578 Holden and Bexon.—On the Seedling 
as was noted in the case of younger seedlings, the behaviour of the midrib 
is somewhat variable. In the majority of cases it bifurcates and the half¬ 
bundles move apart and fuse with the cotyledonary strands ; but occasionally 
the phloem alone bifurcates, the xylem moving as a single unit to one side 
or the other. In none of the seedlings of this age examined does the midrib 
retain its original median position in the intercotyledonary plane. Below 
the level of fusion of the epicotyledonary bundles with those from the coty¬ 
ledons the general vascular structure of the hypocotyl shows no significant 
change from that of the earlier stages and its elements are wholly associated 
with the cotyledonary traces. 
In seedlings in which the first pair of leaves projects about a quarter of 
Figs. 23-6. Camera lucida outlines of hypocotyls from progressively older seedlings (x 20). 
Fig. 23 is from a seedling in which the cotyledons are just fully expanded ; Fig. 24, from one in 
which epicotyledonary growth has commenced; Fig. 25, from one in which the first epicotyledonary 
leaves are expanded ; Fig. 26, from one in which the secondary pair of epicotyledonary leaves are 
expanded. The cotyledonary xylem is outlined ; the epicotyledonary xylem and its connexions are 
solid black ; the phloem is indicated by fine dots, and the pericyclic fibres by coarse dots. 
an inch above the median fissure between the cotyledons considerable 
changes have taken place. The upper part of the elongating epicotyl shows 
six well-differentiated strands derived from the first pair of leaves and, 
alternating with these, six desmogen strands representing the vascular supply 
of the second pair of leaves. At a lower level fascicular cambium is present 
in the larger strands, whilst the smaller strands each possess a group of three 
or four xylem elements. The bundle fusions characterizing the epicotyle¬ 
donary nodes, which will be described in detail below, occur in this instance 
at or near the apex of the hypocotyl, so that there are six large strands in 
two equal groups at this level. These are united at a slightly lower level by 
a band of secondary xylem, and as the diagonally placed cotyledonary 
bundles move inwards they come to lie at the extremities of the arcs of 
xylem so produced (Fig. 24). At or near this level the site of the now 
completely disintegrated protoxylem is frequently occupied by a small 
group of cambiform cells, whilst still farther down the hypocotyl the eight 
strands which result from the bifurcation of each of the cotyledonary bundles 
are linked by a thin but continuous zone of secondary xylem often only one 
cell in width. In some seedlings of this age the continuity of the secondary 
xylem is broken in the lower half of the hypocotyl, but in others it remains 
constant down to root level. 
