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The thread branches more or less as it lengthens, and its various 
filiments grow through and between the cells, soon permeating the 
root with a fine, branching filament, which looks much like the 
mycelium of a mould. This bacteria pouch has been regarded as 
the hypha of some low fungus, but instead of being a mycelium 
growth of a mould the thread is nothing more than a large branch- 
ing colony of bacteria enclosed 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 rapidly 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 contin- 
ues 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 
Prazmowski has not been able to follow the details of the process, it 
is thought that the vesicles in the central cells swell until the mem- 
brane covering the bacteria is so thin that it bursts, and the bac- 
teria 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 is that the 
vesicles burst, and the bacteria coming into the cell plasma are im- 
- mediately checked in their growth by the injurious influence 
of this plasma and begin to undergo involution changes. In- 
stead 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 many 
observers, filling the central cells of the tubercle. The bacteria re- 
tain their power of growth only so long as they remain in the pro- 
tecting covering of the membrane. 
The tubercle by this time is pretty well formed. The outer 
cells have undergone quite an extended growth and differentiation, 
