FIXATION OF ATMOSPHERIC NITROGEN 589 



and to cause a certain benefit or injury. Various attempts have been 

 made to study this physiological property by vegetation experiments. 

 It was found 1215 that the activities (including the ability of fixing nitrogen) 

 of the bacteria that have already penetrated into the plants increase 

 with an increase in the amount of nitrogen available to the plants. 

 Hiltner 124 observed an increased growth of leguminous plants (peas) 

 when grown continuously upon the same soil; he ascribed this not only 

 to an increase in the number of bacteria causing inoculation, but also 

 to an increase in the virulence of the bacteria, similar to an increase in 

 virulence of a pathogenic organism when passed through several animals. 

 Assuming that the nodule bacteria increase in virulence by repeated 

 symbiosis with plants, Hiltner planted peas repeatedly on the same soil, 

 which was sufficiently provided with minerals; he found an increase in 

 the infection by the organism from the first to the fourth generation, a 

 period without change then followed, and finally the continued growth 

 of peas gradually led to a diminution in plant growth. 



On the basis of these results Hiltner proposed the "immunity" theory, 

 according to which substances are formed by the bacteria within the 

 nodules which immunize the plant against further invasion of bacteria. 

 The organism (1) may not get into the plant, (2) it may gain admission, 

 but without producing nodules because of the greater resistance of the 

 plant, (3) it may enter the plant and produce nodules but without 

 fixing any nitrogen, (4) it may fix nitrogen which is assimilated by the 

 plant, (5) the bacterium may become more efficient than the plant, 

 which is then injured, or (6) the bacterium itself may even be killed. 

 According to the "immunity" theory, active nodules impart to the plant 

 an immunity against bacteria of lower or equal virulence than those 

 already found in the plant; only bacteria of higher virulence are capable 

 of penetrating into the plant. The above theory was not confirmed 

 by subsequent investigations. Nodules were found to be transient on 

 biennial and perennial legumes, depending somewhat on the climatic 

 conditions; i.e., there are two crops of nodules in biennial legumes, one 

 each year, while there are many crops on perennial legumes such as 

 alfalfa. When a fresh culture is added to a leguminous plant growing on 

 agar and having already formed nodules, more nodules are formed 

 on the new roots that have grown since the first inoculation. Under 



123 Remy, Th. Ueber die Steigerung des Stickstoffsammlungs-Vermogens der 

 Hulsenfruchte durch bakterielle Hilfsmittel. Deut. landw. Presse, 29: 31-32, 

 37-38, 46-47. 1902. 



124 Hiltner, 1904 (p. 128). 



