458 IVAN E. WALLIN 
These senile forms occupy the older part of the nodule. That 
is, they are present almost exclusively in the cells of the nodule 
nearest to the attachment to the root. Apparently, there is 
nothing indicated concerning their activities. Their position 
in the nodule suggests the possibility that they are senile. It 
is possible that they no longer concern themselves with nitrogen 
fixation. However, one is justified in concluding that they are 
a stage in the morphogenesis of Bacillus radicicola. 
The failure of investigators of this interesting and well-known 
organism to notice the existence of the senile forms is, assuredly, 
an excusable error. The object of my study, as well as my his- 
tological training, suggested the study of the root-nodule cells 
intact. The bacteriologist would crush the nodule and make a 
smear to study the contained bacteria. Again, the bacteriologist 
is interested in the cultural behavior of bacteria. The nodule 
must be crushed to transplant the contained organisms. 
I have never observed the spherical or senile forms of the 
bacillus in smear preparations of the root-nodules. 
Figure 4 is a camera-lucida drawing of a portion of the stem of 
a clover leaf after mitochondrial fixation and staining. Figure 
5 is a camera-lucida drawing of a portion of an unfolded clover 
leaf after mitochondrial fixation and staining. These prepara- 
tions are introduced to furnish a basis for comparison of apparent 
mitochondria in this plant with, particularly, the juvenile forms, 
of bacillus radicicola as represented in figure 3. It is apparent 
from an examination of these illustrations that the mitochondria 
are similar in appearance to the juvenile bacteria. While it is 
obvious that form in itself does not necessarily indicate relation- 
ship, the question may properly be asked: What evidence is 
there to indicate that these structures are cytoplasmic organs 
and not representatives of the organisms that are known to be 
present in the root-nodules of this plant? Again, it may be asked 
with equal pertinence: What evidence may be submitted to 
indicate that these bodies, generally considered cytoplasmic 
organs, are not bacteria that have gained entrance to the plant 
through the root-hairs? The author has not been able to find 
any evidence that would satisfactorily answer these questions. 
a 
