Igtt] SINNOTT—FOLIAR BUNDLE 263 
the primary wood of which is thus entirely centripetal. In the 
petiole and blade of the leaf, the double trace divides into a series of 
parallel strands, each of which possesses a mass of exarch primary 
wood, on the outer side of which, except at the edges of the leaf, is 
a zone of secondary xylem, just as in the leaves of living cycads. 
In Mesoxylon, a new genus recently established by Scott and 
MASLEN (7) to include forms intermediate between Poroxylon 
and the Cordaiteae, we see in the stem the beginnings of the endarch 
condition of the higher gymnosperms. The double leaf trace is 
still exarch, the protoxylem being in seriation with the centripetal 
wood which, in the very lowest part of its course, in several species, 
becomes much reduced and may finally disappear, leaving as the 
only relic of the primary wood the cluster of protoxylem, which 
is separated by parenchyma from the centrifugal secondary xylem. 
The structure of the leaf trace and foliar bundles in this genus 
is the same as in Poroxylon. 
In the case of Cordaites, however, the typical stem structure 
of the modern gymnosperms makes its appearance, for here the 
protoxylem attaches itself to the centrifugal wood, which is almost, 
if not quite, entirely secondary. In the double leaf trace, however, 
well developed centripetal primary wood appears, with exarch 
Protoxylem on its outer face. In the leaf blade the veins are clearly 
exarch. In certain species centrifugal xylem is present here, 
separated by parenchyma from the protoxylem and probably 
Secondary in origin, but this is absent in the simpler forms such as 
C. principalis, where the structure is strikingly like that of a 
Medullosa bundle. 
There are certain other fossil plants, probably rather closely 
related to Poroxylon and the Cordaiteae, which possess a somewhat 
similar type of leaf trace and primary wood, and which have been 
8rouped by Scorr under the family name of Pityeae. 
In the genus Pitys the trace shows signs of bifurcation as it 
Passes out, and it is clearly mesarch, as are the medullary strands 
of primary xylem. The bundles are all small, however, and the 
Protoxylem shows no tendency to align itself with either the centrif- 
ugal or centripetal wood, showing in this respect a primitive con- 
dition, 
