Bottomley .— The Root-nodules of Ceanothus americanus. 609 
grown on agar medium. These spherical bodies therefore are not ‘ spor¬ 
angia ’ as Atkinson interpreted them, but are similar to the ‘ bacteroids ’ 
found in leguminous nodules which are known to be a resting and resistant 
form of the active bacillus produced by unfavourable environmental or 
nutritive conditions. In Ceanothus the alteration in conditions which must 
necessarily occur in the host cell as it becomes depleted of food and crowded 
with Bacteria causes the organism to assume the ‘ bacteroid ’ form which in 
this case is spherical instead of the Y- or V-shape of the leguminous 
nodule. 
Fixation of Atmospheric Nitrogen. 
To test the possibility of these organisms being concerned with the 
fixation of atmospheric nitrogen pure cultures from each species of Ceano- 
thus were grown in Erlenmeyer flasks in the usual non-nitrogenous culture 
solution, one set of flasks being autoclaved immediately after inoculation to 
serve as controls. The flasks were incubated at 36 °C. for seven days, during 
which time the contents of the control flasks remained clear, whilst the non- 
sterilized flasks became cloudy. Kjeldahl nitrogen determinations of the 
contents of the flasks gave the following average figures: 
N. in control. N. in culture. N ' S™ ** 100 c ' c ■ 
oj culture. 
C. americanus 0*24 mg. 2*57 mg. 2*33 mg. 
C.vehitinus 0*14 „ 2*36 „ 2-22 „ 
This gain in nitrogen is very similar in amount to that obtained from 
cultures of the Bacteria in the root-nodules of Alnus> Elaeagnus , and Myrica . 
It is therefore evident that the root-nodules of Ceanothus are definitely con¬ 
cerned with nitrogen assimilation. 
Summary. 
1. The root-nodules of Ceanothus americanus are modified lateral 
roots. 
2. They are perennial and increase in size each year by the formation 
of endogenous outgrowths (branches) similar in structure to the primary 
branch. 
3. Each primary nodule and branch when fully grown shows four 
zones : (a) an apical meristematic zone; {b) an infection zone, where 
the cortical cells are becoming infected with Bacteria ; (< c ) a bacterial zone, 
containing many radially-elongated enlarged cells filled with Bacteria ; 
( d) a basal zone, almost free from bacterial cells. 
4. The younger bacterial cells contain rod-shaped organisms, the older 
ones spherical bodies. These latter are the ‘ bacteroid ’ condition of the 
active nitrogen-fixing rod-shaped bacillus. 
