Weiss. — A Stigmaria with Centripetal Wood . - 225 
the scalariform type, the strong horizontal bars of the lignification being 
connected by fine vertical striae, as has been noted and figured by William- 
son (’ 89 ) for Lepidodendron mundum , as well as for other Lepidodendra. 
We come now to the most important feature of this Stigmaria , and 
that is to the primary wood . As mentioned above, this differs in essence 
from that of other Stigmariae in being centripetal in its development instead 
of centrifugal. Towards the centre of the axis the secondary tracheids 
become considerably smaller, without losing anything of the regularity of 
their arrangement. Then within the cylinder of secondary wood we come 
to a very definite primary wood, beginning with small protoxylem elements 
on the outside, and followed on the inside by a metaxylem consisting of 
about two rows of tracheids, the innermost of which are of very large dimen- 
sions. In radial sections the narrow elements at the outside of the wood 
exhibit often spiral and annular markings, and no doubt this region consists 
of the protoxylem elements. The metaxylem, on the other hand, consists 
of scalariform tracheids, some of which are twice the size (-075 to -09 mm.) 
of the largest elements of the secondary wood. It was no doubt the 
general arrangement of the metaxylem, as well as the small number and 
the large size of its tracheids, which influenced Williamson (’ 89 ) in his identi- 
fication of the axis with secondary wood figured by him 1 with Lepidodendron 
mundum , with which the primary wood shows such singular agreement. 
Within the metaxylem we come to a pith consisting of parenchymatous 
cells of rectangular shape when seen in longitudinal section, just as those 
one finds in Lepidodendron mundum . 
The secondary wood is traversed by numerous parenchymatous rays, 
generally one cell thick, though occasionally two cells in thickness. Their 
thinner tissue has undergone some disorganization, so that they appear on 
the transverse section more like gaps, but they can readily be seen on 
the tangential longitudinal section (Fig. 4, PI. XV). But a feature which 
distinguishes this type of Stigmaria from Stigmaria ficoides and some other 
forms is the absence of the very broad medullary rays which usually break 
up the woody cylinder into distinct wedges, and in which the rootlet bundles 
pass out to the exterior. This is also a character of the Stigmaria with 
centripetal wood, figured by Renault. The absence of these broad rays 
may very well have inclined Williamson to the belief that he was dealing 
with a Lepidodendroid branch. A close examination of the innermost 
portion of the secondary growth reveals the fact that there is a tendency of 
the cylinder of secondary wood to break up into segments, as is charac- 
teristic of Stigmaria. This is seen in Fig. 2, PI. XV, particularly on 
the upper edge of the primary wood. In radial longitudinal sections, 
moreover, the rootlet bundles can be seen taking a horizontal course near 
the middle of the secondary wood, like that figured by Williamson (’ 87 ) for 
1 Fig. I 5> PI. V. 
