DEVELOPMENT OF PLANTS 143 



complex substances into inorganic and simple organic compounds 

 that serve as food for plants. This in brief is the story of the 

 organic world. Plants construct complex compounds from simple 

 ones and die. Bacteria now reduce them to simple substances 

 which become again the food of succeeding generations. Thus 

 bacteria assist in keeping up the fertility of the soil and prevent 

 the burying of the earth under the annual fall of vegetation. The 

 second class, in common with some fungi, live upon sugars and 

 other carbohydrates, causing a decomposition or decay known as 

 fermentation. Vinegar is due to the decomposition of the alco- 

 hol in cider and other weak alcoholic solution by bacteria which 

 appear as slimy masses and are popularly known as mother of 

 vinegar. Milk sours and coagulates through the agency of bac- 

 teria which reduce the sugar to lactic acid which in turn coagu- 

 lates the casein. So also butyric acid, necessary in the manufac- 

 ture of cheese is produced by bacteria. It will be seen from this 

 that the products of decomposition are not necessarily harmful. 

 It is interesting to note that some of the flavors of cheese and 

 of high grade butter are due to the products of decomposition 

 and the excretions from bacteria. It is evident that this might 

 be true in the case of limburger cheese and rancid butter but 

 also remember that species of bacteria are cultivated and used 

 to impart certain flavors to cheese and also to a less extent to 

 butter. It is altogether probable that this will become a common 

 practice in our dairies. Some bacteria bring about decompo- 

 sition and produce secretions that are exceedingly poisonous. 

 The ptomaines found in fish, cheese, ice cream, etc., are due to 

 this cause. 



(e) Bacteria of Disease. — In contrast to the kinds of bacteria 

 mentioned above there is another group that live as parasites 

 on plants and animals producing disease either by destroying the 

 tissue and sapping the vitality or by the production of poisonous 

 compounds, toxins. Among the more terrible of these infec- 

 tious bacteria may be mentioned those producing consumption. 

 These affect especially the lungs of animals and cause over 30 

 per cent, of the deaths of the human race. This is no longer 

 considered so fatal a disease as formerly. A person of ordi- 



