810 Spratt. — The Formation and Physiological Significance of 
that they may attain their full size even when they are not infected 
with it. 
In the old roots there is a formation of periderm (Fig. 14), the phello- 
gen arising immediately outside the endodermis, so that when this takes 
place the whole of the cortex is thrown off, amongst which are some cells 
containing Bacteria. This, however, does not usually occur until the root 
is two or three years old, by which time the formation of new nodules has 
ceased in this region, and the old ones already have a supply of organisms 
for the infection of their new cells. 
These investigations entirely support the theory that the root nodules 
of Podocarpus are actively concerned in the assimilation of atmospheric 
nitrogen, not, however, primarily owing to the presence of a mycorhizal 
fungus, but to their symbiotic association with a nitrogen-fixing bacterium. 
This not only occurs in all the species of Podocarpiis examined, but also in 
Microcachrys , Dacrydium , Saxegothaea , and Phyllocladus , four other genera 
of the Podocarpineae. The production of nodules on the roots is thus 
a constant feature throughout the Podocarpineae, their development and 
morphology being in every case of the same characteristic type, and they 
are always inhabited by apparently the same organism. This organism is 
morphologically and physiologically identical with Pseudomonas radicicola 
found in the nodules of the Leguminosae, Cycadaceae, Elaeagnaceae, Alnus , 
and Myrica . This organism enters the roots of plants of the Podocarpineae, 
and the infection of the meristematic cells of a young lateral root causes its 
transformation into the structure known as a nodule. In this tissue the 
Bacteria multiply very rapidly, and in their growth they have been shown 
to utilize the free nitrogen of the atmosphere, so that by their development 
in the nodules attached to the plant they are rendering that vast store of 
food material available, and enabling the plant to grow even under circum- 
stances which exclude all other nitrogen supply. 
The division of the Coniferales known as the Podocarpineae are inhabi- 
tants of the southern hemisphere, principally Australia. The dominant 
genus Podocarpus has been considered related to the Araucarineae, through 
Dacrydium and Saxegothaea , on certain morphological grounds, and this 
association is supported by the common and peculiar geographical distribu- 
tion of the two groups. Tison supports this idea, from the behaviour of the 
two systems of vascular bundles found in the megasporophyll, but he also 
says that Podocarpus and Saxegothaea are very definitely related through 
Microcachrys. There are many arguments against a union of the Podo- 
carpineae and the Araucarineae, but all the morphological work on the 
various parts of the plants indicates that Saxegothaea and Podocarpus are 
the most widely separated of all the genera placed in the Podocarpineae, 
and Dacrydium and Microcachrys form connecting links between them. 
This conclusion is supported by the morphology of the root nodules in this 
