■ in the Root and Stem of Dicotyledons . 279 
with phloem-islands, which collectively far exceed in area the 
normal phloem. Of these interxylary strands the more external 
lie entirely in the secondary wood. Others, however, occur 
on a level with the primary xylem, and in many roots, 
though not in all, there is a central phloem-strand occupying 
the small pith-area. When this is absent the xylem-groups 
meet in the middle. 
Tracing the development, we find that the bundle-system is 
polyarch, the number of xylem- and phloem-bundles variable, 
often as many as ten of each. 
For a time the protoxylem-groups alone represent the wood, 
often with one vessel only to each bundle, the protophloem 
appearing between them as usual. Later on the centripetal 
development of the primary xylem goes on, but its inner and 
later-formed portion is as a rule separated from the protoxy- 
lem by a few cells, which remain parenchymatous. The 
further development of the primary xylem is accompanied by 
the formation of phloem-strands among the lignified cells, so 
that the more internal phloem is of primary origin. The 
strands situated on the margin of the primary xylem may 
subsequently receive additional elements from the cambium 
(Fig. 10, pA 3 ). The root shown in Fig. 9 has an approxi- 
mately central phloem-group {ph 2 ), the origin of which is 
evidently altogether primary. 
The secondary thickening of the root presents a striking 
peculiarity, which has not, so far as we are aware, been noticed 
elsewhere. In ordinary dicotyledonous roots, as has long 
been known, the cambium first arises within the primary 
phloem, by divisions of the cells of the conjunctive parenchy- 
ma. It then extends round the exterior of the xylem-plates, 
the pericyclic cells dividing to complete the cambium outside 
the protoxylem. In Chironia however, the cambium is, as a 
rule, formed inside the protoxylem-groups, which are thus for 
a time completely cut off from the rest of the wood. This, 
then, is the function of the parenchyma, which remains un- 
lignified between the protoxylem and the more internal wood. 
It divides to complete the cambial ring (see Fig. 10, px). 
x 
