178 Blackburn. — On the Vascular Anatomy of 
strands, whereas in the tree forms, although reduced to three at the point of 
insertion, there is often a larger number arranged in a ring at the base of 
the petiole. The plumular strands become reduced to six at the cotyle- 
donarynode. Each cotyledon supplies one leaf trace, which may be double, 
and a diarch root structure is formed quite high in the hypocotyl, whereas 
in the arborescent forms the hypocotyledonary vascular arrangement is 
frequently like that of the stem. 
Apart from minor deviations from type, there are three main groups of 
exceptions to the general structural ground plan for the seedlings of the 
cohort. 
1. Phyllotaxy. In Clematis the leaves are opposite and decussate in 
the adult, but it has been shown that the two-fifths spiral in the seedlings 
of C. Vitalba and other species changes ontogenetically to the opposite 
arrangement. This, taken in conjunction with the strong likeness between 
seedlings of this species and those of other plants of the family, is strong 
evidence in favour of the primitive nature of the spiral arrangement. 
Calycanthus also has opposite leaves, but Mr. Worsdell has pointed out 
that this is probably derived from a two-fifths phyllotaxy, and the likeness 
between the two cases is strong support for either contention. It would 
seem probable that further data might provide evidence for the derivation 
of the one-half divergence of Anona and P odophyllum from the two-fifths 
condition. 
2. Leaf Trace. The most notable exception to the general arrange- 
ment is found in Lauras , in which, though the material was inadequate, the 
insertion appeared to be unilacunar in the seedling, as in the adult. Paeonia 
is a somewhat curious case in which the first foliage leaf contributes, in 
some species, five strands to the vascular cylinder, while the later ones have 
the usual trilacunar insertion. In some cases the first one or two leaves of 
species belonging to Ranunculaceae contribute a reduced number of strands 
(one or two) either by the approximation of the strands at the point of 
entry, or by loss of one or both laterals. This seems clearly correlated 
with marked reduction in size in the plumular leaves concerned, and may 
throw some light on the very frequent absence of laterals at the base of the 
cotyledonary leaves. 
3. The Stem Cylinder. The main departure from the type is found in the 
genus Ranunculus (also Trollius y Caltha , and, according to Professor Jeffrey, 
Anemone pennsylvanica :), in which there is an internal endodermis and an 
appearance of a tubular stele. The latter has been shown to be a secondary 
formation correlated with the slight production of primary xylem found also 
in Aconitum y but the siphonostelic appearance in Ranunculus is due to the 
absence of the principal medullary rays found in that genus. 
The presence of the internal endodermis in some species of Ranunculus 
is more difficult to explain. At the age examined, the endodermis is not 
