G89 



According to the investigations of Prazmowski the development and 

 growth of the tubercles are as follows : Bacterium radicicola lives nor- 

 mally in the earth and collects in numbers on the outside of the roots 

 of various legumes. Some of the organisms succeed in forcing their 

 way into the tissues of the young roots, though they are not able to 

 pierce rhe older roots. For awhile they may remain in the root as 

 free bacteria, but the plant plasma seems to exert an injurious influence 

 upon them, for very soon a thin membrane is formed arouiul the bacteria 

 masses, inclosing them like a pouch. Prazmowski thinks that this 

 membrane is a product of the bacteria themselves, formed for the pur- 

 pose of protecting them froni the injurious action of the plant tissue. 

 The bacteria which do not succeed in getting into one of these pouches 

 soon cease to grow and degenerate into irregular forms like the bacte- 

 roids which appear later in greater numbers. The bulk of the bacteria, 

 however, become inclosed in the membrane, after which they continue 

 their growth with much vigor. The pouches begin to grow into thread- 

 like masses, and these make their way among the cells of the root. 

 The thread branches more or less as it lengthens and its various fila- 

 ments grow through and between the cells, soon iiermeating tlie root 

 with a fine, branching filament, which looks much like the mycelium of 

 a mold. It was this bacteria i)ouch which was first seen by EricksOn, 

 and which i)revious observers regarded as the hypha of some low 

 fungus. Instead of being a mycelium growth of a mold the thread is 

 nothing more than a large, branching colony of bacteria inclosed in a 

 thin membrane. 



The growth of this colony of bacteria among the cells of the root 

 stimulates these cells to an unusual growth. They multiply more rap- 

 idly than usual, and thus soon produce a swelling on the root which is 

 the beginning of the tubercle. While this rapid multiplication of root 

 cells is going on, the bacteria pouch continues to grow, and swells out 

 into rounded vesicles within the cells which lie at the center of the 

 forming tubercle until most of them become filled with these expanded 

 portions of the bacteria thread. Meantime the root cells of the plant 

 have been rapidly growing, and form around the cells containing the 

 bacteria several layers of smaller cells, which develop into a hard, 

 corky covering forming a coat around the tubercle. This seems to be 

 impervious to the bacteria thread, and confines the bacteria within its 

 limits. 



The bacteria colony now undergoes a change. Although Prazmow- 

 ski has not been able to follow the details of the process, it is thought 

 that the vesicles in the central cells swell until the membrane covering 

 the bacteria is so thin that it bursts, and the bacteria are themselves 

 extruded into the plasma of the root cells. At all events the vesicles 

 disappear and there appears in their place what is called the bacteroid 

 tissue. His interpretation of this is that the vesicles burst and the 

 bacteria coming into the ctll i lasma are immediately checked in their 



