288 BOTANY: PRINCIPLES AND PROBLEMS 



we have discussed in our study of metabolism. Decay, essen- 

 tially the same type of process as fermentation except that it 

 takes place in all sorts of organic substances and is carried through 

 to a complete break-down of these substances into carbon dioxide, 

 water, nitrogen, and mineral salts, is brought about almost entirely 

 through the activity of the many types of putrefactive and 

 decay-producing bacteria. All successful methods of preserving 

 food depend on eliminating these bacteria or preventing their 

 activity. 



Pathogenic Types.- — Those members of the class which are 

 parasitic on other organisms are known as pathogenic bacteria. 

 A particular species is the cause of each of the various bacterial 

 diseases, such as diphtheria, tuberculosis, typhoid fever, pneu- 

 monia, cholera, and many others among animals and man, as 

 well as pear blight, cucumber wilt, black rot of cabbage, and others 

 among plants. These diseases are often communicated from one 

 individual to another, since the pathogenic bacteria responsible 

 for them may be easily transmitted through air, water, food, or 

 contact. Bacteria which are very minute or otherwise difficult 

 to recognize are also probably responsible for many diseases 

 the cause of which is at present unknown. 



The harmful effect of pathogenic bacteria on animals is often 

 not due to the direct attack of the parasite but to the poisonous 

 by-products, or toxins, which they secrete and which enter the 

 blood. The afflicted individual will often produce antitoxins 

 capable of counteracting the poisonous effect of the toxins and 

 of rendering the individual immune for a time to the attacks of 

 this particular parasite. The practice of vaccination consists in 

 inoculating an individual with the parasitic bacteria and thus 

 subjecting him to a mild attack of the disease in order to stimulate 

 the production of sufficient antitoxin in his system to confer 

 immunity upon him for a long time. Vaccination is particularly 

 effective against small pox and typhoid fever. The attacks 

 of certain other disease-producing organisms may also be pre- 

 vented or rendered less virulent by injecting into the circulation 

 a little blood serum from an individual (usually a cow or horse) 

 which has had the disease and whose blood is therefore rich in 

 antitoxin. This serum or antitoxin treatment has been especially 

 successful with diphtheria, tetanus, and hog cholera. 



In our study of the soil we mentioned two other groups of 

 bacteria which are of especial importance to the higher plants 



