STUDIES OF SPORE-PLANTS 277 



are lower than any we have yet studied. Since they have 

 no chlorophyll, some authors class the bacteria with fungi; 

 but they are probably not closely related to yeasts, molds 

 and other fungi. Popularly, the four words which stand in 

 the above heading are used as if synonymous ; but the fact 

 is that the last three may properly be applied to some micro- 

 scopic organisms which are not bacteria, and in certain cases 

 are one-celled animals. In other words, the last three are 

 general words meaning microscopic organisms ; while bac- 

 teria are a special kind of such small organisms, belonging to 

 the plant kingdom. Sometimes it is very convenient to 

 have the general terms. For example, the statement that 

 micro-organisms affect our everyday life in many important 

 ways includes other organisms as well as bacteria. Some 

 of these other organisms will be referred to briefly at the 

 close of this section, because the practical relations to our 

 daily lives of many of them are similar to those of the bacteria. 

 From our studies of yeasts and molds it is evident that 

 foods prepared for human use are likely to ferment (that is, 

 favor growth of yeast), if in a liquid form "containing a cer- 

 tain amount of sugar (e.g., fruit juice, or dilute sugary syrup). 

 If the foods are solid and contain a large amount of starch or 

 sugar (e.g., bread or preserves), molds are likely to develop 

 first. If the foods contain considerable protein (e.g., soups, 

 such vegetables as peas and beans and meats), it is a well- 

 known fact that decay characterized by disagreeable odors 

 begins quickly. This change is caused by bacteria, which 

 may grow so rapidly as to prevent development of the yeasts 

 or molds. In fact, there is a tendency among the bacteria, 

 yeasts, and mold? for the kind which gets the first start to 

 crowd out the others. Thus bacteria do not usually get a 

 chance to develop in fruit juice until after yeasts have used 

 up the sugar ; bacteria get the first start in protein foods; 

 and molds predominate on starchy foods. An apple may 

 first start to rot because filaments or hyphae of molds pene- 



