ON THE NATURE OF MITOCHONDRIA 459 
To recapitulate, thé Bacillus radicicola is a minute organism 
that may be found as a free-living bacterium in the soil. Under 
favorable conditions, it may enter the root-hairs of Leguminosae 
and enter into a partial symbiotic existence. The host plant re- 
sponds to the infection with a production of nodule cells. The 
invading organism comes to lodge in the cytoplasm of the root- 
nodule cells. In a mature root-nodule these forms, apparently, 
represent three stages in the morphogenesis of the bacillus after it 
acquires the symbiotic relationship. 
DISCUSSION 
The morphogenesis of Bacillus radicicola from the juvenile 
to the senile forms is strikingly suggestive of the morphogenesis 
of chloroplasts as described by Guilliermond and other investi- 
gators. Both forms develop from minute structures that have 
similar form and staining reactions. Both forms have a similar 
shape when they have reached their fullest development and are 
apparently alike in fragility. Both forms have a similar rela- 
tionship to the cytoplasm of host cells. The morphogenesis of 
chloroplasts, as described by Guilliermond, appears to imitate 
more closely the morphogenesis of an organism than the develop- 
ment of a cytoplasmic organ. 
The senile forms of Bacillus radicicola have a particular inter- 
est and will serve a useful existence in our conception of the nature 
of mitochondria. It is quite obvious that absence of these forms 
in bacteriological literature is due to the fragility of the organism. 
In ordinary bacteriological technique these forms are destroyed; 
both in smear preparations and planting on culture media they 
would be destroyed in the procedure. I have not used any other 
fixation for root-nodules than the one indicated above (a mito- 
chondrial fixative). It is probable that the senile forms are 
destroyed by many fixatives just as mitochondria are, and this 
condition may account for some of the failures of previous in- 
vestigators to observe them. 
If my interpretation of the senile forms is correct, then, they 
furnish excellent proof of the contention I stated in the ‘ad- 
