53i 
Note . 
II. Heterangium. 
Introduction. 
The genus Heterangium differs conspicuously from Lyginodendron, 
in the structure of the stele of the stem, which in Heterangium con- 
tains no pith, but has a solid axis of primary wood. In most other 
respects the two genera much resemble each other. 
i. Heterangium Grievii ) Will. 1 
The original specimens of this species were derived from the 
Burntisland deposits. At a later date, specimens were found in the 
coal-measures of Dulesgate, Lancashire, which have been referred to 
the same species, though they show some slight differences from the 
original form. 
A. The Stem. 
1. General Structure . — The whole interior of the stele is occupied 
by the primary wood, consisting of tracheides intermixed with con- 
junctive parenchyma. In most specimens a certain amount of 
secondary wood has been formed around the central mass. Outside 
the wood a zone of phloem can be traced, and this again is sur- 
rounded by a parenchymatous belt, which we regard as pericycle. 
The inner cortex is characterized by the presence of horizontal plates 
of sclerotic tissue. The outer cortical zone has a structure similar 
to that of Lyginodendron. In the pericycle and cortex numerous 
leaf-trace bundles are met with. 
2. Course of the Vascular Bundles . — The bundles can be traced 
from the stele into the bases of the leaves. Their arrangement indi- 
cates that the phyllotaxis was three-eighths in the larger and two- 
fifths in the smaller stems. Each leaf received a single bundle. 
The leaf-trace bundles can be followed downwards for some distance 
at the periphery of the stele, where they form distinct strands, 
though united with the axial wood. 
3. Primary Structure of the Stele and Leaf -trace Bundles . — The 
strands at the periphery of the stele, as well as the leaf-trace bundles 
with which they are continuous, have the same collateral and meso- 
1 Williamson, British Association Reports, 1871 ; Organization, Part IY, 1872 ; 
Part XVII, 1890. 
