CARBON AND NITROGEN CYCLES IN THE SOIL 205 



C. B. Lipman and L. W. Fowler ^ have succeeded in isolat- 

 ing the organism from soil, and T. F. Manns and Goheen ^ 

 also claim to have done so. 



The mode of entry into the pea was studied by Prazmow- 

 ski (227), and later by Nobbe and Hiltner (21 5<^). The organ- 

 ism — presumably in the swarmer stage — attacks the root hair, 

 secreting a substance which causes the root tip to curl up ; 

 the membrane of the hair becomes swollen and the bacteria 

 then penetrate,^ A filament, known as the infection thread, 

 and shown to be formed of rapidly multiplying bacteria, 

 gradually extends up into the root where the nodule begins 

 to form ; beyond this, the organisms do not penetrate. The 

 morphological changes have been described by Marshall Ward 

 (294), Miss Dawson (80) and others. Soon the organisms sur- 

 round themselves with slime and appear as bacterial rods, which 

 may then change to the characteristic branched or Y-shaped 

 bacteriods and assimilate free nitrogen. There appear to be 

 some 6 or seven different kinds of nodule organisms which 

 can be distinguished by the agglutination test* and which 

 show characteristic discrimination between the various legumin- 

 ous plants. 



Hiltner (135) regards them as parasites attracted chemo- 

 tactically to the root hair by root excretions, but prevented 

 from getting too far into the plant by excess of the attract- 

 ing material, which now becomes a deterrent. He grades 

 them according to their virulence, the less virulent either 

 being unable to enter the plant, or, if they do enter, being 

 quickly resorbed, or only fixmg little nitrogen ; the more 

 virulent, on the other hand, bring about energetic fixation. 

 As evidence he adduces the well-known fact that infection 

 proceeds best in plants weakened by nitrogen starvation, and 

 scarcely takes place at all in plants growing vigorously on 



^Science, 1915, 41, 256 and 725, where earlier isolations by Nobbe, etc., are 

 discussed. 



^Delaware Ag. Expt. Sta. Bull., 115, 1916. 



^L. Hiltner (135a). 



* M. Klimmer and R. Kruger, Cent. Bakt. Par., 1914, 40, 256-265. 



