582 Worsdell.—Tke Origin and Meaning of Medullary 
As the basal portion of the peduncle is approached the xylem of 
the internal amphivasal bundles gradually becomes extinct. The inner 
xylem, where it is present, disappears first, to be followed eventually by 
the outer xylem, until, in the narrow basal part, the structure is pre¬ 
cisely that of the vegetative stem, with internal phloem only. 
At the node^ representing the point of origin of the peduncle, some 
of the medullary bundles have well-developed outer xylem, 
The occurrence of complete vascular bundles, with inverse orientation, 
in the place of the internal-phloem strands, in the upper part of the fruiting 
peduncle is probably an adaptation to meet the structural requirements 
arising from the attachment of a large fruit. * As the xylem of the internal 
bundle consists in its major portion of fibres, and exhibits very few vessels, 
the function is doubtless mainly a mechanical one, viz. to meet and resist 
tension strains, which would be severest in that part of the peduncle. 
Now, if the internal phloem of the stem and lower part of the peduncle 
represents, on the ordinary academic view, an indissoluble constituent of the 
bundle of the cylinder, of no particular morphological value, the intercala¬ 
tion of xylem between this internal phloem and the xylem of the ring- 
bundle would appear strange and anomalous and no meaning could be 
attached to it, unless the internal phloem is to be regarded as the last 
vestige of a phloem which entirely surrounded the xylem in an original 
concentric bundle. There is, however, no evidence that the bundles of the 
axial organs of Cucurbitaceae were originally amphiphloic in structure. 
If, on the other hand, the internal phloem represents the vestige of an 
original medullary amphivasal bundle, then the reappearance of its xylem 
for the purposes above described is easily understood, for the new mechani¬ 
cal elements are laid down in the place, so to speak, of least resistance, i.e. 
where they formerly existed in the axis of the ancestor, the reversion to the 
primitive condition being easily invoked by the stimulus of the tension-strain. 
As has been mentioned above, the tendency to form large concentric 
amphiphloic bundles does occur, but this is not to be regarded as having 
a reversionary significance, for it is purely adaptational. 
Ecballium Elaterium. 
Stem. 
In the lower part the internal phloem is replaced by large complete 
bundles separated some little distance from the protoxylem of the cylinder 
by ground tissue ; its cambium has formed a very large amount of paren¬ 
chymatous tissue, mostly pertaining to the xylem, amongst which in every 
bundle are from one to several vessels or fibres situated on the outer side 
of the phloem-strand (cf. Fig. 9). 
