FIXATION OF ATMOSPHERIC NITROGEN. 277 



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 ^njhiosis. 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 benefited; and 

 similarly, conditions may arise when the invading bacteria are suppressed 

 by the plants. Making due allowance for the objections raised we still 

 find, nevertheless, that in the broad relation of the two groups of organ- 

 isms 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 organsms 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 pronounced ability 

 to enter the plant roots, but also to fix atmospheric nitrogen. It is con- 

 ceivable that strains of Ps. radicicola may be developed that would grow 

 rapidly and yet possess but a feeble nitrogen-fixing power. In other 

 words, they would possess a high vegetative power and a low physiological 

 efficiency. 



MECHANISM OF FIXATION. It is generally believed that the fixation 



