168 PRACTICAL BOTANY 



are of great importance to agriculture and gardening in other 

 ways, but these are considered in the chapters Further Dis- 

 cussion of Dependent Plants and Further Discussion of Plant 

 Industries. 



156. Relation to other industries. It is impossible to do more 

 in this connection than to suggest a few of the industries in 

 which bacterial processes have an important part. The number 

 and extent of the changes, physical, chemical, or both, that 

 always occur when bacteria grow, suggest the immense indus- 

 trial field that lies before the student of bacteriology ; for bac- 

 teria may, under favorable circumstances, grow upon almost 

 any organic substance. In this immense field there is as yet 

 but a small amount of positive knowledge. There are, how- 

 ever, a few cases in which some of the facts are known. From 

 general knowledge and from what has already been said, it is 

 evident that all our fruits, vegetables, meats, etc., are at times 

 in danger of destruction by bacteria or other organisms. How 

 to prevent this destruction has been one of the important prob- 

 lems since civilization began. Surplus production of food is 

 useless unless some of it may be kept for future needs. If men 

 could not preserve foods, they could live only in regions where 

 there is perpetual food production, or they must constantly 

 migrate into regions where food might be had. 



In preservation of fruits much progress has been made by 

 improvements in methods of gathering them. Most fruits have 

 a natural covering, which, if unbroken or unbruised, and kept 

 clean, will for a long time prevent the entrance of organisms 

 of decay. If two sets of ripe apples are gathered, one carelessly 

 so that bruising and scratching of the surface occurs, the other 

 with sufficient care to avoid these things, and both are placed 

 upon a shelf in the schoolroom, an interesting demonstration 

 will be made of the desirability of care and cleanliness. 



Low temperature and drying were found to prevent decay 

 long before it was known how decay is produced. Refrigeration 

 has now become a leading method of preventing decay, since 

 bacteria do not thrive at or below the freezing temperature. 



