806 Spratt . — The Formation and Physiological Significance of 
The Bacteria enter the cortex of the root by means of the root-hairs, in 
which they have frequently been seen, particularly in some young roots 
of Phyllocladus (Fig. 9). Having once penetrated the cell-wall and entered 
the hair, they multiply rapidly, producing a zooglea thread, exactly com- 
parable to the infection threads of the Leguminoseae. The thread passes 
into a neighbouring cell, and subsequently numerous zooglea threads are 
produced which pass from cell to cell of the cortex (Figs. 9 and 16). When, 
however, a meristematic region arises in the neighbourhood of the Bacteria, 
they appear to be stimulated to further and more rapid development, and 
chemotactically attracted by the meristematic tissue, so that some of the 
infection threads penetrate into the young tissue (Fig. 10). Having once 
entered this new structure the Bacteria divide repeatedly, and the zooglea 
threads so produced ramify through the young cortical cells, causing their 
expansion, but arresting their further division, and consequently the 
immediate growth of the nodule. 
A mature nodule possesses a small stele (Fig. 17), surrounded by 
a definite endodermis, the cells of which are slightly thickened on the outer 
side, although there is no evident radial* dot, and many of them, as in the 
endodermis of the root, contain a deposit of tannin which renders them 
more apparent. In all the species of Podocarpus , where the nodules attain 
the greatest size in the group Podocarpineae, the nodular stele becomes dis- 
tinctly diarch (Fig. 17); so also in Saxegothaea it usually becomes thus 
differentiated, but in Microcachrys and Phyllocladus the vascular strand 
is frequently very rudimentary, the xylem only consisting of two or 
three tracheides, around which are two or three layers of parenchymatous 
cells, and then an endodermis. The stele is continuous with that of the 
root and traverses about half the length of the nodule. The remaining 
nodular tissue is composed of parenchymatous cells, in which there is no 
differentiation of a meristematic zone, all the cells are similar, but the outer- 
most layers after the nodule has emerged from the root for some time 
become slightly more compressed, and their walls become a little thicker, 
the outer ones eventually becoming slightly suberized (Figs. 11 and 17). 
The outermost layer of cells of a mature nodule, like the epidermal cells of 
the roots, have the capacity of growing out to form hairs (Figs. 6 and 7). 
This they characteristically do in Dacrydium , Saxegothaea , and Phyllo- 
cladus in the spring of their second year, and it sometimes also occurs in 
Podocarpus and Microcachrys. 
The cortical cells of both roots and nodules in all the genera appear to 
be traversed by numerous very narrow filaments. These, however, when 
stained with Kiskalt’s amyl gram stain are seen to be composed of numerous 
rod-shaped Bacteria, obviously Pseudomonas radicicola> which are embedded 
in a mass of slime, forming a zooglea, and thus giving the appearance of 
a network of hyphae (Figs. 15 and 16). These threads are capable of pene- 
