(. Tntraxylary ) Phloem in the Stems of Dicotyledons . /. 589 
an amphivasal bundle, for this latter is the typical and primitive con¬ 
dition of medullary bundles wherever they may occur (Fig. 10). 
18. Owing to the fact that some of the outermost amphivasal 
medullary bundles have become approximated to the bundles of the ring 
to form would-be constituents of this latter, only that portion of the 
xylem of the original amphivasal bundle has been retained which is the 
most mechanically serviceable for the ring as a whole, viz. the outer 
portion, or that which is approximated to the xylem of the ring-bundle. 
The inversely-orientated internal-phloem bundle is thus explained. 
19. In the vast majority of cases the xylem which occurs on the 
outer side, or other parts of the periphery, of the internal phloem is 
secondary in origin, i. e. is derived from a cambium. In a few cases, as 
in that of the outer series of bundles in the stipule and stem of Acantho- 
sicyos , it appears to be, at least part of it, primary. The mode of origin 
of the xylem, whether primary or secondary, is, however, a matter of purely 
ontogenetic interest; it cannot affect the morphological question as to the 
origin of the internal phloem, and is, in this connexion, of no importance. 
20. This fact of the existence of vascular bundles replacing the internal 
phloem in the stem and peduncle proves that the ‘ bicollateral ’ bundle has 
no existence in the morphological sense, but is a purely descriptive term. 
21. The £ bicollateral ’ bundle of the Cucurbitaceae is a compound 
structure consisting of the more or less intimate association or attach¬ 
ment of two distinct vascular bundles, of which the innermost has lost 
its xylem. 
22. The collateral bundles composing the two rings or series of the 
cylinder in this order also, in the writer’s opinion, represent reduced amphi¬ 
vasal bundles. In some plants they are very V-shaped, with the phloem 
situated between the arms of the xylem. In this feature and in that of the 
large size of the vessels the bundle is sometimes an exact replica of that of 
some Monocotyledons. 
23. In most Cucurbitaceous stems there may be observed in the region 
outside the zone of the two main series of cylinder bundles, but within the 
sclerotic ring, a few extremely rudimentary phloem-strands. In Cucurbita 
foetidissima evident transitions between these and the bundles of the 
cylinder exist in the form of intermediately-situated vascular bundles. 
24. This last fact shows that the external rudimentary strands repre¬ 
sent the vestiges of a former outermost series of bundles of the cylinder. 
25. The existence of the rudimentary phloem-strands just mentioned 
representing reduced vascular bundles helps, in some degree, to render 
plausible the view that the internal-phloem strands also represent reduced 
vascular bundles. 
26. The sclerotic ring, broken up in some cases into isolated strands, 
marks the limit, as in Monocotyledons, of the central cylinder. 
