METABOLISM 133 



of different types, such as those having for their by-products 

 butyric acid (in the spoihng of butter), lactic acid (in the souring 

 of milk), and various others, many of which are of economic 

 importance. 



In all these cases energy is liberated and may be detected by 

 the consequent rise in temperature, which is often more marked 

 than in aerobic respiration. 



Certain micro-organisms are exclusively anaerobic and are 

 actually killed by the presence of free oxygen. Others, like yeast, 

 may respire either aerobically or anaerobically, depending upon 

 the external conditions. 



The decay of dead organic matter is due almost entirely to 

 the respiration of micro-organisms. If the material is exposed 

 freely to the air, bacteria will break it down rapidly and com- 

 pletely by their aerobic respiration. If free oxygen is unavailable, 

 as is the case within a large mass of dead organic material, the 

 process is carried on anaerobically and is slower and more 

 complicated. Here a whole series of micro-organisms, each 

 specific in its activity, are successively concerned. One type will 

 break down the organic substance partially, extracting a certain 

 amount of the potential energy which it contains. In its altered 

 chemical state and with its diminished supply of energy, the 

 remaining material is now seized upon by another type of micro- 

 organism and, entering into the anaerobic respiration of this 

 form, is still further broken down and loses still more of its 

 potential energy. This process continues until finally the whole 

 of the material (except its mineral constituents) passes into the 

 atmosphere as carbon dioxide, water and nitrogen, the original 

 materials out of which organic substance is constructed. 



We have now completed a study of the root, the stem, and the 

 leaf, and of the functions of absorption, conduction, photosynthe- 

 sis, respiration, and transpiration. These are the main vegetative 

 structures and functions of the plant body, and are graphically 

 set forth in Fig. OS. 



QUESTIONS FOR THOUGHT AND DISCUSSION 



384. Which of the three main types of food makes up the largest 

 part of the diet of animals and man? 



385. What advantage is it to the plant to have its carbohydrate food 

 stored chiefly in tlie form of starch rather than chiefly in the form of 

 sugar? 



