I08 THE STORY OF GERM LIFE. 



of the free nitrogen of the atmosphere. It was 

 found, however, some fifteen years ago that some 

 species of plants, chiefly the great family of 

 legumes, which contains the pea plant, the bean, 

 the clover, etc., are able, when growing in soil 

 that is poor in nitrogen, to obtain nitrogen from 

 some source other than the soil in which they 

 grow. A pea plant in soil that contains no nitro- 

 gen products and watered with water that con- 

 tains no nitrogen, will, after sprouting and growing 

 for a length of time, be found to have accumu- 

 lated a considerable quantity of fixed nitrogen in 

 its tissues. The only source of this nitrogen has 

 been evidently from the air which bathes the 

 leaves of the plant or permeates the soil and 

 bathes its roots. This fact 

 was at first disputed, but sub- 

 sequently demonstrated to be 

 tru e, and was found later to 

 be associated with the com- 

 bined action of these legumes 

 and certain soil bacteria. 

 When a legume thus gains 

 FIG. 2 7 .-Soii bacteria nitrogen from the air, it de- 

 which produce tu- velops upon its roots little 

 bercies on the roots bunches known as root nod- 

 ules or root tubercles. The 

 nodules are sometimes the size of the head of a 

 pin, and sometimes much larger than this, occa- 

 sionally reaching the size of a large pea, or even 

 larger. Upon microscopic examination they 

 are found to be little nests of bacteria. In some 

 way the soil organisms (Fig. 27) make their way 

 into the roots of the sprouting plant, and find- 

 ing there congenial environment, develop in con- 

 siderable quantities and produce root tubercles 



