656 WorsdelL — A Study of the Vascular System in 
form a compact mass at the centre of the stem ; Nymphaeaceae have 
in this sense retained their primitive structure in spite of the aquatic habit, 
although, had they become terrestrial in habit, and developed erect woody 
stems, they would almost certainly have lost it. In Berberidaceae, Caly- 
canthaceae, and Magnoliaceae, all of which have undoubtedly a primitive 
floral structure, the majority of plants are woody shrubs, and hence their 
primitive vascular structure has become largely obscured or lost, but the 
large, sheathing leaf-bases or stipular appendages of such plants as Mag- 
nolia and Berberis Aqidfolinm are an obvious reminiscence of their grandi- 
foliate origin, as is also the ranked arrangement of the bundles in the leaves 
of some genera. In the grandifoliate Berberidaceae, like Podophyllum , the 
ancient vascular structure is present in a most marked degree. The Com- 
positae and Umbelliferae include a great many plants of grandifoliate habit ; 
but, as their floral structure indicates, they are groups advanced in evolution 
i. e. in certain special directions ; hence we find that, unlike the Ranun- 
culaceae, their primitive grandifoliate vascular structure is undergoing a great 
amount of modification, the scattered system of the bundles, where it occurs, 
being always more or less of a rudimentary nature and showing indications 
of extinction. 
In some plants, where the grandifoliate vascular structure has vanished 
from the internodes, it is still preserved at the nodes which are always the 
least modified portions of the shoot. 
Another interesting point is this : that in the bundles of the scattered 
medullary system the xylem always tends to surround the phloem. And 
this amphivasal bundle fs precisely that which is characteristic of Mono- 
cotyledonous stems, although there, as in the Dicotyledonous stems I am 
now referring to, the amphivasal character is very often incomplete, the 
xylem failing to completely enclose the phloem and thereby exhibiting 
a V-shaped structure. The amphivasal character of the bundle in these 
stems is the more complete the more independent the latter is of the 
vascular ring ; the nearer the medullary bundle approaches the ring, the 
more collateral and also the more regularly-orientated does it become ; 
the bundles constituting the ring merely represent the outermost members 
of the scattered bundle-system marshalled and arrayed in line for the 
purpose of forming a cylindrical structure which shall be efficacious in 
resisting bending-strains in the elongating stem ; every transition can, in 
many instances, be seen between the collaterally-constructed members 
of the ring and the most central and independent of the amphivasal medul- 
lary strands. The presence of a cambium, or (as happens in many cases, as 
where the ‘ internal phloem ’-group is without xylem) the rudiments of 
such, is a very important feature of the medullary bundles in these Dico- 
tyledons. It indicates that the latter are descended from ancestors the 
scattered bundles of whose stems possessed secondary thickening to 
