346 



THALLOPHYTES 



that the nitrogen of the compounds decomposed is put in an 

 available form for other plants. The Bacteria use the chemical 

 energy derived from the oxidation of organic compounds in per- 

 forming the work involved in building up their bodies. There 

 are a few exceptional forms of soil Bacteria which can actually 



make their own food, and 

 this they do by using this 

 chemical energy, as green 

 plants do sunlight, in the 

 construction of foods from 

 carbon dioxide and water. 

 There are other kinds of 

 soil Bacteria which have 

 the power of actually in- 

 creasing the nitrogen in the 

 soil. They incorporate the 

 gaseous nitrogen of the air 

 into nitrogen compounds, 

 which they use in building 

 up their own bodies, and 

 when their bodies decay, 

 these nitrogenous com- 

 pounds are added to the 

 soil, which is thereby en- 

 riched. Some kinds of 

 these Bacteria live inde- 

 pendently in the soil, while 

 some kinds are associated 

 with higher plants, espe- 

 cially the Legumes, such as 



FIG. 299. A young Red Clover plant, 

 showing the root nodules that are associated 

 with the nitrogen fixing Bacteria. From 

 Farmer's Bulletin 435, U. S. Dept. of Agri- 

 culture. 



Clover, Alfalfa, Beans, etc. 

 (Fig. 299}. They enter the 

 roots of these plants, and, 

 as a result of the attack, 

 the roots form nodules in 

 which the Bacteria live and carry on their work of fixing nitro- 

 gen. It is due to their association with these Bacteria that the 

 Legumes are important in enriching the soil. 



Pathogenic bacteria. These are the disease-producing forms. 

 They prey upon both animals and plants. The disease is the 



