456 IVAN E. WALLIN 
Further, it offers evidence of the fact that the chemical products 
of the bacterium in this case are essential to the life of the host 
plant. True, the host plant may procure these chemicals by 
absorption from the soil, and it may even do so in spite of the 
nodule organisms. This fact, per se, has no bearing on the im- 
portance of the phenomenon. The point at issue may be clarified 
by an illustration: The thyreoid gland produces a chemical 
substance that is essential to normal metabolism in higher, 
animals. The gland may be removed from an animal, but the 
chemical substance must be supplied artificially if life is to be 
maintained normally. If it were possible to stimulate the pro- 
duction of this chemical substance from another organ of the 
animal, then the thyreoid would be unessential and in all proba- 
bility would degenerate. 
Lewitsky (10), Guilliermond (’12), Regaud (11), and other 
investigators have ascribed to mitochondria the property of 
plastid formation in plants. According to these investigators, 
the original mitochondria transform into plastids. Accompany- 
ing this transformation the mitochondria take on the various 
functions characteristic of plastids. Various kinds of plastids 
are to be found in plants. From the standpoint of evolution, 
the more important of these plastids are the chloroplasts, or the 
plastids containing chlorophyl. 
According to Guilliermond, chloroplasts in higher plants are 
formed from mitochondria. He has, apparently, observed the 
various intermediate stages in the metamorphosis from a minute 
body, the mitochondrium, to a fully formed chloroplast. Such 
a morphogenesis is so strikingly similar to the morphogenesis 
of the Bacillus radicicola in the root-nodules of Leguminosae 
that the writer feels justified in presenting his observations on 
these forms. 
MATERIALS AND METHODS 
Root-nodules found on the roots of the common white clover, 
growing on the University campus, were fixed in the modified 
Flemming’s fixative described in a former paper (Wallin, ’22). 
After they were washed, dehydrated, cleared, and embedded 
