Root Nodules in the Podocarpineae . 805 
a pressure of two and a half atmospheres, for ten minutes. After cooling, 
2 c.c. of the same culture as had been added to the controls was used to 
inoculate each of the other two flasks. All were then incubated at 28° C. 
for ten days. During this period the contents of the control flasks remained 
quite clear, whilst the others had become cloudy owing to the presence 
of a growth of a very slimy nature. The nitrogen content of the flasks was 
determined by the Kjeldahl method of analysis, with the following results : 
Nitrogen found in 
the control. 
1-06 m.grm. 
0-93 m.grm. 
Nitrogen found in 
the culture . 
3*72 m.grm. 
3-46 m.grm. 
Gain in nitrogen 
due to organism. 
2 -66 m.grm. 
2\53 m.grm. 
The increased nitrogen content of the solution in which growth took 
place can only have been produced by the activity of the living organisms 
obtained from the Podocarpus nodules and introduced into the media, and 
must have been caused by the utilization of the free nitrogen of the atmo- 
sphere by these organisms during the processes of metabolism. Hence it 
becomes evident why Hiltner was»able to grow Podocarpus plants in pure 
sand, containing no nitrogen, provided nodules were present on the roots. 
The Bacteria in the nodules assimilate atmospheric nitrogen, and thus render 
it available to the Podocarpus plants. 
Material used for sectionizing was fixed in either Flemming’s orBouin’s 
fixative, or absolute alcohol. Microtomed sections were stained with 
Flemming’s triple, Heidenhain’s iron-haematoxylin, Ziel’s carbol fuchsin, or 
Kiskalt’s amyl gram stain with very satisfactory results. 
The nodules of Podocarpus , Microcachrys , Dacrydium , Saxegothaea i 
and Phyllocladus , as of other non-leguminous plants, are modified lateral 
roots, being developed from the pericycle immediately opposite the proto- 
xylem of the diarch root (PI. LXXVIII, Figs. 10 and 11), and possessing 
a central stele (PI. LXXX, Fig. 17). Owing to some cells of the pericycle 
becoming meristematic (PI. LXXVIII, Fig. 10, a, b), a small swelling is pro- 
duced beneath the endodermis of the root. These cells continue to develop, 
and as the protuberance increases in size a plerome becomes differentiated 
(Fig. 10, c and d ), which later produces a vascular strand directly connected 
with the vascular cylinder of the root (PI. LXXVIII, Figs. 11 and 12, and 
PI. LXXIX, Figs. 13 and 14). This structure gradually pushes its way 
through the cortex of the root and may become a lateral root, but very fre- 
quently, as indicated by the enormous number of nodules present on the 
roots, its growth is arrested, with the result that it produces a small almost 
spherical structure known as a nodule. This arrest in its development is 
caused by the infection of the young cells with the nitrogen-fixing organism 
Pseudomonas radicicola , before it emerges from the cortex of the root 
(Fig. 10). 
