568 WorsdelL — The Origin and Meaning of Medullary 
been hinted at by a few writers in the past, but no serious attempt ever 
seems to have been made to establish the theory, or to view this peculiar 
anatomical feature in its true light. That the surest way of determining 
the nature of a structure or organ is to trace its phylogenetic origin, hardly 
needs demonstration. 
In the case of medullary phloem an attempt to do so will be made in 
a few natural orders where it is not hopelessly difficult. In others, however, 
the structure has become so stereotyped and the intermediate stages in 
its evolution so utterly extinct that the task is impossible. 
As the first of a series of concrete illustrations of the above thesis, 
the order Cucurbitaceae will be taken. 
Historical. 
The following notices of work on this subject represent the more 
salient and relevant points brought out by the various authors: 
Gerard concludes, from a study of the transition from root to stem in 
Cucumis Melo and Cucurbita maxima , that the internal phloem is a part 
of the external which becomes situated on the inner face of the bundle. 
Petersen, in a general paper on the occurrence of bicollateral bundles, 
states that ‘ while the outer soft bast always forms an integral part of 
the bundle, this is not in the same degree the case with the inner soft bast \ 
He refers to a continuous series of phenomena, ranging from a ring 
of bicollateral bundles to a ring of normal bundles with a whole system of 
medullary bundles within. He found in the creeping stem of Alsomitra 
sarcophylla that the four larger bundles of the stem almost meet in the 
centre, the pith being crushed, and the intraxylary phloem replaced by 
cambiform tissue. 
Van Tieghem found in the roots of Cucurbita with a large pith, 
especially the large adventitious ones, that on the medullary side of each 
primary and secondary collateral bundle one or several longitudinal series of 
pith-cells divide actively to form a phloem-bundle. 
Weiss determined that all the bundles in Cucurbitaceae are leaf-traces. 
The phloem-bundles scattered in the pith of the stem of Cucumis perennis 
are branches from the phloem-bundles of the leaf-traces which have passed 
into the outer ring. In the petioles, where the bundles are arranged in 
a half-moon shape, at the point where the leaf-veins branch off from the 
main trace, the internal unites with the outer phloem, so that the smaller 
bundles are no longer bicollateral. 
Fischer, in a paper on the sieve-tube system of this order, traced, 
in Cticurbita Pepo> the transition from hypocotyl to root structure, and 
found that the medullary phloem gradually died out, ending blindly below. 
In the female peduncle the small collateral and the phloem-strands, which 
are associated with the bundles of the inner of the two rings, frequently 
