620 Spratt.—The Root-Nodules of the Cycadaceae. 
The first part of the present series of investigations was carried out with 
material of Cycas circinalis and Encephatartos Hildebrandtii kindly supplied 
by the Curator of Chelsea Physic Gardens ; and was extended to comprise 
the genera, Stangeria, Macros amia, Zamia , Ceratozamia, Dioon, and Bowenia , 
through the kindness of the Director of the Royal Botanic Gardens, Kew, 
both of whom I desire to thank. 
Root-nodules have been found to occur throughout all the cycadean 
genera, and as in other non-leguminous plants they are perennial modified 
lateral roots which have diverged from their normal growth owing to their 
having been infected with the nitrogen-fixing organism Bacillus radicicola. 
In Cycas circinalis they are confined to the surface of the soil and the 
layers immediately below. It has, however, been ascertained that the 
lower roots frequently become infected with Bacillus radicicola , which 
penetrates the root-hairs and enters the cortex, where its presence stimulates 
the root to become negatively geotropic (Pig. i), and when near the surface 
of the soil the tip becomes swollen and subsequently profusely branched, 
giving rise to a coralloid mass (Fig. d). Large masses of these structures 
are visible on the surface of the soil, and they characteristically contain 
a very definite green ring, the algal zone. The same conditions occur 
in C. revoluta and C. seemanni , but some species in cultivation at Kew have 
not developed these nodules very abundantly. 
Encephalartos Hildebrandtii and all the species cultivated at Kew 
exhibit a similar phenomenon, but the individual nodules are larger, and 
thus, although the branching is very profuse, the clusters are less dense 
(Fig- 3 )- 
Examination of a large number of nodules from both Cycas and 
Encephalartos showed that the nodule tip may become quite large and even 
branched several times, while no algal zone is present (Fig. 3, a , b). The 
Alga therefore lives in the soil, and must at some period gain an entrance 
to the nodule. The roots which have become negatively geotropic have 
Bacillus radicicola in their cortical cells, but they retain their normal appear¬ 
ance. Very soon, however, a ring of structures, apparently lenticels, is 
formed a little distance from the tip, which subsequently becomes swollen 
(Figs. 1 and 3 a). The lenticels consist of large masses of very loosely 
arranged parenchymatous cells formed from the phellogen. They soon 
become infected with Bacillus radicicola and also Azotobacter , the latter 
at first making their way between the cells, and later entering them (Fig. 9). 
Gradually in the nodule, i. e. the portion above the ring of lenticels, the outer- 
cells produced by the phellogen increase in size owing to infection with 
Bacillus radicicola , and are pushed apart by masses of Azotobacter , some of 
which subsequently penetrate into them, with the result that there is a zone 
on the outside of the nodule which contains the two nitrogen-fixing organisms 
associated together (Fig. 9). This zone extends from the basal lenticels 
