^FIXATION OF ATMOSPHERIC NITROGEN 409 



hollow thread or tube that seems to consist of a gelatinous material 

 (Fig. 134) . The tubes branch out as they pass from cell to cell and carry 

 the invading organisms with them. The bacteria which may be readily 

 detected within the tubes and cells are the involution forms of Ps. 

 radicicola and assume various irregular shapes. They are designated 

 as bacteroids. Stefan has suggested that bacteroids may be produced 

 within the tubes and, possibly, from the buds or swellings that appear 

 on the tubes. While still young, the bacteroids are capable of dividing, 

 but as they grow they swell up and finally degenerate. 



RESISTANCE, IMMUNITY AND PHYSIOLOGICAL EFFICIENCY. The 

 invasion of legumes by Ps. radicicola and the acquisition by the plant, 

 thanks to this invasion, of the power to fix elementary nitrogen are cited 

 as a case of symbiosis. However, some writers would regard the pres- 

 ence of Ps. radicicola in legume roots as a case of parasitism. According 

 to them symbiosis presupposes the living together of two organisms 

 with resulting benefit to both. In the present instance, however, 

 conditions may arise when the host plant is injured, rather than bene- 

 fited; and similarly, conditions may arise when the invading bacteria 

 are suppressed by the plants. Making due allowance for the ob- 

 jections raised we still find, nevertheless, that in the broad relation of 

 the two groups of organisms there is an apparent benefit to both plants 

 and bacteria. The former gain an adequate supply of nitrogen and 

 the latter a supply of carbohydrates and of mineral salts. 



A more detailed study of this relation shows that the plants resist 

 the entrance of bacteria. When an abundance of available nitrogen 

 compounds is supplied tubercle formation may be largely or wholly 

 suppressed. In that case the plants secure their nitrogen from the soil 

 and are not only independent of the bacteria, but are strong enough to 

 resist their entrance. It is further claimed by Hiltner that tubercle 

 bacteria differ in their virulence, and that the more virulent the organ- 

 isms, the more readily will they penetrate the root tissue. Moreover, 

 he believes that when a plant is invaded by organisms of any degree of 

 virulence, the host plant becomes immune to a large extent and can keep 

 out all but the most virulent bacteria. The use of the term virulence, 

 in this connection, has been objected to, since it is borrowed from 

 animal pathology and is likely to be misleading. It is better to employ 

 the term physiological efficiency as implying not only a more pro- 

 nounced ability to enter the plant roots, but also to fix atmospheric 



