DEVELOPMENT OF PLANTS 161 



the principal source of nitrogen in nature for the plant would be 

 lost. 



You have noticed that free nitrogen (N) may also appear in 

 decomposition. A few forms of bacteria have been discovered 

 that are able to synthesize this gaseous element into a compound 

 that is of use to plants in a manner quite as remarkable as in the 

 case of the nitrifying bacteria. Because they are able to fix the 

 free gaseous element nitrogen in a stable compound these bacteria 

 are referred to as the nitrogen-fixing bacteria in contradis- 

 tinction to the nitrifying bacteria that only act upon nitrogen 

 when it is already in combination, as in ammonia. The most 

 important of the nitrogen-fixing bacteria are those that live in 

 the roots of plants belonging to the bean family. They cause 

 minute gall-like swellings, termed tubercles, upon the roots of 

 beans, clovers, alfalfa, etc., and are able to combine the free 

 nitrogen of the air into a nitrogenous compound suitable for the 

 nourishment of the plant. These bacteria are of inestimable 

 value in maintaining the fertility of the soil (see page 65). 

 Less important economically than the tubercle-forming bacteria 

 but more wonderful biologically are those forms that live in the 

 soil quite independent of higher plants and effect a somewhat 

 similar fixation of free nitrogen. In this work they are associated 

 with two other forms that take no part in the fixation of nitrogen 

 but simply remove the oxygen and probably the organic matter, 

 for only in the absence of these substances can these remarkable 

 bacteria live. In this manner they are able to fix the nitrogen 

 and build up complex compounds, probably of a proteid nature, 

 which are subsequently decomposed and eventually converted 

 into available nitrates by other bacteria. 



As a final example of these synthesizing bacteria mention 

 should be made of the iron and especially of the sulphur bacteria. 

 They are able to oxidize simple compounds of iron and sulphur 

 so that deposits of these minerals may often be seen as slimy 

 white, red or yellow masses in springs and streams. The sulphur 

 bacteria are of especial interest because they are able to oxidize 

 one of the products of decomposition, H 2 S, into a sulphate one 



of the valuable crude plant foods. These plants are practically 

 ii 



