816 W. T. GORDON ON RHETINANGIUM ARBERI, A NEW GENUS OF 
irregular (Pl. III. fig. 19), and even in the smallest divisions of the petiole observed 
the same irregular outline may be seen in the trace (PI. IIL. fig. 20). 
One example of an adventitious root has been discovered in organic continuity with 
the stem, but the preservation leaves much to be desired. The root-trace is represented 
in transverse section in Pl. II. fig. 12. 
HisToLoGy oF THE STEM. 
Although both specimens are to some extent crushed, yet the preservation is suth- 
ciently perfect to allow of a minute examination of the tissues. Starting with the 
primary wood, we find that there is conjunctive parenchyma between the anastomosing 
xylem strands, and this ground tissue is interesting on account of the numerous 
secretory ducts and sacs contained in it (Pl. I. fig. 7, s.s.). The ducts may result from 
the fusion of several sacs or they may be elongated sacs. The former notion seems to 
be supported by the fact that the dark contents do not occur as continuous masses but 
are, in every case, broken up into small pieces (PI. III. fig. 25, s.s.). This appearance, 
however, may be explained otherwise, for the resinous contents may have contracted 
prior to fossilisation, and the fragments have been subsequently separated by the 
petrifying medium. The whole duct would thus seem to contain a series of dark 
coloured bodies in its interior, and appear to result from the fusion of separate cells. 
In transverse section the ducts may be clearly seen in Pl. I. fig. 7, s.s; the dark 
contents are here seen contracted into the centre of the lumen. ‘Taking all available 
data into account, however, it seems more probable that these ducts are produced by 
the elongation of single elements. No epithelium has been noticed surrounding the 
secretory elements. 
The primary wood consists of tracheides, on the walls of which multiseriate pits are 
arranged in a reticulate manner (Pl. I. fig. 5, x). These elements vary in size from 
130 to 150» in diameter, and are united in groups of from twenty to sixty. The 
number of tracheides in each group is thus much greater than in the corresponding 
groups in Heterangium. There are, however, fewer xylem groups in the primary 
wood of Rhettnangvum than in the former genus. 
Round the periphery of the primary wood the xylem groups contain much smaller 
tracheides. These are the protoxylem elements, and they are seen in longitudinal sec- 
tions to have scalariform or sub-spiral thickenings on their walls (Pl. I. fig. 6, pra.). 
They are exarch in position and not mesarch as in Heterangium. 
Each peripheral group of the primary xylem is bounded externally by a wedge of 
secondary wood, and so the principal medullary rays are in direct communication with 
the ground tissue of the central primary wood. The elements of the secondary zone 
are much smaller than those of the primary xylem, being from 45 to 85m in diameter, — 
and they are arranged, as usual, in radial rows. Secondary medullary rays occur in 
each wedge, and usually a ray occurs between each two or three rows of xylem, though 
