172 



BOTANY 



Many other diseases have been traced to bacteria. Diphtheria 

 and asiatic cholera are tlie best known. Grippe, pneumonia, 

 whooping cough, and colds are believed to be caused by bacteria. 

 Other diseases, as malaria, yellow fever, and probably smallpox, 

 scarlet fever, and measles, are due to the presence in the blood of 

 a one-celled animal parasite. 



Methods of Fighting Germ Diseases. — As we have seen, dis- 

 eases produced by bacteria may be caused by the bacteria being 

 transferred from one person directly to another, or the disease 

 may obtain a foothold in the body in food, or water, by breath- 

 ing in the germs in the air, or by taking them into the blood 

 through a cut or wound. 



In the prevention of germ diseases we must fight the germ 

 by attacking the parasites directly with poisons that will kill 

 them (such poisons are called germicides or disinfectants), and 

 we must strive to make the persons coming in contact with the 

 disease unlikely to take the disease. This insusceptibility or 

 immunity may be either natural or acquired. Immunity may be 

 acquired by means of such treatment as the anti-toxin treatment for 

 diphtheria. This treatment, as the name denotes, is a method of 

 neutralizing the poison {toxin) caused by the bacteria in the system. 

 It was discovered a few years ago that the serum of the blood of 



an animal immune to diphtheria is 

 capable of neutralizing the poison pro- 

 duced by the diphtheria-causing bac- 

 teria. Horses are rendered immune by 

 giving them large doses of the diph- 

 theria toxin or poison. The serum of 

 the blood of these horses is then used 

 to inoculate the patient suffering from 

 or exposed to diphtheria, and thus the 

 disease is checked or prevented alto- 

 gether. 



A lichen {Physcia stellaris); 

 a, spore-bearing organs. 



Lichens. — Lichens may be found incrust- 

 ing rocks, tree trunks, or other exposed locali- 

 ties. They have in general a grayish color, although they may be red, 

 yellow, or black. The form of the body is usually that of a thallus, being 



