186 THE PLANT WORLD 



bacterial enzymes into gases, fluids, and some solids. It has an 

 offensive odor, and contains poisons which come as a result of the 

 work of the bacteria. 



Demonstration 4. To show how light affects the growth of bacteria. 



Cover with black paper one of two Petri dishes in which bacteria 

 are growing. Place the dishes in a light warm place for a few days. 

 Compare the growth of bacteria in the exposed dish with the growth 

 in the covered dish. 



Bacteria and air. We have seen that plants need oxygen in 

 order to perform the work that they do. This is equally true of 

 all animals. But not all bacteria need air to live ; in fact, some 

 are killed by the presence of air. Bacteria which live without free 

 oxygen are called anaerobic bacteria. They need oxygen, as do all 

 other living things, but they obtain it by breaking down the foods 

 on which they live, and utilizing the oxygen freed in this process. 

 Those that grow or thrive in the presence of oxygen are called 

 aerobic bacteria. 



Self -Testing Exercise 



Bacteria are found almost (1). Bacteria can be obtained 



for study by exposing a (2) medium containing (3) 



to the (4) for a few minutes. Under proper conditions of 



(5) they grow rapidly. There are three common (6) 



of bacteria : the (7) form, called coccus, the (8) ba- 

 cillus, and a (9) form, the spirillum. They prefer (10) 



to all other food. Many forms are (11) by (12) 



to light. Some forms obtain their (13) by breaking down 



the food substances on which they live. 



PROBLEM III. WHAT ARE SOME USEFUL THINGS THAT 

 BACTERIA DO? 



Bacteria cause decay. Imagine a world cluttered up with dead 

 plants, dead animals of all kinds, dead bodies of fish in the waters, 

 insects in the grass, cattle in the fields. Did you ever think 

 what this world would be like if nothing could decay? Bacteria 

 are responsible for decay. These bacteria are most numerous in 

 rich, damp soils containing large amounts of organic material. 



