262 BOTANICAL GAZETTE [APRIL 
primitive portions of their leaf bundles to conditions which prevailed 
in the foliar strands of the early Botryopterideae, and it seems 
reasonable to suppose that this resemblance indicates a phylo- 
genetic relationship between the two groups. These cycad-ferns, 
however, show their advanced condition by the possession of a 
much more complicated petiolar system, and by displaying the 
tendency shown by modern ferns for the protoxylem to become 
continuous with the centrifugal wood and to be at last completely 
endarch. 
The remaining Cycadofilices show a very different structure 
in their leaf bundles. Here the progression, instead of being 
toward endarchy, is all toward exarchy, and the primitive mesarch 
trace with protoxylem near the outside loses its narrow zone of 
centrifugal wood, and its protoxylem becomes continuous with 
the centripetal wood, thus forming a direct contrast to the trace of 
Lyginodendron. The Myeloxylon type of bundle characteristic 
of the petioles of Medullosa and Sutcliffia shows clearly this exarch 
condition, though Colpoxylon, which often possesses only a single 
vascular cylinder in its stem and which is therefore probably more 
primitive than the other two genera, shows traces of centrifugal 
wood and the ancient mesarch structure. In all these genera 
the petiolar vascular system consists of a large number of bundles 
more or less irregularly arranged, but, in M edullosa at least, the 
base of the leaf trace is a single strand possessing one or more groups 
of protoxylem which seem to be slightly imbedded in the metaxy lem. 
In Megaloxylon the base of the leaf trace is the only part of che 
foliar strand of which the structure is known, and it is here a single 
exarch bundle. 
Ptychoxylon Levyi, one of the Cycadoxyleae, shows a closer 
approach to the primitive Cycadofilices, for here the trace 1S j 
double one, each part of which, however, instead of being mesare 
is clearly exarch. oo q 
In those primitive gymnosperms of the Palaeozoic include 
in the cordaitean alliance, we find the same exarch structure ™ 
the leaf bundle, which almost always consists at its base of ae ; 
strands. In Poroxylon this pair of monarch bundles, which 
mately unite at their very base, are carried well down into the steer 
