96 SOIL CONDITIONS AND PLANT GROWTH 



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 

 (293), Miss Dawson (80, 81) and others. Soon the organisms surround 

 themselves with slime and appear as bacterial rods, which then change 

 to the characteristic branched or Y-shaped bacteroids and assimilate 

 free nitrogen. The organisms have a remarkable power of discrimina- 

 tion and! only enter in any quantity the particular species of plant to 

 which theyiare accustomed ; they can, however, train on to other species, 

 but they then lose the power of attacking their original hosts. 



Hiltner (134-, and 135) regards them as parasites attracted chemo- 

 tactically to the root hair by root excretion, but prevented from getting 

 too far into the plant by excess of the attracting 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 fixing little nitrogen ; the more viru- 

 lent 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 I vigorously on rich soils. The parasitism is beneficial to 

 both parties : the plant gains nitrogen and the organism gains carbo- 

 hydrates. 



In its general outlines the process has been reproduced artificially. 

 Leguminous plants can be fed with nitrogen compounds and made to 

 grow perfectly without the organism. On the other hand, the organism 

 can be grown on artificial media containing carbohydrates,^ made to 

 pass through all its stages from swarmers to bacteroids, and to fix nitro- 

 gen.^ The change to bacteroids is conditioned by the presence of car- 

 bohydrates or of small quantities of various acids, such as are known to 

 occur in the plant (277). The fixation of nitrogen rapidly comes to a 

 stop unless the resulting compound is removed, as in the plant. Golding 

 has attained this end by an ingenious filtering device, and has thus 

 succeeded in fixing considerable quantities of nitrogen. He has also 

 shown that the reaction of the medium during actual fixation is alka- 

 line, but changes to acid when fixation is stopped by the accumulation 

 of nitrogen compounds. An actual loss then seems to set in (108, 109). 



The chemistry of the process is unknown ; even the changes in the 

 carbohydrates of the culture medium have not been worked out Nitro- 



^ Harrison and Barlow (126) used maltose : other observers have used an infusion of the 

 host plant. Neumann suggests pentosans (213). 

 ' See also (13). 



