§1] 



UPON THE RATE OF GROWTH 



311 



of nitrogen, but the nitrogen is usually obtained from tbe soil 

 in the form of nitrates. It is feasible to determine by analysis 

 the amount of nitrogen in the soil at the beginning of the ex- 

 periment and the amount in the seed planted, and then after 

 the experiment to determine the quantity in the soil and in 

 the plant. But the difficulty comes in interpreting the results- 



Fig. 86. -r- Parallel cultures of peas in the symbiotic and the non-symbiotic conditions. 

 Each series comprises three culture vessels : B, the symbiotic plants in soil with- 

 out nitrogen ; C, the non-symbiotic plants in like soil ; A, for comparison, non- 

 symbiotic plants after addition of nitrate to the soil. After a photograph. (From 

 Frank, '92.) 



of the analyses. Thus the fact that the sum of nitrogen in the 

 plant and the soil is greater at the end than at the beginning 

 of the experiment does not prove that the plant has taken in 

 free nitrogen ; for, as we have seen, the soil contains nitrifying 

 bacteria, which intermediate between the free nitrogen of the 

 air and the nitrates absorbed by the green plants. This diffi- 



