578 Worsdell .— The Origin and Meaning of Medullary 
cylinder occur great numbers of small phloem-strands ; of these some, 
viz. the most rudimentary, occurring immediately within the sclerotic ring, 
are those met with in this region in most Cucurbitaceae ; but others, repre¬ 
senting an extension of this system, are larger, of varying size, irregularly 
grouped, and occurring chiefly between the protrusions of the bundle-ring. 
Those, however, which approximate to the ends of the latter, and thus tend 
to form part of the bundle-ring, possess some xylem . It is thus evident that 
there are transitions between the tiny phloem-strands of the extreme outer 
edge of the cylinder and the vascular bundles of the ring. It was not ascer¬ 
tained whether the tiny outer vascular bundles unite with the bundles 
composing the ring-protrusions or whether they die out at a higher level; 
Fig. 2. Cucurbitafoetidissima. Transverse section of a small portion of the central cylinder 
of the stem, showing a transition between the bundles of the ring and the small phloem-strands ( ep ) 
of the extreme periphery of the cylinder; these latter representing vestiges of former vascular 
bundles, ip , ‘ internal phloem’ ; sr, sclerotic zone, x 27. 
this is immaterial ; it is certain, however, that the tiny phloem-strands 
do die out at a higher level. 
We may conclude that the tiny phloem-strands and bundles described 
represent the vestigial remains of one or more outermost series or rings 
of bundles. In other words, there are in this stem clear traces of an 
ancestral scattered bundle-system, such as occurs in Monocotyledons. The 
peculiar sinuous contour of the bundle-ring, so characteristic of the Cucurbi¬ 
taceae, can also be explained. It represents an attempt to condense all the 
rings or series into a single ring, the contour of which becomes more 
and more cylindrical and even and the individual bundles (or some of them) 
larger as the base of the stem is approached. The sclerotic ring, situated, 
