264 BOTANICAL GAZETTE [APRIL 
Dadoxylon Spenceri shows considerable resemblance to Poroxy- 
lon, for its leaf trace, at first single, becomes double before leaving 
the stele, and possesses well developed primary centripetal wood, 
on the outer face of which occurs the exarch protoxylem. At the 
very base of the trace, however, this primary wood disappears and 
an endarch condition results. 
It seems highly probable that the living Coniferales have been 
derived from plants closely related to the Cordaitales, but that, 
in the course of their development, the modern members of 
the group have almost entirely lost the centripetal primary wood 
so characteristic of their ancestors. In the cretaceous genus 
Prepinus, however, JEFFREY (2) has recently described an inter- 
mediate condition between ancient and living forms, for in the 
leaf of this primitive pine there is abundant centripetal wood. 
Though the protoxylem is adjacent to this, it is apparently m 
seriation with the centrifugal part of the bundle, which is very 
probably modified secondary xylem, as indicated by the presence 
of abortive medullary rays. In this case, apparently, the first 
formed tracheids had assumed the typical coniferous position 
before the disappearance of the centripetal xylem. We shall find 
a very similar condition at the base of the petiole in living cycads. 
Among the higher modern gymnosperms, the only occurrence 
of centripetal wood is in Ginkgo, where it is present as scattered 
tracheids, not in connection with the protoxylem, in the cotyledons, 
vegetative leaves, and anthers. This genus, which on other ey 
dence is accounted one of the most primitive of the existing 
gymnosperms, also possesses a double leaf bundle and a wide, 
parallel-veined, rather cordaitean lamina. : 
The occurrence of a double foliar strand, in the more ancient 
of the living conifers, seems without much question to be the per 
sistence of such a double trace as is found at the base of the leaf in 
the Cordaitales. The double foliar bundle found by Miss Tao 
(12) in the seedlings of many angiosperms is probably also an indi- 
cation of a primitive condition. The leaf trace in the adult mem 
bers of this group takes on a great variety of forms, and eee 
displays the true centripetal xylem or “cryptogamic wood” © 
the early gymnosperms. 
