compared with other genera of Cycadeae . 615 
the protoxylem, more or less disorganized (Figs. 14, 15 spx 1 ). 
The normal part of the secondary wood only attains a com- 
paratively small development, and its tracheides have the 
ordinary bordered pits (Figs. 14, 1 5 nx l ). 
The explanation of the peculiar structure of these bundles 
lies in the fact that, from the direction of their course in the 
cortex, they would inevitably be submitted to an extreme 
tension in the tangential direction, so that an adaptation in 
a portion of the secondary wood, in the form of greater 
extensibility in the walls of the tracheides, became an im- 
perative necessity, and thus we find the curious appearance 
in the secondary wood of tracheides with dense spiral 
thickenings. These bundles are orientated so that the xylem 
and phloem are directed towards the inside and the outside 
of the stem respectively. In many cases the outward course 
of the bundles was interrupted and cut short by the periderm. 
It is a fact worth noting that, as soon as the bundle leaves 
the tangential and assumes a more or less radial course, this 
abnormality in their secondary wood becomes very much less 
marked and tends to disappear altogether. 
Uniting the various girdles in different regions of the 
cortex are ‘ radial connexions/ consisting of bundles equal 
in size to those of the girdles, and often fusing with them by 
means of a curious network of elements which are sometimes 
quite contorted and involuted. 
On the outer limit of the cortex, the periderm forms a con- 
spicuous layer, recognizable with the naked eye as a narrow 
white zone with an extremely irregular, sinuous course. It 
consists, on its outer side, of a layer of crushed cork-cells, 
and on its inner side, of a thick zone of phelloderm, built up 
of large square or variously-angled idioblasts or stone-cells, 
alternating with much smaller, thin-walled, colourless cells. 
The cells of this zone were seen to be arranged in radial rows 
continuous with those of the cork-layer on the outside. 
The periderm by successive layers cuts off, firstly, bit by 
bit of the leaf-bases, as shown in Fig. 1, where the dark 
lines ( pd) represent this tissue, the deeply-shaded portions 
