430 Worsdell— Origin and Meaning of Medullary 
Tolpis barbata, Gaertn. 
Stem. 
There are two kinds of medullary bundles occurring as an invariable 
feature of this plant. Minute bundles and phloem-strands arise de novo in 
each internode opposite the bundle of the ring which eventually passes out 
as the median bundle of the leaf-system ; they form an arc around its xylem 
(Fig. 4, ms). After the leaf-bundle has passed out, they unite with the ring 
on each side of the gap. 
At each node, at the mouth of the bay formed by an outgoing branch, 
similar medullary strands arise de novo and, after the exit of the branch, die 
out in situ , though one or two may join the ring of the stem. 
In another stem, in one of the shortest, lowest internodes, eight or ten 
small, variously-orientated, amphivasal bundles arise de 7 iovo all round the 
periphery of the pith ; in succeeding internodes above they gradually 
die out. 
Here also the same small medullary strands arise opposite the outgoing 
median leaf-bundle. 
Besides the above-mentioned medullary strands, another set, of quite 
different origin, sometimes occur in certain internodes. These are well- 
developed bundles of the ring which pass into the pith and become amphi- 
vasal (Fig. 4, mb). More often, they are seen as constituent parts of the 
ring projecting into the pith. In fact, this plant is one of the best for 
showing how the vascular ring of the stem of Compositae has been derived 
from an original scattered system of bundles. 
Leaf. 
As the three main leaf-bundles pass outwards into the cortex they 
each acquire on their ventral (inner) side a number of small inverted bundles ; 
these are derived from the stem-ring ; they persist upwards into the leaf, 
and represent the peripheral medullary system of strands of that organ. 
Lactuca virosa, L. 
Stem. 
Medullary strands occur throughout the stem. In the higher part of 
this organ phloem-strands only occur (Fig. 6). In the basal region they 
acquire xylem and become bundles (Fig. 5). In this basal region, at a still 
lower level, they lose the xylem, and eventually either die out in sitii or 
join the vascular ring. The amount of xylem in the medullary strands 
varies with the thickness of the stem, and is always secondary in origin. 
The fate of the strands, if traced upwards, is fourfold. Some of them end 
blindly in the pith. Some pass into the vascular ring. Others pass out to 
form the medullary system of a branch ; but in one and the same plant 
