190 Education through Nature 



mately associated for mutual aid and protection. 

 The small algi inside are supposed to manufacture 

 food for the fungus outside, and this latter serves as 

 protection for the algi. Hence this is an interesting 

 case of different organisms associated together, like 

 slave and master, for mutual benefits. Such a rela- 

 tion is called symbiosis, and the parties to the associa- 

 tion are called symbionts. 



BACTERIA. If a handful of pond-scum or a handful 

 of hay be put into a jar of fresh water and allowed to 

 stand for a week or ten days, the water turns black 

 and emits a foul odor, due to the decaying organic 

 substances in the water. There will then gradually 

 appear on the surface of the water a thin film which 

 gradually thickens into a crust floating on the surface. 



If this surface film be examined under the micro- 

 scope, it will be found to consist of innumerable tiny 

 bodies moving, whirling, wriggling rapidly through 

 the water. To see the individual bodies well requires 

 the highest magnifying powers of the microscope. 

 They are the so-called micro-organisms, germs or 

 bacteria. 



These micro-organisms are very important ones, 

 notwithstanding their minuteness; for many of them 

 are very useful (nonpathogenic) while many are 

 very harmful inasmuch as they are the cause of many 

 contagious diseases (pathogenic), such as scarlet 

 fever, cholera, yellow fever, diphtheria, lockjaw, 

 and anthrax, etc. They seem to be closely related 

 to yeast, the organism used for raising bread and in 

 the fermentation of liquors. It is not definitely 

 known how yeast does its work as a ferment whether 

 it be by means of its secretions acting when it is present 

 or whether it be by taking substances such as the 

 starch of the grains used for such purposes into its 

 body and there in a sense digesting it, giving off as a 

 result of disorganization a gas, carbon dioxide, and 



