Bottomley .— The Root-nodules of Ceanothus americanus. 607 
the centre of a young nodule or branch shows that the relation of the 
different tissue systems differs but little from that of a normal root. There 
is a central stele surrounded by an abnormal development of cortical cells 
and a protective corky layer on the outside. The xylem forms a solid 
mass in the centre of the stele. As all the elements are small it is difficult 
to distinguish the separate protoxylem groups, but where determined they 
appear to vary in number from three to five in different nodules. The xylem 
is surrounded by several layers of parenchymatous cells, the inner layers 
consisting chiefly of elongated cells (phloem), and the two outer layers 
of iso-diametric cells (pericycle). 
When the nodule branches, secondary thickening occurs in the primary 
stele, whereby a certain amount of secondary xylem is produced each year, 
thus forming a strong supporting stalk for the annually increasing cluster of 
branches. The stele is bounded by a very definite endodermis, which, as in 
the nodules of Alnns, Elaeagnus , and Myrica , is characterized by the cells 
being filled with reserve material, chiefly proteid and oil drops. 
Outside the endodermis the cortical cells are arranged in two zones—■ 
a narrow inner zone of small cells, always free from Bacteria, and an outer 
broad zone of largeTells, most of which are elongated radially, and contain¬ 
ing much enlarged cells crowded with Bacteria. This outer zone is enclosed 
by a layer or layers of suberized cells. 
A median longitudinal section shows that the young nodule is sur¬ 
rounded by a protective layer of suberized cells. In certain localized areas 
these cells occur in relatively large masses. This is seen especially at the 
apex of the nodule, where they function as a kind of root-cap. 
In the older nodules and branches a definite phellogen is present, which 
produces a few very regular layers of periderm, the cells of which always 
contain a dark brown substance. 
The apex of the young nodule, below the protective layer, is occupied 
by a group of meristematic cells which covers the end of the stele. Imme¬ 
diately below this meristematic tissue many of the cortical cells are infected 
with rod-shaped Bacteria, readily seen in situ in sections cut from material 
fixed in alcohol and stained with Kiskalt’s amyl gram stain. The organisms 
occupy at first only a small portion of the cytoplasm of the cell. Here, 
however, they actively divide, and a little further from the apex of the 
nodule the infected cells are seen to have increased in size and are filled 
with Bacteria. Lower down the nodule these cells become still more 
enlarged and the rod-shaped Bacteria are replaced by large spherical bodies 
containing either two or four denser portions. These are evidently the 
‘ globose sporangia ’ of Atkinson. 
Towards the base of the nodule the infected cells gradually lose their 
contents, and are finally seen as dead empty cells. 
