ASSIMILATION OF NITROGEN 77 



enlarge and become branched, thus becoming mature bacterioids. At this time 

 the vascular bundles develop in the tubercle. The bacterioid tissue becomes 

 depleted after a time, its contents being used up by the plant. The bacterial 

 cells collect in groups in the remaining portions of the infection-filaments and 

 become enclosed in a hard sheath. The spore-like colonies thus formed fall 

 away after the destruction of the tubercle and are capable of infecting other 

 roots the following spring. 



Kossovich 1 sought to solve the question as to what organs of legumes 

 absorb atmospheric nitrogen. He carried out two series of experiments, in 

 one case depriving the leaves, and in the other case the roots, of nitrogen. He 

 came to the conclusion that nitrogen is absorbed by the roots. 



Infection of legumes with cultures of Bacterium radicicola does not always 

 have a favorable influence upon the growth of these plants. If the inoculation 

 occurs late in the growing season (in July), the result is an abundant formation 

 of root tubercles, but the plants, instead of growing better, grow more poorly 

 than do uninfected individuals. The action of the bacteria is merely parasitic 

 in this case. Microscopic investigation shows that the transformation of 

 bacteria into bacterioids does not occur here, and it was for this reason that 

 Nobbe and Hiltner 2 believed assimilation of atmospheric nitrogen to be corre- 

 lated with the formation of the bacterioids. Long-continued cultivation upon 

 nutrient gelatine (from spring until midsummer) is said to make Bacterium 

 radicicola more vigorous and to deter its transformation into bacterioids after 

 it enters the root. Plants inoculated late in the season, being already partially 

 exhausted at this time, are too weak to produce this change in the infecting 

 organism. 



Investigation of the tubercle bacteria of various legumes leads to the conclu- 

 sion that there are many varieties of these organisms. In order to obtain a 

 healthy development of Robinia pseudacacia in soil without available nitrogen, 

 inoculations must be made with cultures from Robinia tubercles; infection with 

 bacteria from pea and lupine tubercles has no effect at all. But inoculation 

 with cultures from Cytisus tubercles has almost as good an effect as inoculation 

 with cultures of the bacteria of Robinia itself. 3 



Certain non-leguminous plants also assimilate atmospheric nitrogen by 

 symbiosis with bacteria, and the tubercles may be formed in other regions of 

 the plant besides the root system. For example, the leaves of some of the 

 tropical Rubiaceae are characterized by numerous rounded, tubercle-like thicken- 

 ings, which contain peculiar bacterial cells (Mycobacterium rubiacearum) . These 

 bacteria fix nitrogen from the air in the same general manner as do the root- 

 tubercle bacteria of legumes. 4 (See Fig. 48.) 



1 Kossowitsch, P., Durch welche Organe nehmen die Leguminosen den freien Stickstoff auf ? Bot. 

 Zeitg. 50: 697-702, 713-723, 720-738, 745-756, 771-774- 1892. 



2 Nobbe, F., and Hiltner, L., Wodurch werden die knollchenbesitzenden Leguminosen befahigt, den 

 freien atmospharischen Stickstoff fur sich zu verwerten? Landw. Versuchsst. 42 : 450-478. 1893. 



8 Nobbe, F., Schmid, E., Hiltner, L., and Hotter, E., Versuche iiber die Stickstoff-Assimilation der 

 Leguminosen. Landw. Versuchsst. 39: 327-359. 1891. 



* [Faber, F. C. von, Das erbliche Zusammenleben von Bakterien und tropischen Pnanzen. Jahrb. 

 wiss. Bot. 51: 285-375. 19 1 2.] 



