ASSIMILATION OF NITROGEN 



75 



of potassium and phosphoric acid being sufficient to produce normal growth. 

 In this case the total need of nitrogen is supplied from the air (Fig. 45). 



The results of Lawes and Gilbert and those of Wagner thus seem to dis- 

 agree with the conclusions reached by Boussingault (see page 64). This is 

 explained by the fact that Boussingault used sterilized soils, whereas the other 

 authors, just referred to, worked with unsterilized soil under natural condi- 

 tions. The reason for the entirely different behavior of legumes in sterilized 

 soils and in unsterilized soils has been discovered in a series of remarkable in- 

 vestigations conducted by Hellriegel and Wilfarth. 1 In their experiments, 

 various legumes grew quite normally in soils that lacked nitrogen, provided these 

 soils were not previously sterilized. Growth was checked, however, in sterilized, 

 nitrogen-free soils, because of lack of nitrogen. Addition to the sterilized 

 soil of a small quantity of an infusion from unsterilized soil produced normal 

 growth of the plants and resulted in a crop rich in nitrogen. If the added 

 infusion was previously boiled, however, 

 then its addition produced no effect at all; 

 the plants were retarded in their develop- 

 ment and the harvest showed no increase 

 in nitrogen. The soil used in preparing 

 the infusion must be taken from a field 

 upon which plants of the kind used in the 

 experiment have been cultivated; for ex- 

 ample, if peas are employed the soil used 

 for the water extract must be obtained 

 from a field where peas have previously 

 been grown. 



Legumes growing under natural condi- 

 tions have small tubercles upon their roots 

 (Fig. 46). Hellriegel and Wilfarth ob- 

 served that these tubercles developed only 

 in unsterilized soil, or in sterilized soil only 

 if it had been treated with infusion of un- 

 sterilized soil. Tubercles never develop in 

 uninoculated sterilized soils. 



From their studies Hellriegel and 

 Wilfarth came to the conclusion that the 

 formation of root tubercles is the result of 

 a symbiosis between the legumes and lower 

 organisms, and that these very tubercles are directly influential in the assimila- 

 tion of atmospheric nitrogen by leguminous plants. 



A cross-section of a legume root, through one of these tubercles, shows that 

 the greater part of the tubercle consists of parenchymatous tissue (Fig. 47). 

 The inner cells are very different from the outer ones. The former constitute 



1 Hellriegel, H., and Wilfarth, H., Untersuchungen uber die Stickstoffnahrung der Gramineen und 

 Leguminosen. Beilage Zeitschr. Rubenzucker-Indust. d. deutsch. Reich. 234 p. November, 1888. 



Fig. 46. — Root system of pea plant, with 

 tubercles (if.) 



