588 Worsdell.— The Origin and Meaning of Medullary 
7. The fact that the xylem of the medullary bundle unites with or 
becomes attached to the internal-phloem strand, and not with the xylem of 
the corresponding bundle of the cylinder, proves that the internal-phloem 
strand represents a distinct vascular bundle of the medullary system which 
has lost its xylem. No case is known in any plant of a bundle fusing with 
the phloem only of a collateral bundle. 
8. In some cases, as in Citrullus ecirrhosus , where the internal phloem 
of the stem is normal, that of the fruiting peduncle has xylem attached 
to its outer side. 
9. In this case a physiological need has been the stimulus evoking 
what is regarded as a reversion to an ancestral structure. 
10. I11 the peduncle of Acanthosicyos the xylem of the internal phloem 
of the outer series of bundles is very greatly developed, much surpassing 
that of the inner series. In the twig-like stipule of this plant the 
internal phloem of the outer bundles is clearly seen to be a distinct and 
individualized bundle, for the reasons above given. 
11. The imperfect or rudimentary structure of the intrapericyclic and 
medullary phloem-strands and of the medullary bundles shows them all to 
be ancestral vestiges and not structures in the course of evolution ; for such 
imperfectly functional structures would not have been evolved and preserved. 
Hence the view that the cylinder has been derived from a former scattered 
system of bundles is correct. 
J 2 . The internal-phloem strands, although, as the writer considers, 
vestigial, are as well developed and functional as the phloem of the ring- 
bundles, owing to their having been retained as a useful and necessary 
adjunct to the conducting system. This has led a few authors to suppose 
that they represent a new product of evolution. 
13. The fact that the internal phloem arises in the course of ontogeny, 
at a later period (as a general rule) than does the phloem of the ring- 
bundles, is in favour of its being vestigial. 
14. In the only cases investigated for this purpose, viz. Lagenaria , 
Citrullus, and Cucurbita , it was found that during the ontogeny of the 
stem, viz. in the hypocotyl, the internal-phloem strand arises (as a whole in 
the case of the first two genera, in part in the case of Cucurbitai) from 
the phloem of the ring-bu 7 idle . This, of course, is the natural and only 
mode of origin in view of the fact that the internal phloem is still fully 
functional and well developed. 
15. The internal-phloem strand (at any rate in the cases of Lagenaria 
and Citmdlus ), as it passes inwards from the phloem of the ring-bundle, 
revolves on its axis through an angle of 180°. 
1 6. This affords the ontogenetic origin of the inversely-orientated 
internal-phloem bundle. 
17. The morphological origin of this internal-phloem bundle is from 
