129 



115. Bacteria are of great practical Practical 

 importance for the following reasons :- 



(1) Many species possess the power of more or less de- 



composing, and altering the composition of, the 

 material from which they obtain their food. 



Bacterium acidi lactici breaks up milk causing it to 

 become sour. 



Bacterium aceti, the Vinegar Bacterium, converts alco- 

 hol into acetic acid. 



Bacillus vulgaris is the most common cause of the 

 decomposition of meat. 



Such decomposing actions are sometimes spoken of as 

 decay or rotting, sometimes as fermentation, and 

 sometimes, owing to the evolution of evil-smelling 

 gases, as putrefaction. 



(2) Many species cause very virulent and dangerous animal 



diseases. 



Bacillus tetani is found in the soil and causes tetanus. 

 Bacillus typlii causes typhoid fever. 

 Vibrio cholerae causes cholera. 



(3) Many species help other plants to obtain the supply 



of valuable nitrogenous materials which are neces- 

 sary for their existence. 



Bacillus radicicola lives in tubercles on the roots of vari- 

 ous plants and is able to fix the free nitrogen 

 of the air and to pass it on in a form in which it 

 can be utilised by these plants, this source of 

 nitrogen being otherwise not available for them. 



The so-called Nitrifying Bacteria live in the soil and con- 

 vert ammonia to nitrous and nitric acid. Nitrates are thus 

 eventually formed which are the most valuable source of 

 nitrogen for the majority of the higher plants. 



It will be seen that, although bacteria are responsible 

 for many of the most virulent animal diseases, they seldom 

 affect other plants injuriously, this being partly due to the 

 difficulty which such minute organisms find in passing 

 through the cell walls of plant tissues. 



116. Fungi. In this group are in- Fun ? ! - 



eluded a great variety of plants which are popularly known by 

 various names, such as moulds, rusts, toadstools, mushrooms, 



etc. They are of special importance to the Forester on 

 account of their being responsible for the majority of the 

 most destructive of known plant diseases. Some fungi are 



