LIVING MATTER 257 



279. The Need of Food. Response to outside influ- 

 ences, motion of a part of the body or the whole body, diges- 

 tion of food, and formation of other bodies like themselves, 

 are the physiological properties of plants and animals. Each 

 of these processes is work, and requires energy. Energy can 

 be obtained by living things only from oxidation; oxidizable 

 material (which is provided by food) and oxygen are neces- 

 sary for oxidation. Therefore all living things require food 

 and oxygen. (LABORATORY MANUAL, Exercise XXIV). 



280. Amounts of Food Required. Plants require less 

 food than animals because they do less work in performing 

 their functions than animals do. The temperature of a 

 growing plant is slightly, if at all, higher than that of the 

 surrounding air. Its temperature averages much lower than 

 that of many animals living under the same conditions. 

 Hence less oxygen is needed by plants in respiration and less 

 food material is needed as fuel. 



All animals do not require the same amount of food. The 

 robin is an active, rapidly moving organism; a toad is inac- 

 tive much of the time. Therefore a robin needs more energy 

 than a toad and, consequently, more food. 



281. Waste Products from Oxidation. When the coal 

 in a furnace is brought to the kindling temperature, its prin- 

 cipal constituent, carbon, unites with oxygen, and heat energy 

 is released. The gas, carbon dioxide, which is not only of 

 no assistance to combustion, but is *a positive hindrance, 

 passes off. It is called a waste product. 



A similar process takes place in the body of a plant or ani- 

 mal. Carbon is an element in many kinds of food and be- 

 comes a part of the body itself. This body material unites 

 with the oxygen taken in by the plant or animal to produce 

 energy and, just as in the furnace, carbon dioxide is formed. 

 As this gas cannot be used and is injurious when retained in 

 the body of the plant or animal, it is a waste product and is 

 given off from the breathing organs of animals and from the 



