in the Root and Stem of Dicotyledons . 267 
(10-12) of phloem groups. Some of these are placed 
opposite the primary xylem of the bundles, but on the 
whole there is little regularity. Their distance from the 
protoxylem-elements also varies, some of the groups lying 
near the centre of the pith. It is noticeable, that in the 
hypocotyl the number of internal still exceeds that of ex- 
ternal phloem-groups, the dimensions of each group being 
about equal. 
On tracing the hypocotyl downwards to the tap-root, the 
changes which we find in the position of the tissues are as fol- 
lows 1 : — The pith gradually thins out ; the two lateral bundles 
disappear, becoming confluent with those of the cotyledons. 
The primary xylem-groups of each cotyledonary pair ap- 
proach each other and ultimately unite, turning their proto- 
xylem outwards. In the transitional region the strands of 
internal phloem successively pass out between the converging 
xylem-bundles and one by one reach the strands of external 
phloem, with which they fuse. The external phloem-strands 
concentrate themselves on the two sides of the vascular 
cylinder, between the two centripetal xylem-groups, which 
now represent the cotyledonary pairs. Finally, these two 
groups themselves unite at the centre of the root, forming the 
diarch xylem-plate, and at this point the last of the internal 
phloem-strands passes out and joins the normal phloem. 
So far we have considered the course of the primary bundles 
without reference to the occurrence of secondary growth. In 
the upper part of the hypocotyl, the cambium, being formed 
immediately outside the primary xylem, lies necessarily far 
outside the internal phloem-groups. Lower down, where 
some of these groups on their outward course are already 
level with the xylem, the cambium forms in contact with 
their outer surface. Then we come to the critical region of 
transition, where the internal approaches the external phloem, 
and here it is plain, that for a certain distance the cambium 
must actually cut through the outgoing strand. Lower down 
1 It can. hardly be necessary to explain that the succeeding description relates 
simply to the course of the bundles and has no reference to developmental changes. 
