460 THE president's address. 



exercised is termed Chemotaxis. When substances such as 

 peptone, potassic chloride, asparagin, etc., attract bacteria, the 

 Chemotaxis is positive, and negative when repelled, as by free 

 acids or alkalies, alcohol, etc. The innumerable ills of man 

 and beast attributed to "germs" must be passed over in silence, 

 and the brief period of time remaining at our disposal devoted 

 to a sketch of the direct or indirect influence of bacteria on 

 plant life. 



The final decomposition of various substances present in the 

 soil, or intentionally placed there for the purpose of forming 

 manure or plant food, was until recently attributed entirely to 

 chemical activity. The disintegration of such varied bodies 

 as manure, urine, gas-lime, green crops ploughed into the 

 land, etc., are now definitely known to be due to innumerable 

 bacteria present in the soil, and invariably resulting in the 

 liberation, along with other substances, of free nitrogen and 

 ammonia. 



The free nitrogen escapes directly into the atmosphere, where 

 some of it is used up by bacteria, in a manner to be described 

 further on. 



The ammonia is immediately attacked by the so-called 

 nitrifying bacteria. These organisms are divided into two 

 groups, representing a division of labour in the production of 

 nitrates. First come the nitrite bacteria, which convert the 

 ammonia into nitrous acid ; this acid is then converted by the 

 nitrate bacteria into nitric acid, which eventually combines with 

 a base to form a nitric salt. It is from such soluble nitric salts 

 formed by bacteria that all green plants except the Leguminosae 

 obtain their nitrogen by absorbing the dissolved salt through the 

 roots. Nitrate of soda and nitrate of ammonia are examples 

 of such salts. 



Saltpetre or nitre — nitrate of potash — whether produced 

 naturally, as in India and South America, or artificially in 

 saltpetre works, depends on the presence of bacteria. For the 



