Phloem in the Stems of Dicotyledons. II. 437 
behave as in stem 1. But in all higher branch-nodes no medullary strands 
exist at all. 
A feature of the stem of this species is the very narrow cortex, the 
wide pith, and the existence of minute bundles (sometimes reduced to phloem 
only) between, and slightly farther to the outside of, the large bundles of 
the ring (Fig. 12). These rudimentary bundles, which can hardly represent 
an adaptive, but rather a vestigial character, and their relative position 
furnish evidence of the primitive scattered arrangement of the bundles of 
the ring in this plant, it being merely a 
rather good example of what has happened 
in the stems of most other Compositae, 
viz. a pushing outwards of the larger, inner- 
most bundles towards the periphery, whereby 
the outer smaller ones inevitably become 
reduced and on the way to extinction owing 
to lack of adequate-space. 
The xylem of all the bundles in the 
nodal region is very V-shaped (Fig. 12); 
this means that each of these bundles per- 
tains partly to the pith and partly to the 
vascular ring, hence their semi-amphivasal 
structure ; in other words, that they are 
the constituents, in this conservative region, 
of an original scattered system of bundles. 
This is but typical of what occurs in 
most Compositae ( 1 'olpis barbata and Picris 
strigosa are merely cases of this in more primitive form). 
P. strigosa, Bieb. 
Stem. 
Herbarium material only was available of this species. The vascular 
ring, as seen in transverse section, consists of bundles with rather V-shaped 
xylem, with very numerous tiny ones between them at the same level. 
The medullary system of bundles is very striking and pronounced, consist- 
ing of a ring of large, perfectly circular amphivasal bundles with protoxylem 
in varying positions (Fig. 13). They occur at the periphery of the pith, 
and are connected with the ring by two or three bundles of an intermediate 
series. This species, therefore, shows the more primitive structure of 
the two. 
Leaf. 
In the typical part of the rachis (there is no petiole) of P. hieracioides 
there is an arc of alternately large and small bundles, some of the latter 
K k 
Fig. 12. Picris hieracioides. 
Vascular ling of stem, showing its 
vestigial outermost constituents 
{vvr) and two peripheral medullary 
bundles {mb), x 9. (In this figure 
the small vestigial strands have been 
placed slightly farther to the outside 
than they are in the actual specimen.) 
