324 
Holden. — Observations on the 
below the apex of the hypocotyl, so that four exarch xylem groups, 
with alternating phloem, traverse the hypocotyl until the base is approached. 
This primary structure becomes very considerably modified in older seed- 
lings, owing to secondary changes. Prior to the giving off of the whorl of 
lateral roots, to which reference has been made earlier, the typical vascular 
structure of the hypocotyl is obscured, even in the youngest seedlings 
examined, by the development of a solid plug of xylem consisting of short 
tracheidal elements, and this persists until it is gradually replaced by the 
tetrarch xylem star characterizing the main root (cf. Fig, 99). 
2. The vascular system of the young epicotyl. In the normal seedling 
the leaves borne at the first two epicotyledonary nodes are decussate and 
opposite, the lower pair being in the intercotyledonary plane : those produced 
at subsequent nodes are usually in whorls of three. Although the anatomy 
is subject to minor variations the essential features are stereotyped, and it is 
proposed to describe the course of the vascular strands from above down- 
wards in a typical specimen at a stage in which two whorls of leaves are 
developed. For the sake of brevity the leaves will be referred to as the 
upper and lower whorls and the upper and lower pairs respectively. The 
petiolar vascular supply consists of three collateral bundles, one median and 
two lateral, the latter often being somewhat flattened transversely. There 
is thus at the apex of the young stem a ring of nine bundles (Fig. 23). This 
number becomes doubled by the entry of the three bundle-systems of the 
lower whorl, the eighteen bundles thus produced being reduced, first to 
twelve by the fusion of the laterals (Fig. 24), and then to nine again by the 
subsequent union of the median bundles of the upper whorl with one of the 
adjacent compound laterals (Fig. 28). This fusion of the median bundles is 
generally regular in either a clockwise or a counter-clockwise direction, but 
in some specimens it is irregular, two fusing in a clockwise direction whilst 
the third fuses in the opposite direction, or vice versa. The insertion of 
this trimerous system on to that of the members of the upper and lower 
pairs” gives rise to an interesting disturbance of symmetry. The bundle- 
systems of the upper pair do not enter the stem exactly in the cotyledonary 
plane, but slightly obliquely, forming with those of the whorl above an 
irregular ring of fifteen bundles (Figs. 24, 25). This number is ultimately 
reduced by the fusion of the adjacent laterals, but often before this occurs 
an additional complication arises owing to the insertion of the components 
from both nodes on to the vascular bundles of the lower pair. 1 The latter 
enter the stem in the intercotyledonary plane, but one group shows a retarded 
rate of entry compared with its fellow, a feature which is to be correlated 
with the difference in the methods of insertion of the vascular strands from 
1 The level at which the fusion of the adjacent laterals occurs is somewhat variable. It may 
occur soon after the entry of the bundles of the upper and lower pairs into the stem (Fig. 26), or it 
may be delayed until a relatively late stage (Fig. 27). 
