Bottomley.— The Root-nodules of Myrica Gale . 113 
closely crowded together sometimes only two branches arise from the 
end of the nodule, instead of the normal three, owing to lack of space 
for development. 
The branching is apparently associated with the outgrowth of lateral 
roots, and is not due to dichotomy of the apical meristem of the nodule as 
in the case of the branched nodules of Alder, Elaeagnus , and Cycas. 
Internal Structure of the Nodule. 
A transverse section of a single nodule (Fig. 3) shows a central tetrarch 
stele like that of the normal root, and indicates that the nodule itself is 
a modified root. The stele is surrounded by an endodermis characterized 
by neither radial dot nor thickened walls, but by the cells being filled with 
oil drops. Outside the endodermis are several layers of cortical parenchy- 
matous cells covered on the outside by a definite small-celled cork layer. 
In mature nodules this cortical tissue is characterized by the presence of 
(1) somewhat enlarged cells filled with Bacteria, (2) cells filled with oil drops. 
In sections cut from material fixed in alcohol and stained with Kiskalt’s 
amyl-gram stain the Bacteria can be seen in situ in the enlarged cells as 
small rods. In transverse sections cut near the apex of the nodules zoogloea 
threads of Bacteria are seen passing from cell to cell. These are comparable 
to the ‘ infection threads ’ seen in leguminous nodules. 
In a median longitudinal section of a nodule (PI. XII, Fig. 6) four fairly 
well defined areas can be distinguished : 
1. Meristem zone. An apical mass of meristematic cells beneath the 
outer protective cork layer. This crowns the end of the stele and extends 
some distance down the sides (Figs. 4, 5, 6, mi). 
2. Infection zone. A zone of cells in which the infection threads are 
seen passing from cell to cell. In a young nodule some of the cells contain 
starch grains (Fig. 8). 
3. Bacterial zone . This area includes the bulk of the cortical tissue of 
the nodule and consists of parenchymatous cells. The majority of the cells 
are enlarged and crowded with Bacteria. Amongst these are scattered cells 
containing reserve food material in the form of oil drops (Fig. 3, 4, 5, 6, 
b. and 0.). 
4. The basal zone. In this region the bacterial cells are few in number, 
and in fully matured nodules are quite absent, but the oil-containing cells are 
numerous. 
As the individual nodule matures and branches, the seat of bacterial 
activity is transferred to the branch nodules, and the Bacteria gradually 
disappear from the bacterial zone. 
True fungal hyphae are often found ramifying through the cells of the 
basal zone, and in the old nodules they may sometimes be seen filling the 
majority of the cortical cells. These fungal hyphae were considered by the 
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