Bowenia spectabilis , Hook . f. 
483 
Transition from Root to Stem. 
The normal structure of the root is retained to within a few 
mm. of the insertion of the cotyledons. The first indication 
of change is seen in the reduction of the pentarch structure 
to triarch : this is brought about by the disappearance of two 
protophloem-groups (see Fig. 5) and the subsequent union 
of the two adjacent pairs of protoxylem-groups to form two 
distinct masses. The result is, three groups of phloem, and 
three alternating groups of xylem, two of which represent 
a fused pair each. The diameter of the vascular cylinder 
meanwhile diminishes, as the constituent bundles of xylem and 
phloem pass in slightly towards the centre. The third and 
smallest group of protoxylem next passes inwards into the 
centre, and its elements become scattered in the pith between 
the two larger groups of xylem (Fig. 21) ; the two groups of 
phloem which were formerly separated by this third xylem- 
group (Fig. 21, pk. 1 , pk. 1 ), pass towards one another, and 
eventually become united. 
The structure of the cylinder at this stage is as follows : — 
It is of an elliptical form (Fig. 21), two well-defined 
protoxylem-groups being situated at the ends of the longer 
axis of the ellipse (Fig. 21, p. xy.), and the third group of 
protoxylem represented by a number of tracheids which are 
scattered in the pith between the polar groups of xylem 
(Fig. 21, c. xy.). On one flank of the xylem-plate is a single 
large group of phloem (Fig. 2 L,pk.) showing a distinct band 
of crushed protophloem (p. pk.). On the opposite side of the 
xylem-plate are two smaller groups of phloem (ph. 1 ,pk. 1 ) which 
were separated by the third group of protoxylem, and which 
will shortly unite to form a single bundle. Each of the three 
phloem-groups is bounded on its inner face by cambium, 
to which is due a considerable development of secondary 
phloem and a few secondary tracheids (Fig. 21, xy. 2 ). As far 
as could be ascertained, this structure is always to be 
found in the region of transition, though it is not always 
