622 Spratt. — The Root-Nodules of the Cycadacecte. 
root. The root-tip may also develop a ring of lenticels and form the begin¬ 
ning of a cluster of nodules. At the base of the nodules which are produced 
laterally the phellogen becomes very active, and gives rise to a complete 
ring of lenticellular tissue (Fig. 12). Between this basal ring of tissue and 
the nodule large groups of Azotobacter and sometimes Anabaena collect, 
both of which are also associated with the outer cells of the nodule, as 
in Cycas and Encephalartos. As the nodule develops, other lenticellular 
areas may appear, which are very out-curved with a strong tendency 
to form a zone of parenchyma parallel with the surface of the nodule, and 
frequently small localized areas, containing the three organisms so charac¬ 
teristically associated with the outer cells, become completely enclosed 
(Fig- 13)- 
Nodules in which an algal zone is developed become very much 
elongated, always remaining very slender, and branching repeatedly 
(Fig. 4, b). In these nodules the zone occupies a larger proportion of 
the cortex, and is traversed by many more papillate cells than in Cycas and 
Encephalartos. The three symbiotic organisms extend through the outer 
cortex to the phellogen usually, and are often associated with the outer 
cells. Nodules have been examined which had enclosed, in addition to an 
algal zone, certain other areas of Algae and Bacteria on the surface. In 
branched nodules the two algal zones of respective branches have been found 
separated from one another by a band of meristematic tissue. 
The nodules of Macrozamia Macleyi (Fig. 6) and M. Dennisoni resemble 
very closely those of Encephalartos in which no algal zone has developed, 
and no algal zone has been found in these nodules. Correlated with this 
is the scarcity of nodules on the surface of the soil in the plants at Kew. 
Their development corresponds with that described above, but the outer 
cells, in which Azotobacter and Bacillus radicicola are found together, are so 
large and loosely arranged that the surface of the nodule presents an uneven 
spongy appearance. In some cases the infection with Azotobacter extends 
well into the cortex throughout which Bacillus radicicola is distributed. 
In Ceratozamia mexicana (Fig. 7), and Zamia lindeni (Fig. 5), 
the nodules observed remain quite small although branched, and are 
arranged very regularly in two rows along the root. In Ceratozamia large 
numbers are visible on the surface of the soil. A few roots of Bowenia 
spectabilis were also obtained upon which were some very young nodules. 
In these three genera Bacillus radicicola is present in the cortical cells of the 
root and nodule. The basal phellogen produces an envelope of parenchy¬ 
matous tissue as in Stangeria , and in some of the older nodules a second 
phellogen appears in that area inside the old one. The loose outer cells 
usually have Azotobacter associated with them, but no Alga has been found 
present in these genera. 
The roots of Dioon spimdosum and Dioon edule produce successive 
