FUNGI 



377 



pounds to produce a good crop. That is, if a good crop is 

 to be produced, the soil bacteria are absolutely necessary. 

 They must be present and actively at work throughout the 

 growing season. Just as we must depend upon the activity 

 of green plants for our food, so it apptars that green plants 

 must depend upon the 

 activity of soil bacteria 

 for some of the mate- 

 rials out of which they 

 make food. 



That great group 

 of plants, the Legumi- 

 nosce, to which peas, 

 beans, alfalfa, and 

 clover belong, have 

 a special relation to 

 certain soil bacteria 

 which is of much ec- 

 onomic importance. 

 On the roots of these 

 plants are found 

 swellings called nod- 

 ules or tubercles. (See 

 Figures if 4 and 147.) 

 Within these tuber- 

 cles are found bac- 

 teria. The tubercles 

 are caused by the presence of these bacteria. These bac- 

 teria of the tubercles have the power to use in their nutri- 

 tion the nitrogen of the air, a power which green plants 

 do not possess. The activity of the bacteria in the tuber- 

 cles results in the presence there of certain nitrogen com- 



Fig. 174. — Roots of clover showing the numerous 

 nodules. These nodules or tubercles contain 

 bacteria which are of much benefit to the plant 

 that bears them, as described in the context. 



