152 Worsdell. — The Structure and Origin of the Cycadaceae. 
dendron , &c., as the most primitive of the two types of petiolar bundle- 
systems, for the simple reason that it is the most Fern-like ; one has only 
to recall the single, concentrically-constructed petiolar bundles of such 
Ferns as Zygopteris , Osmund a, Ophioglossum , &c., to render this view 
reasonable. 
The fact that the leaf-trace of Medtdlosa, on its first leaving the 
vascular system of the stem, is a single concentric bundle which subse- 
quently becomes fragmented into a number of collateral bundles, each 
having precisely the structure of the petiolar bundles of modern Cycads, 
is further evidence as to the original ancestral character of the petiolar 
vascular system of these modern plants ; this is another good example of the 
first-formed tissues in the ontogeny exhibiting ancestral characters, for we 
must regard the lowest part of the leaf-trace as, ontogenetically, the most 
primitive. 
The Sphenopteroid foliage of Lyginodendron and Heterangium may 
also be said to be less Cycad-like 
than the Alethopteroid and Neuro- 
pteroid foliage of the Medulloseae. 
Another point is worth re- 
marking: there is always a ten- 
dency for the separate collateral 
bundles, which I regard as the 
individual units resulting from the 
above-described fragmentation, to 
become each one concentric in 
structure, as if partaking of the 
character of the parent-bundle in 
the stem from which they sprang. This is the probable cause of the arc- 
shaped phloem of all foliar bundles in Cycads. Hence the concentric 
bundles of the cotyledons of Stangeria and Bowenia, of many sporophylls 
and, in some cases, of the integuments. 
It is not unlikely that in the vascular systems of the petioles of the 
Cycadophyta 1 all stages and types occur between the single large con- 
centric bundle of Lyginodendron , &c., and the numerous collateral bundles 
of Myeloxylon , just as in the vascular system of the stems of the 
Medulloseae and Cycads the types range from the single solenostele 
of M. porosa , through the ripg of numerous concentric bundles or steles, 
to that of the numerous collateral strands of a modern Cycad. 
In the case of the sporophylls the primitive concentric structure 
tends wholly to disappear, as a rule, in those bundles which are functional 
1 I am here using this term, for the first time, in a new sense, viz. to include the Pteridosperms, 
the Mesozoic Cycadales, and modern Cycads ; whereas Nathorst’s use of the term included only the 
two latter groups ; the Cordaites are, for the time being, at any rate, left out of consideration. 
MJh 
^ ^ cy 
h 
Fig. 17. Zctmia Lindeni: diagrammatic trans- 
verse section of vascular systems of fertile (a) and 
sterile ( 6 ) sporophylls respectively, showing differ- 
ence in orientation and structure of the strands in 
the two systems. 
