ASSIMILATION OF NITROGEN 



71 



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 HeUriegel and Wilfarth.* 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 the kind of plants used in the 

 experiment has 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). HeUriegel 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- 

 steriUzed soil. Tubercles never develop in 

 uninoculated sterilized soils. 



From their studies HeUriegel 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 

 the so-called bacterioid tissue and are characterized by thin cell walls and high 

 content of protein. The protein substances occur in the cells as small, bacteria- 

 Uke rods, which are branched in the older tubercles. These are the so-called 

 bacterioids. The cells of the outer parenchyma layers contain little reserve 

 material, and only those adjacent to the bacterioid tissue are filled with starch 



' HeUriegel, H., and Wilfarth, H., Untersuohungen fiber die Stickstoffnahrung der Gramineen und 

 Leguminosen. Beilageh. Zeitsohr. Rubenzucker-Indust. d. deutseh. Reich. 234 p. November, 1888. 



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

 tubercles {w). 



