820 W. T. GORDON ON RHETINANGIUM ARBERI, A NEW GENUS OF 
The cortical tissues in the petiole are quite similar to those in the stem. The 
inner cortex is a delicate parenchyma with secretory sacs and ducts, while the outer 
zone contains the sclerotic hypoderma with its secretory ducts. 
HistToLogy oF THE Root. 
One example of an adventitious root has been found in organic connection with the 
stem, and several similar detached roots have been noted. ‘The root-trace seems to leave 
the stem close to the point of emission of the petiole-trace, or it may be that it comes 
from the petiole-trace itself. This particular specimen occurs at a part of the stem 
where the outer cortex has been stripped off and the root is seen in the inner cortex. 
Although the trace is not very well preserved, one can see that it was probably tetrarch. 
A considerable amount of secondary wood has been developed round the primary xylem 
(Pl. IL. fig. 12, x, and x, respectively). : 
There are still some points in connection with the anatomy of the petiole and root 
which require further elucidation, and it is to be hoped that additional well-preserved 
examples may be discovered. 
_There is no indication in the specimens of their foliage or fructifications. 
SUMMARY. 
Rhetinangium arberi had a stem of considerable length, and was probably of 
scrambling habit, with adventitious roots at intervals on the stem. Petioles were 
emitted in spiral sequence, and three of them may be seen intersected at different levels 
in any transverse section. 
The vascular axis is protostelic, the primary wood is of the Heterangium type, 
but the protoxylem is exarch, and many secretory sacs and ducts are present in the 
conjunctive parenchyma. ‘The secondary wood is of cycadean type. The protoxylem 
elements have scalariform or sub-spiral thickenings on their walls; the other tracheides 
are reticulately pitted. The phloem consists of elongated elements and parenchyma, 
while the inner cortex is formed of a zone of delicate tissue with numerous secretory 
bodies scattered through it. The outer cortex is made up of thick-walled parenchyma, 
and contains a sclerotic hypodermal zone round which the cortical parenchyma is much 
elongated radially. 
The leaf-trace is peculiar in form and is produced by the union of several peripheral 
groups of the primary xylem. The protoxylem elements are on the lower (abaxial) 
surface of the trace and continue throughout in that position. In no case does the 
petiole-trace divide up into numerous small bundles. Even in the smallest division 
noticed the trace is a single strand. The cortical tissues are similar to those of the stem. 
Close to the emission of a petiole-trace an adventitious root has been observed to 
leave the stem. 
