98 SOIL CONDITIONS AND PLANT GROWTH 



signs of bacteroid formation, and "swarmers" 0-9 p, long and 0*18 //, 

 wide, these being among the smallest organisms known. 1 



None of these organisms, however, could be found in the soil, nor 

 indeed has any one yet succeeded in finding them there although their 

 existence cannot be doubted. Their mode of entry into the pea was 

 studied by Prazmowski (228), and later by Nobbe and Hiltner (216). 

 The root hair is attacked, presumably by the " swarmer," and 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 

 (293), Miss Dawson (80) 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 they are 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 che'mo- 

 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 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, 2 made to 

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

 gen. 3 The change to bacteroids is conditioned by the presence of car- 

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



1 Golding has shown that they will even pass through a porcelain filter and has pre- 

 pared pure cultures in this way. 



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

 host plant. Neumann suggests pentosans (213). 



3 See also (13). 



