664 Worsdell. — A Study of the Vascular System in 
which become inversely orientated, are separated off (Figs. 21, 22) ; some 
of these die out below — a fact, coupled with that of their minute and insig- 
nificant size, which proves their vestigial ancestral character ; others pass in 
and unite with the stem-cylinder. At about this time also the large median 
bundle of the petiole unites with the stem-cylinder, followed shortly after 
by the two which are second in number from each end of the arc. The two 
large concentric bundles are the last to join the stem-cylinder, for before 
doing so they pass a considerable distance down the internode, forming the 
characteristic cortical bundles of this plant. 
My interpretation of the above structure is this : that the extreme 
basal portion of the petiolar bundle-system containing the concentric 
bundles 1 and the minute inverted bundles alone represent the primitive 
cylindrical structure. The cylindrical structure occurring in the upper part 
of the petiole, which is formed by incurving of the ends of the dorsal 
arc of bundles, I regard as a secondary phenomenon. If our knowledge of 
events was confined to Paeoniaceae alone, we should never be able to 
recognize in the concentric structures the passage from the primitive 
cylinder to the dorsal arc of bundles ; it is only by comparison with similar 
structures in the closely allied order Magnoliaceae and the allied genus 
Helleborus (see above) where transitional stages are available for our obser- 
vation, that we are able to interpret the facts in Paeoniaceae in the same 
way as I have done in the case of the concentric petiolar bundles of 
Helleborus . It is certainly interesting to find that the cylindrical structure 
of the upper or typical portion of the petiole of Paeonia is not strictly 
homologous with that of the petiole in Ranunculaceae, Magnoliaceae, and 
Anonaceae, but is secondarily derived. However, another mode of inter- 
preting this structure may be as follows : that the same primitive cylindric 
conformation constituting the extreme base of the petiolar system, viz. in the 
stem-cortex, after completely disappearing for a time as we pass upwards, 
the dorsal arc of bundles only existing there, reasserts itself in the upper or 
typical region of the petiole ; or that the primitive structure of the upper 
part of the petiole reasserts itself in the cortex of the stem. The com- 
paratively narrow free base of the petiole, in the great majority of plants, 
seems to show the mechanical necessity of the presence there of a dorsal 
arc of collateral bundles ; in the cortex of the stem, in the region where the 
leaf-base forms part of the latter, this mechanical necessity does not, 
probably, arise ; hence the partial assertion there of the primitive cylindric 
structure ; the reassertion of this primitive cylindric structure occurs in the 
upper or typical part of the petiole ; but it is there brought about by a 
somewhat different constructive agency than is the case with that part of 
the system which is in the stem ; yet this cannot prevent our regarding the 
1 Paeonia shows its alliance with Calycanthaceae in the fact that the ventral portion of the 
primitive cylinder does not occur in the leaf proper but only in the cortex of the stem (see infra). 
