in progress, oxygen is discharged into the air as a prod- 

 uct of the process. This oxygen adds to the atmospheric supply 

 and is used by animals in respiration. The carbon dioxide dis- 

 charged by respiration of plants and animals is made use of by 

 plants in this synthesis of material. The excretory products of ani- 

 mals contain nitrogen which is easily transformed into a soluble 

 form and absorbed by plants to be combined with the simple carbo- 

 hydrate, already described, to produce protein. 



In general, plants, by utilizing the radiant energy of the sun, and 

 chlorophyll, as a catalyst in causing the combination of water and 

 carbon dioxide, form the potential material for the animals, because 

 they extract simple substances from the earth and unite them into 

 complex foodstufl^s, such as the starches and proteins. Plants are 

 devoured by animals, and some animals are in turn devoured by 

 others. The complex substances are then broken down with libera- 

 tion of energy, and the by-products excreted are again incorpo- 

 rated in the earth to be available to other plants. 



Attributes of Life 



Most of us think we know what life is, but if asked to define it, 

 we find ourselves confronted by an almost hopeless task. The ques- 

 tion. What is Life?, is the greatest riddle in the biological world. 



The term life is an abstraction with no objective reality except as 

 it is a phenomenon related to the activities of living units. The 

 following statement has been given and is probably as nearly a 

 definition as can be found : Life is a continuous series of reactions 

 in a complexly organized substance, by means of which the organi- 

 zation tends to adjust itself to a constantly varying environment. 

 Numerous attributes of living material may be given. Living mate- 

 rial has the ability to carry on active chemical reactions without 

 losing its body form. It is responsive to changes in the environ- 

 mental conditions ; therefore, it is said to be adaptive. Living 

 material is able to sustain and reproduce itself under favorable con- 

 ditions. The size of living organisms varies within definite limits. 

 Much more will be said of living material in the following chapter. 



