6 58 Worsdell. — A Study of the Vascular System in 
part of the axis of the plant, and hence that part where the primitive 
vascular anatomy will be most likely to portray itself. Here we find the 
Monocotyledon-like, scattered system of bundles well shown. There are 
3-4 ranks of bundles, though some irregularity obtains here and there. 
The largest and earliest-formed bundles are innermost, the bundles 
diminishing in size and development of their tissues from the centre of the 
axis outwards. Each bundle possesses a circular primary phloem-group 
situated in a fork of the xylem. All the bundles are leaf-traces. A pith 
is present, the diameter of which is about equal to the radial distance from 
the protoxylem of the innermost bundles to the epidermis. There is no 
sclerotic zone present in the stem. 
The peduncle also possesses the Monocotyledonous medullary bundle- 
system, being a stoutly-developed organ. 
If this plant has been derived by reduction from an aerophytic ancestor 
with an elongated woody stem containing a vascular cylinder with the 
bundles compactly grouped in one rank, one would think that the elongated 
leafless peduncle of this species is just the kind of organ in which such a 
one-ranked vascular cylinder would have been retained, unless one admits 
the somewhat forced idea that this peduncle has been redeveloped out of a 
more humble organ ; but if this latter is the case how is it that the 
aerophytic or parvifoliate structure has not yet been acquired. 
The flowering stem of A. rivularis , Buch.-Ham. also shows beautifully 
the grandifoliate or Monocotyledonous structure. 
Small forms like A. nemorosa L. and A. apennina L. have probably 
been reduced from larger ones like A. japonica . There is anatomical 
evidence of this ; for in the petioles of these species a remnant of the 
medullary bundle-system occurs in the form of a few small rounded phloem- 
groups. And in the bract-node of the stem of A. nemorosa I observed four 
very small bundles pass into the pith, there assume an inverted orientation 
(two of them having completely lost their xylem) and again pass outwards 
into the ring. 
Thalictrum flavuni L. 
This plant affords an excellent illustration of the mode of transition in 
both leaf and stem-structure from the ‘Monocotyledonous’ to the ‘Dicotyle- 
donous ’ type. 
Leaf : In the upper part of the petiole a complete cylinder of bundles 
obtains, the latter, although of three distinct sizes or primitive ranks, being 
all in one (rather sinuous) ring ; this is owing to the excessively wide hollow 
pith (PI. XXXII, Fig. 8). Lower down, two ventral, rather filamentous wings 
begin to form, and two smaller bundles pass out of the ring at this point, 
quite on to the dorsal side of the larger bundles there situated. As the wings 
enlarge, small bundles, presumably arising from the division of the just- 
