Relation of Pteridosperm Anatomy to that of Cycads. 65 
intra-fascicular primary cylinder of bundles. These bundles are 
feebly-developed and are scattered and variously orientated, or 
concentrically grouped. Worsdell regards this inner series as the 
remnants of the inner cylinder of primary bundles in some 
Medulloseae. Similarly in the peduncle of Enceplialartos there are 
two primary cylinders, the outermost being divided into groups of 
concentrically-arranged bundles (Fig. 19). 
system of peduncle, showing outer primary ring consisting of groups of bundles. 
(From Worsdell, 1906, after Matte). 
Briefly considering the support for WorsdelPs theory derived 
from a study of the foliar organs :—A comparison of the petiolar 
structure of Enceplialartos and Myeloxylon shows that in the two 
types the structure, orientation and number of the vascular bundles 
are so similar as to indicate that the petiolar vascular systems of 
the recent and fossil forms are homologous. Further, Worsdell 
shows how a relation may exist between the vascular systems of 
the petioles of Lyginopteris and Heterangium on the one hand, and 
of Medulloseae or Cycads on the other. The (usually) single large 
mesarch bundle of Lyginopteris may represent “a number of 
collateral mesarch bundles united by means of their centripetal 
xylems,” their fused phloems surrounding the whole. If these 
bundles become separate, while retaining their original position, 
the grouping so characteristic of Cycad petioles, particularly of 
Stangeria, is obtained (Fig. 20). Myeloxylon and some species of 
Enceplialartos may represent extreme cases of sub-division of an 
original vascular strand. Cases of fusion in Cycad petiolar bundles 
may be regarded as a reversion to an ancestral condition. 
