Spratt .— The Root-Nodules of the Cycadaceae . 623 
layers of cork which gradually become split and peel off. The cells themselves 
are rich in tannin. In addition to, and perhaps correlated with, this somewhat 
extensive development of periderm, large fan-shaped masses of parenchyma 
are produced at frequent intervals on the roots. There is a greater develop¬ 
ment of cells resembling lenticellular complementary cells than has been 
observed elsewhere. Large masses are present at the base of all lateral 
roots and also nodules (Fig. 14), which are abundant on roots a little below 
the surface of the soil. The nodules are repeatedly branched ; in some 
cases two rows of small branches are produced from a central portion, 
but eventually large spherical masses are produced (Fig. 8). These small 
nodules show the typical structure described above where no algal zone is 
present. In addition to these, very much enlarged root-tips are visible on 
the surface of the soil, which also contain Bacillus radicicola and have 
a thick periderm in which numerous very large lenticels are developed. 
One nodule was obtained which had a definite algal zone (Fig. 8, b) occupy¬ 
ing a large area. Since only those nodules, to which the sun’s rays can 
penetrate, can contain a living green organism, it seems probable that it is 
in the swollen tips which reach the surface that the Anabaena finds its home. 
From these investigations the conclusions have been reached that root- 
nodules are universally present throughout the Cycadaceae, and they 
are primarily produced by infection with Bacillus radicicola , and are there¬ 
fore concerned with nitrogen assimilation. They are adapted to utilize 
both the nitrogen-fixing organisms Bacillus radicicola and Azotobader which 
are known to fix a greater quantity of atmospheric nitrogen per unit of 
carbohydrate when growing together than separately. 
There is also sometimes a fourth symbiont present—the Anabaena . 
It is not, however, essential to the formation of the nodule, since in Macro- 
zamia , Zamia , Ceratozamia , and Bowenia it has not been found, and only in 
one instance in Dioon . The presence of lenticels is evidently correlated 
with the formation of the algal zone, since they are much more abundant 
on nodules in which it is developed, and the zone itself is always inter¬ 
rupted immediately below some part of the lenticel. These facts, together 
with the universal occurrence of a ring of lenticels, or a,basal ring of paren¬ 
chyma with an active phellogen, and the presence of Azotobader and 
Anabaena amongst the outermost layers of cells, suggest very strongly that 
the outer layers of the original nodule become broken down to form the 
algal zone, and the phellogen of the basal and lenticellular areas is stimu¬ 
lated by the presence of the Anabaena to develop a zone of cells, parallel 
with the surface of the nodule, which gradually extends and forms a pro¬ 
tective layer. This is supported by ( a) the formation of the local enclosed 
areas on the surface of the nodule, containing the three organisms in 
Stangeria ; (b) the algal zone retaining a localized position in the nodule 
which has been observed in Cycas, Encephalartos , and Stangeria , and also 
