1 1 6 Bottomley . — The Root-nodules of Myrica Gale . 
four made no fresh growth, and after a time showed evident signs of starva- 
tion. At this stage two of these starved plants were watered twice, at an 
interval of seven days, with a liquid culture of Myrica nodule organisms. 
Soon these plants commenced active growth, and caught up and surpassed 
the two plants possessing root-nodules. By the end of the summer they 
were larger than any of the Myrica plants in the greenhouse. The two 
starved plants, which were not inoculated, lingered on for a time and were 
quite dead by the end of June. 
It is evident from these results that the root-nodules of Myrica are 
definitely concerned with nitrogen assimilation, and the Bacteria in the 
bacterial zone of the nodule are the active agents in nitrogen fixation. 
Summary. 
1. The root-nodules of Myrica Gale are modified lateral roots. 
2. The young primary nodules give rise by branching to the character- 
istic ‘ cluster ’ nodules, surrounded by rootlets which grow out through the 
end of each branch. 
3. Three branches or secondary nodules arise from the end of each 
primary nodule, and, like it, are modified lateral roots. After the formation 
of these branches the stele of the primary nodule elongates and grows 
through the apex of the nodule, giving rise to the hair-like rootlet. 
4. I11 each mature nodule four zones can be distinguished : 
(a) The apical meristern ; 
(b) The ‘ infection thread ’ area ; 
(c) The ‘ bacterial ’ zone, which includes most of the cortical tissue 
of the nodule, and consists chiefly of the enlarged cells containing Bacteria ; 
(< d ) The basal zone. The lower end of the nodule, devoid of 
bacterial cells, but containing numerous cells filled with oil drops. 
5. After the nodules have branched and reached their full size the 
Bacteria disappear from the cells of the bacterial zone, and the basal zone 
gradually encroaches upon and finally replaces all the other zones. 
6. In old nodules, and sometimes in the basal zone of younger nodules, 
mycorrhiza filaments are found. 
7. Pure cultures of the Bacteria from the ‘ bacterial ’ cells were made, 
and on examination were found to be identical in structure and growth with 
Pseudomonas radicicola , the organism of the root-nodules of the Leguminosae. 
8. Nitrogen determinations of liquid cultures, incubated for 7 days at 
25° C., showed a fixation of 2*05 mg. of nitrogen per 100 c.c. 
9. Young Myrica plants grown in sterilized nitrogen-poor soil did not 
flourish unless they possessed root-nodules. Plants devoid of nodules, after 
inoculation with a culture of Myrica nodule Bacteria, developed root-nodules 
and grew well. 
