432 Worsdell. — Origin and Meaning of Medullary 
In the leaf of L. virosa there is a simple dorsal arc of bundles ; but 
vestiges, in the form of minute bundles, of the adaxial portion may occur. 
The medullary system consists of small phloem-strands occurring on the 
immediate inner side of the dorsal bundles ; they arise both from these 
latter and de novo in the pith. 
In L . Scariola medullary phloem has vanished from the leaf. In 
L. macrophylla the typical region of the petiole (about half-way up) has 
a complete vascular ring, some of the adaxial bundles of which are very 
minute and end blindly below , while the rest unite with those on the dorsal 
(abaxial) side. The same thing happens in the upper region of the petiole. 
Thus, we see in one and the same organ the evolution of the more recent 
simple arc of bundles (in other species characteristic of the entire petiole) 
from the more primitive ring (characteristic of the entire petiole of still 
other species). In one section a minute rudimentary medullary strand was 
observed. In those species in whose stems medullary strands are either 
absent or almost absent, they often occur, in the form of small phloem- 
strands, on the ventral side of the leaf-bundles in the cortex in or near 
a node. It is important to note that that segment of the stem-cortex in which 
bundles occur which have passed in from a leaf, the place of insertion of 
which latter may be a considerable vertical distance above, must be regarded 
as constituting part of the leaf, viz. its decurrent base. Further, the small 
phloem-strands occurring on the ventral (inner) side of leaf-bundles in 
this region are to be regarded as belonging to the peripheral medullary 
system of the leaf. In some species, as in L . perennis, these small phloem- 
strands occur on the inner side of the median leaf-trace bundle while it still 
forms part of the stem-cylinder ; they end blindly in the pith-tissue if 
traced downwards. These strands in that species do not pass out into the 
cortex with the median leaf-bundle, but they fill up the gap in the outer 
portion of the vascular ring of the axillary branch caused by the median 
leaf-bundle passing outwards from it. Much the same sort of thing was 
observed in Z. macrantha . but, at least in one node, the median leaf-bundle 
was devoid of internal phloem at every stage. In all these species the 
lateral leaf-bundles also have internal-phloem strands which are derived 
from the ordinary external phloem of the stem-ring . 1 Before the bundles 
pass out into the free petiole the internal phloem fuses completely with the 
external phloem of the bundle. 
This nodal region of the stem, where the leaf-bundles are passing in 
towards the ring, is a conservative region where ancestral characters would 
be likely to persist. 
The presence of phloem-strands on the ventral side of the median 
leaf-bundle while it still constitutes part of the stem-ring is a fact of the 
1 In L. Bourgaei it was noted that the more lateral of the internal-phloem strands of the median 
leaf-bundle have the same origin. 
