690 



growtb by the iDJurious influence of tliis plasma and begin to undergo 

 involution changes. Instead of multiplying in the normal manner, 

 they assume 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 bac- 

 teria retain their power of growth only so long as they remain in the 

 protecting covering of the membrane. 



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

 have undergone quite an extended growth and differentiation, so that 

 the tubercle is really a structure of a rather high grade of plant tis- 

 sue. The tubercle itself is thus really a growth of the root cells of the 

 plant and not a growth of bacteria. But in the center of this mass of 

 plant tissue are a large number of cells, which are completely 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 com- 

 pletely 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 are beginning 

 to be absorbed by the plant. The cell plasma soon assumes a network 

 structure, and from this time the bacteroids entirely cease their activi- 

 ties 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 somewhat 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 still 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 its seeds, and the root is beginning to 

 die. The tubercle is immediately attacked by the putrefactive bacteria 

 in the soil and becomes decomposed. 



Frank has also published an extended series of observations upon 

 the same subject.* While he differs from Prazmowski in some impor- 

 tant particulars, his later results are, on the whole, a good confirmation 

 of those of the latter writer. He, too, finds the tubercles produced as 

 infections by some organisms in the soil, and the organism to which he 

 attributes them is described by him as a micrococcus or short rod, and 

 is very probably the same as that studied by Prazmowski. His 

 explanation of the hypha? and the bacteroids is different from the one just 

 noticed. The hyiihse he finds filled with bacteria, as does Prazmowslu, 

 but he regards the membrane that surrounds them as a product of tiie 

 root cells rather than of the bacteria. He thinks that the root cells 

 produce these peculiar threadlike forms in which the bacteria multii)ly, 

 and that by means of the threads the bacteria are conducted into the 



• Luudw. Jabr., Bil. 17, 1888, pp. 4^>l-552, and 19, 18D0, pp. 523-640. 



