786 The American Naturalist. [September, 
dwarfed and soon disappear. The plant can no longer use 
them as organs of absorption on account of the impervious 
layer of cork. The tubercle receives its nourishment by means 
of the vascular bundle system and the parenchymatous con- 
ducting tissue of the root which is continuous with that of the 
tubercle. The outline of the infected area is wavy, there being 
a depression before each vascular bundle as seen in a cross 
section. Single, or groups of noninfected cells are generally 
distributed through the infected area. In the centre may be 
found a noninfected area of considerable size, as in Robinia 
pseudacacia. The infected cells resemble those of the nonin- 
fected cambium in that they are angular and closely united, 
there being no intercellular spaces. They differ in that they 
are much larger and in that they have undergone peculiar 
protoplasmic and nuclear changes. ‘The cells are entirely 
filled with Rhizobia and protoplasm. The Rhizobia feed upon 
the cell protoplasm and in turn appropriate for the use of the 
plant the free nitrogen of the air. In the case of Rhizobium 
mutabile the abundant food supply causes it to become much 
enlarged and to assume various forms during the season, hence 
the name. No other species seem to undergo such extensive 
changes. As to the position the Rhizobia take in the cell, I 
have noticed some peculiarities. In colonies of Rhizobium 
mutabile in Melilotus alba the long axes are all directed toward 
the nucleus. Trifolium which contains the same species of 
Rhizobium does not show the same arrangement. Here they 
are placed in all concievable positions. Why they should take 
this position in Melilotus alba I am unable to state. In 
Phaseolus vulgaris and Piswm sativum they are often collected 
into Zoogloeae as is clearly shown in thin sections. 
In Amphicarpaea comosa and Phaseolus vulgaris I have always 
found well developed lenticels on the tubercles. They are 
developed from a clearly marked lenticular phellogen, show- 
ing tangential cell division. The phellogen of the lenticels as 
well as that of the cork layer is developed from a large celled 
parenchymatous tissue lying above the cambium layer. The 
lenticular phellogen is not depressed into the underlying tissue, 
as is usually the case, but somewhat elevated above it. ‘he 
