Structure of Acer Pseudoplatanus . 
579 
In older seedlings a steady increase in the thickness of the secondary 
xylem occurs, the cambium adding elements external to the cotyledonary 
system and linking up with a small collateral strand which fills the position 
formerly occupied by the isolated cotyledonary protoxylem in each half of 
the hypocotyl (Fig. 25). By the time the second pair of epicotyledonary 
leaves have expanded there is a continuous ring of secondary xylem five or 
six elements in width throughout the hypocotyl 
and the lower half of the first epicotyledonary 
internode (Fig. 26). 
Accompanying these changes there occurs 
a development of pericyclic fibres. These first 
appear as clearly differentiated arcs in the 
hypocotyl lying outside the four diagonal 
cotyledonary bundles” about the time the first 
epicotyledonary leaves are fully expanded. 
They are succeeded in older seedlings by 
others which develop both in the epicotyl and 
hypocotyl opposite the strands derived from the 
midribs of the first pair of leaves (Figs. 25, 26). 
It is interesting to note that traumatic stimulus 
may induce their formation in younger seed¬ 
lings even though the wound is relatively slight 
and local in character. Somewhat deeper but 
still superficial wounds frequently result in an 
excessive development of secondary xylem 
adjacent to the injured area. The cork cam- 
bium is sub-epidermal in origin (Fig. a 7 ). 
The period of its inception is variable, since contents of three outer cells only 
many seedlings gathered in 1921, with the fig. 28. Pericyclic cork formation 
first pair of epicotyledonary leaves alone ex- in the region of the collet involving 
r more than one layer of cells. Imme- 
panded, had a well-developed cork cambium, diately outside these is a layer of 
whilst in the wetter season of 1922 it was only c ™shed. and disorganized cells (mdi- 
J # J cated in solid black), and beyond 
just commencing to show in seedlings in which these the dead outer cortical tissues. 
the second pair of epicotyledonary leaves in xLTayer^soUdTkck 
were expanded. It first appears at the top of indicates the same region as in Fig. 
the hypocotyl and often involves the bases 
of the cotyledons, the cells of which divide to a depth of several layers. 
From this region it spreads downwards and in some cases is remarkably 
irregular, frequently leaving one or two vertical strips of green tissue 
bounded by the epidermis when all the rest of the hypocotyl shows the 
characteristic grey-brown resulting from cork formation. The cork cambium 
in the root is pericyclic in origin, as is normally the case (Fig. 29), but it is 
interesting to note that in the collet, where the pericycle is becoming several 
