621 
Spratt .— The Root-Nodules of the Cycadaceae. 
nearly to the meristematic apex. In many cases Anabaena filaments are 
associated with these large loose outer cells, especially in the angles produced 
by the nodule branching (Fig. 9). 
Other nodules have a very definite green algal zone traversing a corre¬ 
sponding area of the nodule, but enclosed by a few layers of cells (Fig. 11). 
The three organisms Bacillus radicicola , Azotobacter , and Anabaena now all 
become concentrated in this area, which consists of a large air space 
inhabited by the Anabaena and Azotobacter , and traversed by elongated 
papillate cells connected with both the inner and outer tissues, thus keeping 
the zone intact, and being themselves rich in protoplasmic contents, and 
containing Bacillus radicicola (Fig. 10). The cortical cells between the 
algal zone and stele are remarkably free from infection, although some contain 
the two Bacteria, most of them are filled with starch grains or calcium 
oxalate crystals. Outside the algal zone is a very definite phellogen which 
produces a few layers of parenchymatous cells towards the zone, and a few 
very regular cells on the outside. The algal zone always begins very abruptly 
immediately above a lenticel, but at the apex of the nodule the Anabaena 
appear to make their way gradually between the cells into which also the 
Bacteria penetrate, thus continuing the growth of the zone with that of the 
nodule. Sometimes, however, it is localized to quite a small area, perhaps 
on one side of the nodule. Associated with its production is an extended 
development of lenticels at some portion of which the zone is always inter¬ 
rupted (Fig. 11). In nodules without an algal zone only a basal ring 
of lenticels occurs, but where it is present they are produced in connexion 
with every branch and at quite frequent intervals on the branches themselves 
(Fig. 3). The nodule often branches dichotomously, but lateral branches 
also arise, particularly beneath the primary ring of lenticels, which may 
become nodules or lateral roots (Fig. 5). 
Whilst visiting Kew Gardens, one of the Stangeria Schizodon plants 
was observed to have a piece broken from the pot quite near the bottom, 
and from this hole a number of roots upon which were numerous small 
nodules were seen to be emerging. This aroused the question whether the 
nodules in other members of the Cycadaceae were confined to the surface of 
the soil or not. Their formation here might have been induced by the 
exposure to light and air resulting from the presence of the hole, or it might 
be that they were more or less uniformly distributed throughout the pot. 
On inquiry the latter was found to be the case. 
In Stangeria small nodules are produced all along the root, each 
of which branches dichotomously, and thus little clusters are formed 
(Fig. 4). Infection of the root with Bacillus radicicola occurs through the 
root-hairs, and these organisms infect the young lateral roots whilst they are 
traversing the cortex, and thus their growth in length is arrested, and they 
are stimulated to branch almost immediately they are free from the parent 
