DEVELOPMENT OF PLANTS 157 



meat because it is injurious to bacterial life and smoking also 

 introduces volatile products that prevent their growth. So too 

 acids are generally harmful to bacteria. You have heard much 

 regarding the use of salicylic acid, benzoate of soda, etc., in pre- 

 serving foods and certain beverages. Whether these substances 

 are harmful to us or not, certain it is that they are fatal to the 

 existence of bacteria. So also meats, fruits and vegetables are 

 kept in a wholesome condition for considerable periods in ice 

 boxes and for months in cold storage since the bacteria grow 

 more slowly as the temperature is lowered and their activity 

 ceases at the freezing point. It should be remembered, however, 

 that no amount of cold will kill all of the bacteria and that food 

 spoils very quickly when removed from cold storage. 



(c) Economic Importance of Bacteria. Insignificant and simple 

 as are the forms and structures of bacteria it is safe to say that 

 no plants are so vitally related to our welfare. Numerous forms 

 are instrumental in effecting changes in various substances termed 

 fermentation; others assist in the decay of dead plants and 

 animals; a comparatively small number of species build up or 

 construct substances that are of vital importance to plant life; 

 finally many diseases of plants and animals are due to bacteria. 

 The work performed in all the above mentioned processes, save 

 the third, is of the same nature. These plants have the power 

 of breaking down into simpler compounds the various sub- 

 stances with which they may be brought in contact. Thus 

 vinegar is formed through the decomposition of certain alcohols 

 into simpler substances, as acetic acid and a gas, carbon dioxide. 

 These changes are effected by substances, termed ferments or 

 enzymes, that are excreted by the bacteria. These enzymes are 

 very remarkable compounds (p. 20). They do not enter into per- 

 manent union with any substance but by their presence cause it to 

 break down into some of its component parts. When this decom- 

 position is attended with the liberation of gas, as in certain sugar 

 solutions, we term the process fermentation but when in an 

 exactly similar way bacteria break down the substances compos- 

 ing the bodies of plants and animals we term the process decay. 

 So also in disease bacteria simply cause the decomposition of 



