232 EXPERIMENTAL GENERAL SCIENCE 



this way they usually destroy any tissue they attack. Nearly 

 all the diseases of animals and plants are due to the activities 

 of bacteria. On the other hand, there are many helpful forms 

 of these plants. Certain kinds give the flavor to butter, 

 cheese, and tobacco, others turn cider to vinegar, and still 

 others cause milk to sour. The retting of flax, by means of 

 which the fibers for linen thread are obtained, is also due to 

 the action of bacteria. Though the bacteria of decay often 

 injure substances which we value, it is probable that even 

 these must be placed among the helpful forms. To realize 

 the helpfulness of such forms, we have only to consider what 

 would happen if all the dead leaves and other refuse which 

 falls on the earth were to lie where they fell without decaying. 



FIG. 85. Various forms of bacteria. (Gager.) 



Moreover, there are a number of other forms in the soil which 

 steadily break up organic compounds into simpler substances 

 which the plants can use. Others, in connection with the 

 group of flowering plants called legumes, add to the soil in 

 available form nitrogen obtained from the air. 



198. Toxins and Ptomaines. Bacteria are of the highest 

 interest to man because of the capacity for harm which certain 

 species possess if allowed to thrive unchecked. When growing 

 in protein foods, they may form poisonous substances called 

 ptomaines which are very difficult to neutralize. Cooking 

 the food may kill the bacteria, but it does not destroy the 

 ptomaines. Other forms may secure entrance into the human 

 body through the air passages, the alimentary canal, or through 

 breaks in the skin due to abrasions or insect bites, and so cause 



