124 Spratt. — -The Morphology of the Root T^lbercles of 
rapid expansion and growth of the nodular tissue when reawakened to 
activity. 
In the tubercles of Elaeagnus the cortical cells situated immediately 
behind the meristematic zone appear to contain a protoplasmic network, 
while those a little further back are often somewhat abnormally enlarged 
and are filled with densely staining spherical bodies. This appearance 
suggests the idea which has been held by Woronin, Brunchorst, Shroter, 
and others, that the tubercles are produced by one of the Plasmodio- 
phoraceae. When, however, the sections are stained with Kiskalt’s 
amyl gram it becomes evident that the cortical cells contain the nitrogen- 
fixing organism Pseudomonas radicicola (Fig. 8). 
The apparent network is then seen to be a zoogloea thread in which 
are embedded the small rod-shaped Bacteria (Fig. 8, b, z). This is fur- 
ther demonstrated by applying a little ethyl alcohol, when the stain is 
instantly removed from the network, remaining only in the nuclei of the 
host cells. This condition always prevails in the youngest cells inhabited 
by the Bacteria, and is undoubtedly the form in which the organism passes 
from cell to cell, being exactly comparable with the so-called ‘ infection 
threads * produced in leguminous tubercles, and, as in them, seems to be 
attracted by the host nucleus around which it twines. The contents of 
the slightly older, enlarged cells are also very clearly defined by Kis- 
kalts amyl-gram stain, as distinct spherical bodies, having a definite wall 
and a strong resemblance to the organisms obtained in the old cultures 
and on the plates (Fig. 8, c). Detailed examination of a large number 
of sections of nodules of various sizes and ages, at different seasons of 
the year, showed that these spherical organisms are produced from the 
zoogloea after it has attained certain dimensions with regard to the host 
cell, and consequently has reached a particular stage in its development. 
In addition to the cells containing only the small rods in the zoogloea, 
others were examined in which there were a number of larger structures 
in the slime thread, and amongst these some very definite spherical 
bodies, the number of which evidently increases in proportion, until the 
host cell contains only these, the network being at first masked, but after- 
wards entirely used up as the bacilli assume the coccoid form (Fig. 8). The 
nuclei of the host cells under the influence of the zoogloea frequently assume 
a somewhat amoeboid form, become very vacuolate, may have several 
nucleoli, and in some cases appear to disintegrate (Fig. 8). 
In order to ascertain the connexion between the two structures 
described above, the following methods of cultivation were tried. Part 
of the internal nodular tissue was crushed and put into a hanging drop so 
that it could be watched under the microscope, or incubated as desired. 
Some of these preparations were subsequently dried and stained. Thin 
hand sections were treated in the same way. The most satisfactory method 
