Il8 MAINE AGRICULTURAL EXPERIMENT STATION. 



cess, 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 interpre- 

 tation of this is that the vesicles burst and the bacteria coming 

 into the cell plasma are immediately checked in their growth by 

 the injurious influence of this plasma and begin to undergo 

 involution changes. Instead of multiplying in the normal man- 

 ner, they asume various abnormal forms which have no further 

 power of growth. They become, in short, the bacteroids which 

 have been found by so many observers, filling the central cells 

 of the tubercle. The bacteria retain their power of growth only 

 so long as they remain in the protecting covering of the mem- 

 brane. 



"The tubercle by this time is pretty well formed. The outer 

 cells have undergone quite an extended growth and differentia- 

 tion, so that the tubercle is really a structure of a rather high 

 grade of plant tissue. The tubercle itself is thus really a growth 

 of the root cells of the plant and not a growth of bacteria. But 

 in the centre of this mass of plant tissue are a large number of 

 cells, which are complete!}' filled with the so-called bacteroids. 

 These bacteroids give to the tubercle at this stage a flesh-red 

 color. Some of these central cells are so completely filled with 

 them that nothing else can be seen, while others may show the 

 nucleus. In others, spaces begin to appear in the body of the 

 celL The appearance of the spaces marks a new stage in the 

 history of the tubercle, and indicates that the bacteroids entirely 

 cease their activities and begin to disappear rapidly. After a 

 little they are completely absorbed by the substance of the plant 

 and the tubercles are left as empty pouches. The tubercles 

 have now changed their appearance again and assume a some- 

 what grayish green color. 



"This practically ends the history of the tubercle. In most 

 cases some of the bacteria seem to remain within their original 

 membrane, and therefore are capable of growing. These may 

 now set up a secondary growth, but it amounts to little, for 

 by this time the plant has usually blossomed, ripened the seeds, 

 and the root is beginning to die. The tubercle is immediately 



