ABSORPTION OF FREE NITROGEN 105 



material is, as has been discussed in Chapter X. At any 

 rate, there is as yet no definite knowledge of any other 

 method of transformation. Up to the present no intentional 

 practical utilization of this valuable property of these free- 

 living forms has been made. 



Nitrogen Nutrition of Green Plants.— It is the belief of 

 botanists that green plants obtain their nitrogen chiefly 

 in the form of nitrates, though ammonium salts may be 

 utilized to some extent by certain plants at least. Excep- 

 tions to this general rule are those plants provided with 

 root tubercles (and the bog plants and others which have 

 mycorrhiza). These plants obtain their nitrogen in the 

 form of organic compounds made for them by the bacteria 

 growing in the tubercles. That nitrogen circulates through- 

 out the structure of plants in organic combination is certain. 

 There does not appear to be any reason why similar com- 

 pounds which are soluble and diffusible (amino-acids) 

 should not be taken up through the roots of plants and 

 utilized as such. It seems to the author that this is very prob- 

 ably the case. Arguments in favor of this view are: (1) The 

 nitrogen nutrition of leguminous and other plants with root 

 nodules. (2) The close symbiosis between " Azotobacter" 

 and similar nitrogen-absorbing bacteria and many species 

 of algse in sea water at least. (3) The vigorous growth of 

 plants in soils very rich in organic matter. This inhibits 

 the production of nitrates by the nitrous nitric bacteria 

 when grown in culture. It probably does so in the soil, so 

 that nitrates may not account for the vigorous growth. 

 (4) The effect of nitrate fertilizers is to add an amount of 

 nitrogen to the crop much in excess of the amount added as 

 nitrate. (5) The most fertile soils contain the largest num- 

 bers of bacteria. The doctrine that nitrates furnish the 

 only nitrogen to plants was established before the activities 

 of bacteria in the soil were suspected, and, so far as the 

 author is aware, has not been supported by experiments 

 under conditions rigidly controlled as to sterility. 



It would seem that one of the chief functions of soil bac- 

 teria is to prepare soluble organic compounds of nitrogen 

 for the use of green plants and thus to make a "short cut" 



