676 Worsdell. — A Study of the Vascular System hi 
of all from the stem and is replaced by a hollow cylinder of bundles 
arranged in a single ring or rank ; later on this also vanishes from the leaf 
leaving a similar hollow cylinder, which in its turn may become reduced to 
a mere dorsal arc of bundles. In the other orders the medullary system of 
bundles has largely disappeared ; yet we see most evident relics of it in the 
peduncles of certain Anonaceae and distinct traces yet remaining in the 
petioles of some Magnoliaceae and Berberidaceae. In these two latter 
orders we are able beautifully to trace the steps by which the complete 
cylinder of the petiole has degenerated into a dorsal arc of strands, in some 
genera all signs of the former having completely disappeared (Text-fig. 2). 
Text-fig. 2. — Schematic representation of the stages in the evolution of the petiolar vascular 
structure in Dicotyledons A-C will also apply to the stem. A is not quite the earliest stage. 
Numerous variants of this typical general scheme occur. 
The undeveloped, imperfect structure of the medullary bundle system, 
as also, so often, of the ventral portion of the petiole-cylinder, as compared 
respectively with that of the vascular ring and the dorsal part of the 
petiole-cylinder, is an index to their vestigial 1 character ; and this is the 
1 If the ventral portion of the cylinder were not an ancestral character, due to the essentially 
