72 PHYSIOLOGY OF NUTRITION 



grains. The tubercle is covered on the outside by a layer of cork, and branches 

 of the vascular bundles of the root extend into the tubercle. 



Beijerinck^ and Prazmovskii^ have succeeded in securing tubercle bacteria 

 in pure culture. When transferred to a nutrient solution, the young bacteria, 

 or the modified cells called bacterioids, begin to divide and multiply rapidly. 

 The newly formed organisms appear to be in no way different from ordinary 

 bacteria, and they show the same' kind of movement. Prazmovskii has given 

 them' the name Bacterium radicicola. 



This writer has studied the developmental history of the tubercles of the 

 pea plant. If sterilized soil in which young pea seedlings are growing is inocu- 

 lated with a pure culture of Bacterium radicicola, an accumulation of bacteria 

 in the root-hairs becomes noticeable after several days. This mass of bacteria 

 then becomes enclosed in a sheath, forming a sack-like body that enlarges and 



Pig. 47. — Cross-section of a root tubercle of lupine, showing bacteroid tissue (the elon- 

 gated area below) surrounded by root parenchyma. The dark lines above the bacteroid area 

 represent vessels that penetrate from the uninjured root to the hypertrophied tubercle. 



penetrates through the root-hair into the root parenchyma as a bacterial fila- 

 ment. Having advanced into the root, this filament begins to branch rapidly. 

 A lively division of the cells of the root parenchyma proceeds at the same time, 

 in the neighborhood of the bacterial filament, which results in a swelling in this 

 region of the root and in the formation of a tubercle. The branches of the fila- 

 ment occupy the central portion of the tubercle. The filament sheath finally 

 disintegrates and the bacteria thus liberated enter the cell sap. Here they 

 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 



'Beijerinck, M. W., Die Bacterien der PapiUonaceen-KnoUchen. Hot. Zeitg. 46: 725-735, 741-750, 

 757-771. 781-790, 797-802. 1888. 



2 Piazmowski, Adam, Die WurzelknoUchen der Erbse. I. Teil. Die Aetiologie und Entwickelungs- 

 geschichte der KnoUchen. Landw. Versuchsst. 37 : 161-238. 1890. 



