26 
A. S. Marsh. 
connects the two systems which are disunited in the higher regions 
of the leaf. This connecting portion is probably primary and thus 
connects up the Cycadean foliar bundle with the truly mesarch 
bundles of the Cycadofilicales. Even in the bundles of the main 
part of the petiole of Stangeria we have a vestige of this primary 
centrifugal wood in the median line of thickening which often 
stretches out from the protoxylem towards the centrifugal wood 
(Fig. 8). 
Fig. 8.—Transverse section of the petiole of Stangeria, between the leaf- 
base and the insertion of the lowest pinnae. 
Structure of the Wood Elements of the Petiole. 
Longitudinal sections were cut both near the base (but above 
the transition region) and at some distance up the petiole and in 
each case the structure of the elements was the same (Fig. 10). The 
centripetal wood showed a spiral protoxylem, while the metaxylem 
had pitting of a close scalariform type, quite in contrast with the 
annular-reticulate or open scalariform pitting of the centrifugal 
xylem, which resembles the wood of the stem. 
Since in the stem of Stangeria there is so little distinction 
between primary and secondary wood one may be justified in 
assuming that the centrifugal elements of the petiole, even though 
