222 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



and seed was known, the loss or gain of nitrogen could then be 

 easily calculated. Some results are as follows : 



The experiment showed not only that the lupine could utilize 

 the nitrogen of the air, but proved that the soil extract contained 

 something which brought it about. Observations showed that 

 the lupines assimilate nitrogen only when nodules are present on 

 their roots. Examination of the nodules showed that they con- 

 tained bacteria, and further experiments, similar to the one 

 described above, proved that plants grown in soil inoculated with 

 these bacteria evolved nodules and attained the power of 

 assimilating elementary nitrogen. By experiments similar to 

 those we have described, it was proved that alfalfa, vetch, clover, 

 cow peas, and other leguminous plants have the power of utilizing 

 the free nitrogen of the air when the proper bacteria are present ; 

 but corn, wheat, oats, and most plants other than legumes, can 

 take up only nitrogen in combination. 



Formation of the Tubercles. When a lupine seed is planted in 

 inoculated soil free from combined nitrogen, the plant grows 

 until the nutrient in the seed is consumed, then ceases to grow, 

 and shows all signs of nitrogen starvation. In the meantime, 

 tubercles are forming on its roots. In a few days, it begins to 

 grow vigorously, and appears to possess an abundance of nitro- 

 gen. The tubercles themselves pass through three stages. There 

 is first vigorous growth, producing the tubercle filled with large 

 numbers of small, rod-like bacteria; then the bacteria change to 

 bacteriods, assuming a T or Y form; and finally the bacteriods 

 begin to disappear and after a little they are absorbed almost com- 

 pletely by the substance of the plant and the tubercles are left as 

 empty pouches. The plant begins to receive benefit from the 



