438 Worsdell. — Origin and Meaning of Medullary 
being reduced to phloem. As in the case of Crepis , the median bundle 
abstricts off a small phloem-strand on either side. There are no central 
medullary bundles or phloem-strands. But the small bundles or phloem- 
strands alternating with the larger bundles of the arc really represent the 
outermost series of a medullary system, as shown by their situation slightly 
nearer the ventral side than the large bundles and also close to the edge 
of the lacuna. But these last, again, represent the outermost (dorsal) of the 
whole system of scattered bundles which once existed in the leaf of all 
Compositae. Ihese remarks also apply to the structure of the petiole of 
Crepis and I^actuca. 
Helminthia echioides, Gaertn. 
Stem i (Wild). 
The characteristic feature of this plant is the large number of very 
small phloem-strands which are scattered uniformly throughout the pith 
(Fig. 14). No instance was seen of the occurrence of any xylem in these 
strands, and they are of very uniform size. A few arise de novo in the pith 
Fig. 13. Picris strigosa. Vascular ring 
and medullary bundles ( mb ). x 10. 
Fig. 14. Helminthia echioides. Segment 
of cylinder of stem, showing vascular ring and 
the scattered medullary phloem-strands {mps). 
x 12. 
of the extreme thickened base of the stem, where they branch up rapidly 
to form the large number seen at a higher level in the thinner aerial 
portion. They have connexions with the phloem of the ring through the 
leaf-trace rays. 
The medullary phloem-strands are entirely devoid of hard bast. 
At the node, in the bay formed by an outgoing branch, the medullary 
strands increase in size ; some of them here join the phloem of the stem- 
ring, others pass out and become the medullary strands of the branch. 
Here we see that where a medullary system is well developed in the stem 
it is continuous with a similarly well-developed system in the branch. 
In three cultivated stems, which are more robust than those of the 
wild plant, the structure is precisely the same throughout. 
In the peduncle the medullary phloem-strands are very minute and 
rudimentary and sometimes completely absent. 
