Structure of Acer Pseucioplatanus. 
577 
(ii) The Anatomy of the Older Seedling. < 
The period following the full expansion of the cotyledons is one of 
rapid anatomical change which is to be correlated with the growth and 
expansion of the plumular structures. The change, as far as it affects the 
hypocotyl, consists essentially of the development of a purely cauline system 
of secondary xylem, the upper portion of which connects directly with the 
epicotyledonary leaf-traces. The cotyledonary vascular supply and its con¬ 
nexions have attained practically their full dimensions at the beginning of 
this period and only add slightly to their bulk subsequently. The further 
vascular history of the seedling is one of steady change in the relative 
importance of cotyledonary and epicotyledonary constituents, the former 
Fig. 22. Transverse section of the midrib of the cotyledon when completely unrolled. Note 
the disorganized and partly resorbed protoxylem and the fact that the bulk of the functional xylem is 
secondary. The large cells are secretory sacs, x 750. 
assuming a subsidiary position as the latter increase in size. Although it is 
impossible to correlate precisely the various stages in vascular development 
with the morphological changes, since these vary slightly from seedling to 
seedling, the broad features are as follows : At the time that the cotyledons 
are first expanded the isolated cotyledonary protoxylems are disorganized 
and in some cases partly resorbed (cf. Fig. 15), but are still recognizable. 
This disorganization and resorption is shared to some extent by the older 
elements of the primary metaxylem which lie on the inner faces of the four 
large diagonally situated bundles. The epicotyl, which has not yet begun 
to elongate, contributes six well-differentiated strands to the hypocoty¬ 
ledonary vascular system, namely, the midrib and laterals of the first pair 
of epicotyledonary leaves. Near the apex of the hypocotyl the lateral 
bundles are reinforced by desmogen strands representing the vascular supply 
of the axillary buds of the cotyledons. The three epicotyledonary bundles 
of each side are united by a continuous band of phloem which, at a slightly 
lower level, connects with that of the cotyledonary traces (Fig. 23). The 
xylem of the lateral bundles also soon unites with that of these traces, but, 
Q q 2 
