1 14 Bottomley . — The Root-nodules of Myrica Gale . 
earlier investigators of Myrica to be the cause of the formation of the 
nodules. This view, though now seen to be erroneous, was quite natural 
before the bacterial nature of the nodule was demonstrated, and it is possible 
that these hyphae, whilst not the cause of nodule formation, may be of 
a mycorrhizal nature and benefit the Myrica plant. 
Origin of the Nodules. 
The nodules evidently arise as modifications of lateral roots. The 
method of infection has not yet been observed, but it is presumably similar 
to that of leguminous plants. The Bacteria enter a root-hair and pass into 
the cortical cells as an ‘ infection thread \ It is possible that the presence 
of these Bacteria in the cortical cells stimulates the stele to produce a lateral 
branch at this point, for every nodule possesses a well-defined tetrarch stele. 
The cortical cells become enlarged by the growth of the infecting Bacteria, 
and a short swollen nodule is formed instead of a typical lateral root. 
The tip of the nodule is rounded and does not possess a root-cap, for the 
stele of the lateral root does not break through the surrounding cortex 
until later. 
When the nodule has reached its mature size further expansion ceases, 
but the stele, which up to this stage has remained completely within the 
nodule, ending a short distance below the apex, now grows on and out 
through the apex, and forms a thin hair-like rootlet (Fig. 6, r.) with a typical 
tetrarch stele (Fig. 7) surrounded by a few layers of cortical cells, but 
possessing no definite root-cap. 
Around the rootlet three secondary nodules or branches arise from the 
primary nodule in a similar manner to the formation of the primary nodule 
from the root. These secondary nodules repeat the growth and branching 
of the primary nodule, a rootlet and three tertiary nodules arising from 
each. By repeated branching in this manner the peculiar ‘ cluster ’ nodules 
are produced. 
Bacteria — Isolation and Cultivation. 
Small nodules were removed from the roots and sterilized by being 
placed for two minutes in the sterilizing fluid used by Harrison and Barlow 
in their investigations on leguminous nodules. This fluid consists of hydro- 
chloric acid 2*5 c.c., mercuric chloride crystals 1 grm., distilled water 500 c.c. 
After removal and washing in distilled water one of the nodules was 
crushed on a slide. The exuded matter adhering to the slide was then air- 
dried under sterile conditions and stained with Ziel’s carbol fuchsin. On 
microscopic examination numerous small rod-shaped Bacteria were seen. 
These were evidently similar to Pseudomonas radicicola , the organism found 
in the root-nodules of the Leguminosae. They further showed the charac- 
teristic staining reaction with Kiskalt’s amyl-gratn stain, the aniline gentian 
