422 Worsdell. — Origin and Meaning of Medullary 
the principal argument are omitted, while stress is laid upon other facts 
which most authors, from their standpoint, would probably deem unworthy 
of notice. 
Historical. 
Petersen, in a general paper on bicollateral bundles in various orders, 
describes a number of Compositae amongst the Cichoriaceae which exhibit 
intraxylary phloem, e.g. Sonchus , Lactuca , and Tragopogon , and mentions 
the presence of some xylem in connexion with this tissue. He gives a long 
list of Cichoriaceous genera in which he could find no trace of medullary 
phloem. Crepis, L contodon 3 Picridium , and Rodigia are included in this 
list, in all of which medullary phloem has since been found. 
Weiss, in the following year, 1883, contributed an important treatise on 
the subject. He seems to have been one of the first to show that the 
bundles of the main vascular system of the stem possessing internal phloem 
are not true bicollateral bundles in the original acceptation of that term. The 
result of his investigations is that the medullary phloem of the stem is in 
every case a leaf-trace. 
For instance, with reference to Scorzonera hispanica he states : ‘ that 
the phloem-bundles to the inside of the protoxylem cannot be regarded as 
an integral part of the bundle extending in the same radial line outwards is 
clear, for they arise later, and they are also the continuation in the pith of a leaf- 
trace occurring in the cylinder in the internode above the node concerned.’ 
With reference to the case of Lactuca saliva he held that the two smaller 
lateral phloem-strands occurring near each protoxylem-group of the bundle 
of the cylinder are the direct continuation of those occurring opposite 
a bundle in the leaf-base. Here the medullary phloem is directly a leaf- 
trace. But other phloem-strands pass into the pith from the ordinary 
external phloem of the cylinder, and as this phloem has entered the cylinder 
from a leaf at a higher node, such medullary phloem-strands may be 
regarded as indirect leaf-traces. 
Kruch’s lengthy thesis on the medullary bundles of Cichoriaceae affords 
us the results of by far the most thorough and complete investigation of the 
matter extant. In Part I he deals with the distribution and diffusion of the 
strands, whether they occur exclusively at the periphery of the pith, are 
partly peripheral and partly central, or scattered irregularly and uniformly 
throughout the pith. In Part II the course of the strands is dealt with ; the 
conclusion, after an intricate and painstaking study, being that, for the most 
part, the medullary strands of the stem are either the direct continuation of 
flower-traces or of the medullary strands of branches , as in Tragopogon . In 
a minority of instances they are leaf and cauline strands, as in Lactuca , 
where there is no medullary phloem in the branches (at least in their basal 
part) and that of the leaf- bundle enters the pith of the stem when the leaf- 
bundle joins the vascular ring (as seen in transverse section), while other 
