374 Hickling . — The Anatomy of Pataeostachya vera . 
The average size of the cells increases towards the centre, and they are 
all vertically elongated except at the nodes (Figs, i, 2, 13). Only two or 
three layers immediately round the cavity were thin-walled, the remainder 
sclerized. The medullary rays consisted likewise of regular thick-walled 
cells. The rays between the bundles of a pair merge insensibly into the 
medulla internally, and the ‘ disc ’ externally. Those between the pairs 
of bundles stop short at the ‘ cortical canals.’ 
Vascular System . The wedge-shaped xylem bundles form a ring 
round the medulla. Of these bundles there are usually eighteen, occasion- 
ally from sixteen to twenty, arranged normally in pairs, which lie alternately 
with the ‘ large canals ’ of the ‘ nodal discs.’ This pairing of the bundles 
is the most striking feature in the anatomy of the axis. It is perhaps more 
apparent than real, and seems to be due to the abnormal orientation of the 
bundles necessitated by the mechanical arrangements at the nodes, where 
the phloem must be turned so as to lie within the 4 canal,’ while the xylem 
must correspondingly rotate in the opposite direction. If the bundles were 
normally orientated the disc would obviously be much weakened. Thus 
the pair of bundles lying between two canals have their xylem masses 
turned towards one another, and so approximated, while the phloem 
masses are turned away. Each bundle is end arch, and has a ‘ carinal ’ 
canal representing the degenerated protoxylem exactly as in Catamites and 
Equisetum . Traces of the broken tracheids may be seen in the longitudinal 
section of the canal. 
At the node the protoxylem remains, obliterating the canal at that 
point (Fig. 1, ptxy .). In a mid-internodal section, each xylem bundle is 
seen to consist of some twenty to forty very small tracheids. The whole 
of this wood appears to be primary and centrifugal, though one section 
shows one tracheid apparently centripetal, but only, I think, because the 
section does not cut the carinal canal medianly at that point. 
The tracheids are all either spiral or annular. On approaching the 
node the bundles increase rapidly in size by the addition externally of 
a considerable mass of secondary wood. As in Catamites these secondary 
tracheids were, in the youngest condition, limited to the nodes only, while 
the later-formed tracheids extended successively further and further into 
the internodes. It was this characteristically Calamitean development of 
secondary xylem which first convinced Williamson of the affinities of this 
cone. These secondary tracheids are larger than the primary ones, and are 
scalariform or pitted with two to four rows of eye-shaped pits (Fig. 14, B } C). 
Where this secondary wood is thickest, there are about ten to fifteen layers 
of tracheids (Fig. 3). 
The node, using this term to indicate the level at which the 
vascular bundles give off their branches to the appendages, and at which 
there is the usual shortening up and disturbance of the course of the 
