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 
Fic. 299. — A young Red Clover plant, Clover, Alfalfa, Beans, ete. 
showing the root nodules that are associated (Fig. 299). They enter the 
with the nitrogen fixing Bacteria. From 
roots of these plants, and, 
Farmer's Bulletin 435, U.S. Dept. of Agri- Oe ; , 
as a result of the attack, 
culture. 
the roots form nodules in 
which the Bacteria live and carry on their work of fixing nitro- 
gen. It is duc 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 
