ON THE NATURE OF MITOCHONDRIA 455 
IV. A COMPARATIVE STUDY OF THE MORPHOGENESIS OF 
ROOT-NODULE BACTERIA AND CHLOROPLASTS 
In a growing conception of a bacterial nature of mitochondria, 
one naturally would seek an example of an undisputed symbiotic 
bacterium forstudy. The root-nodule bacteria, Bacillus radicicola 
offers an example of a relationship between two organisms that is, 
at least, of a partial symbiotic nature. The bacterial organism 
in this case, apparently, exists as a free-living organism in the soil. 
Under favorable circumstances, it enters the root hairs of Legum- 
inosae and ultimately may be found in the cytoplasm of the cells 
of the root-nodules. The host plant responds to the infection by 
developing the root-nodules. 
The degree of symbiosis in this example is, perhaps, not ab- 
solute.! The bacterium can live as an independent organism 
in the soil. Its symbiotic qualities are of the nature of partial 
adjustment. In other words, the organism has not changed to 
such a degree that it cannot exist independently of the host 
organism. This status of its existence is undoubtedly respon- 
sible for the ease with which the organisms from a root-nodule 
may be grown on artificial culture media. 
Compared to absolute parasitism, the root-nodule bacteria 
are not as dependent on the host as is an absolute parasite. In 
the case of an absolute parasite, as well as an absolute symbiont,? 
the adjustment is so complete that the organism does not nor- 
mally live outside thehost. However, the Bacillusradicicola offers 
an illustration of a microscopic organism that may live and 
flourish within the cytoplasm of the cells of a higher organism. 
1 Various classifications of symbiosis may be found in the literature. The 
terms employed in these classifications have been based upon individual examples 
and conceptions and consequently can not be employed with clarity in an en- 
larged conception of symbiosis. Schneider’s (1897) terms ‘“‘mutualism,” “indi- 
vidualism,” and “‘contingent mutualism’”’ are vague and misleading. The terms 
‘absolute’ and ‘incomplete’ symbiosis have been introduced by the author on 
account of their simplicity, clearness, and direct significance. 
2 The author chooses to use the term ‘“‘symbiont’’ employed by Schneider (1897) 
in preference to the term ‘‘symbiote’’ introduced by Portier (1918) for the 
reason that ‘‘symbiont’’ refers to either one of the two organisms entering into 
symbiosis, while ‘“‘symbiote” refers particularly to mitochondria in a bacterial 
conception of their nature. 
