256 GENERAL BOTANY 



ferment secreted by a saprophytic bacterium which causes the 

 sugar of milk to be transformed into lactic acid. 



Milk sugar 4- lactic ferment = lactic acid + lactic ferment. 



Similarly, cider is changed into vinegar by certain ferments 

 secreted by bacteria. This process is brought about by the oxi- 

 dation of alcohol in the cider to form the acetic acid of vinegar. 



Alcohol -f oxygen + acetic ferment 



= water + acetic acid -f- acetic ferment. 



Many other familiar instances might be given in which sapro- 

 phytic bacteria cause profound changes in organic bodies by the 

 secretion of ferments which enable the bacteria themselves to 

 digest food or produce energy for their own life processes. 



Disease. Certain parasitic bacteria, commonly designated as 

 disease germs, cause diseases in animals and plants by the secre- 

 tion of poisonous substances called toxins. The toxins are 

 products manufactured, like ferments, by the protoplasm of the 

 bacterial cells. These toxins then diffuse out into the blood and 

 into the tissue cells of the organism attacked by the bacteria. 

 When thus liberated in the system of another organism, the 

 toxins cause the various symptoms characteristic of particular 

 bacterial diseases. The toxins resemble the ferments, or enzymes, 

 in being cell secretions ; but they do not cause fermentations or 

 digestion, and they have no known function for the bacterial 

 cells secreting them. Similar poisonous toxins are secreted by 

 snakes, by the castor bean, and by poisonous mushrooms. The 

 toxins are not, therefore, confined to bacteria, although the bac- 

 terial toxins are the most important on account of the diseases 

 which they cause in man, other animals, and plants. A large 

 number of plant diseases are now known to be due to bacteria. 



Antitoxins are secretions of the cells of the organism attacked 

 by toxic bacteria, which tend to neutralize the toxins secreted 

 by the bacterial cells. Diphtheria, for instance, is produced by 

 bacteria growing on the lining membrane of the throat. The 

 diphtheric bacteria do not enter the body of the diseased organ- 

 isms, but secrete the diphtheric toxin, which is circulated by 



