1 5 ° Worsdell . — The Structure and Origin of the Cycadaceae. 
outer part of the cortex one or two of the bundles supplying the barren 
sporophylls possessed three distinct groups of xylem ; this is probably 
a relic of a similar tripartite bundle as that above described and as 
is also found in Medtdlosa. Quite frequently a perfectly concentric 
sporophyll-trace occurs in the neighbourhood of other collateral traces 
and too far out in the cortex to belong to the cauline system of strands 
(Fig. 14). Here, then, we see some last obscure remnants of the ancestral 
leaf-trace bundles in precisely that part of the plant where we should most 
expect to find them. 
In Cycads the leaf-traces leave the cylinder, as a rule, in groups 
of two 1 ; in the peduncle of Enccphalartos villosus Matte observed groups 
of three to five leaf-trace bundles passing out. The bundles have very often 
their xylems slightly directed 
towards each other . Each 
group of one, two, or more 
may be regarded as the equi- 
valent of the single concentric 
leaf-trace of Medidlosa ; in the 
case of Cycads fragmentation 
and reduction of the leaf-trace 
has taken place congenitally 
(i. e. before the trace left the 
cylinder) ; in the case of Me- 
dullosa it has occurred post- 
genitally (i. e. since leaving the 
cylinder). 
The bundles in the petiole 
of, e. g. Encephalartos , are pre- 
cisely identical, both as regards orientation, number, and individual struc- 
ture, with those of Myeloxylon. The similarity, indeed, is so close as to 
render an homology between the two systems of bundles practically 
certain. If this be admitted, it must then of necessity follow (a con- 
clusion which appears hitherto to have been completely overlooked) that 
the vascular systems of the stems of Medidlosa and Cycads respectively 
must also be homologous. And if this be so, what becomes of the theory 
which regards the Cycadean central cylinder as having been derived from 
the single Heterangium stele ? 
At first sight it would seem as if no relation whatever existed between 
the vascular structure of the leaf-petiole of Lyginodendron or Heterangium 
and that of the same organ in Medidlosa and the Cycads. The single 2 
1 In the case of the male sporophyll a single bundle leaves the cone-cylinder which subsequently 
in the cortex branches into two. 
2 In some cases there are two bundles. 
Fig. 15. Diagrammatic representation of method of 
derivation of the numerous, separate, collateral bundles 
of petiole of Cycads from the single, large, concentric 
bundle of petiole of Lyginodendron , &c. (transverse 
section). 
