Structure of Acer Pseudoplatcimis. 
587 
and only becomes well defined in the region of the collet. The site of the 
other is occupied, a little way below the apex of the hypocotyl, by a small 
but well-defined collateral bundle which persists for a little over a centimetre 
but gradually dies out. As it disappears a similar bundle appears in the 
intercotyledonary plane at one side and persists for about a centimetre and 
a half before it too dies out. Meanwhile, the four diagonally situated 
xylem groups, with one exception, have each divided into two in the normal 
way. The one showing delay is in close proximity to a wound involving 
the greater part of the cortex in that particular part of the hypocotyl, and 
CpxA A 
Figs. 61-7. Series of semi-diagrammatic outlines, showing the transition features of seedlingy. 
The small crosses in Figs. 63 and 64 mark the positions of the single protoxylem-like elements which 
appear sporadically in the hypocotyl; the arrow in Fig. 64 marks the position which would 
be occupied by Cpx. 2 if it had persisted. 
the traumatic stimulus has led to a precocious development of secondary 
xylem which obscures the behaviour of the affected strand. Transition to 
root structure, however, apart from this disturbing factor, is typical. Sub¬ 
sequently there appear, at slightly different levels, three xylem groups lying 
inside three of the four root phloems and, apart from the repetition, resembling 
the condition found in seedling F. Of the three groups of xylem one per¬ 
sists for about half a centimetre and then dies out, whilst the other two are 
still well developed in the lowest sections obtained. 
Seedling J is one which stands quite apart from the remainder and is 
peculiarly complex. Only one epicotyledonary leaf is developed, the fusion 
of the cotyledons being slightly earlier on the side opposite to this. In one 
cotyledon the vascular strands behave in the normal manner, so that a 
protoxylem strand ( Cpx . 1) and two widely separated collateral groups 
(A and B) enter the hypocotyl from it. In the second cotyledon the 
vascular strands of one side fuse in the usual manner to form one large 
bundle (C), whilst on the other side the half-midrib bundle fuses with the 
