CHAPTER XIII 



EFFECT OF EXTERNAL CONDITIONS ON BACTERIA 

 (Continued} 



Light. The relation of protoplasm to light varies with 

 different types of organisms. Light as a form of radiant energy 

 is utilized by certain living forms in building up their food 

 material. In the green plants, for instance, the chlorophyll 

 enables the organism to absorb and use the sun's energy in 

 constructing its food material, and the greatest amount of 

 assimilation occurs in the absorption bands of the chlorophyll. 

 Forms that derive their energy from organic sources do not 

 need light. Colorless plants and animals obtain their energy 

 by breaking down highly complex food substances and are thus 

 independent of the energy of light. Since the bacteria are 

 colorless organisms we would naturally expect that light was 

 not essential to them, and this is true in the main. The ma- 

 jority of bacteria grow best in the dark. A small group of 

 bacteria that will be referred to later obtain their energy much 

 in the same way as the green plants do and are therefore 

 dependent upon sunlight for their energy. The great majority 

 of bacteria, however, prefer darkness rather than light, and for 

 these forms light is not only unnecessary but it may act as a 



