BACTERIA 31 



repeated. (2) By spores, which are termed endogenous, 

 when formed within the cell ; arthrogenous, when portions 

 separate from the cell and gradually develop into distinct 

 individuals. 



The changes effected by bacteria on the substance on 

 which they are living are very varied, and included under 

 the term metabolism. Bright colours are often produced ; 

 odours agreeable or otherwise are also common; some 

 again induce phosphorescence. Putrefaction and fermen- 

 tation also result. 



Certain species possess the property of forming nitrates 

 by the conversion of ammonia into nitric acid, saltpetre 

 potassium nitrate being one of the ultimate products. 



A most important feature to remember, from the stand- 

 point of the cultivator of the soil, is the fact that what has 

 been termed ' nitrification ' of the soil, or the fixation of 

 nitrogen in a form that can be readily assimilated by 

 plants, to whose existence it is indispensable, is effected 

 by bacteria present in the soil ; hence it is important, in 

 attempting to destroy fungous germs present in the soil, 

 not to make use of substances also capable of destroying 

 the indispensable bacteria present. 



FUNGICIDES 



The various preparations applied to plants for the pre- 

 vention of diseases caused by parasitic fungi are called 

 fungicides. 



A fungicide to be of value should possess the following 

 points : (i) Destroy the parasite without injuring the host. 

 (2) Be easy to prepare and to apply. (3) Cost moderate 

 or even cheap. 



