608 Bottomley.—The Root-nodules of Ceanothus americanus . 
Origin and Branching of the Nodules. 
From the material available it was not possible to trace the earliest 
stages in the formation of the nodule, but sections of young nodules in¬ 
dicate very clearly that they are modified lateral roots. 
They consist of an apical meristematic region below which is a branch 
from the root-stele surrounded by cortical tissue containing many infected 
cells, the endodermis of the nodule-stele being continuous with that of 
the root-stele. Presumably certain cortical cells of the root became infected 
by Bacteria as in the formation of leguminous nodules, and these stimulate 
the formation of a lateral root at this point. This lateral root never breaks 
through the cortex, but as it extends into the infected area the outer cortical 
cells above this area become meristematic, and by their subsequent growth 
a long cylindrical nodule is formed instead of a typical lateral root. 
The branches of the primary nodule are very evidently endogenous 
in origin. They arise from an active development of the pericycle cells 
opposite a protoxylem group. A stelar branch is thus formed, and as this 
extends into the cortex an apical mass of meristematic cells becomes 
differentiated which soon produces a visible branch to the nodule. 
This method of branching is quite different from that which is charac¬ 
teristic of the nodules of Alnus , Elaecignus , and Cycas where there is a di- or 
trichotomy of the apical meristem. In Ceanothus as in Myrica the branches 
are primarily due to the formation of stelar outgrowths, the essential differ¬ 
ence between the two being that in Ceanothus the stele never emerges from 
the nodule, and the apex remains meristematic, continuing its growth each 
year, whilst in Myrica the stele in its second year pushes through the apex 
of the nodule and forms a small hair-like rootlet, thus preventing further 
apical growth. 
Isolation and Cultivation of the Bacteria. 
As described above, two different structures are found in the infected 
cells, rod-shaped organisms near the apex of the nodule and spherical bodies 
lower down. The rod-shaped organisms when isolated by the methods 
described in detail for Myrica nodules (Ann. Bot.,xxvi, p. 114) appear to be 
identical with the Bacillus radicicola group found in leguminous nodules, 
giving the characteristic staining reaction with Kiskalt’s amyl gram, and 
typical colonies when grown on nutrient agar. The spherical bodies are 
very similar in appearance to the ‘ coccus ’ forms described by Spratt 
in Alnus and Elaeagnus nodules. When grown in ordinary B. radicicola 
culture solution they soon lose their spherical condition and break up into as 
many typical rod-shaped forms as there are dense portions present. The 
reverse change from rods to spherical bodies can be seen in old colonies 
