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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. In the cen- 
ter of this mass of plant tissue are a large number of cells which 
are completely filled with the so-called bacteroids. These bac- 
teroids 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 still show the cell nu- 
cleus. 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 
activities and begin to disappear rapidly. After a little they are 
completely absorbed by the substance of the plant, and the tuber- 
cles 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 or may not set up a secondary growth, but it amounts to 
little, for by this time 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. 
Since the work of Prazmowski other observers have studied 
the same problem. Pure cultures of the root organism have been 
obtained and used by artificial inoculation. Water cultures have 
been made where the process can be better studied. The general 
result has confirmed the idea above outlined, and proved conclu- 
sively that the tubercles are the result of the action of micro-or- 
ganisms in the soil. 
There has been more or less dispute in regard to the actual 
nature of these organisms. Fora time they were thought to be 
parts of the roots of the legumes, but this idea was soon aban- 
doned, and they were regarded as the hyphz of moulds. Later 
their relation to bacteria was rendered extremely probable, and 
