236 Bailey . — The Relation of the Leaf -trace to the Formation of 
bundles to a hollow woody cylinder is assumed to exist in the so-called 
‘ embryonic stem \ This hypothetical condition, however, does not occur 
in woody plants, and the study of the development of seedling plants reveals 
the fact that in numerous species conditions exactly the reverse of those 
which should be expected in accordance with the Sachsian theory occur. 
The Cupuliferae illustrate the truth of this statement very clearly and 
diagrammatically. In the mature twigs of Red and White Oak there are, 
as has been shown in Fig. 25, five smaller depressed segments. These 
segments, and the rays which demark them, are considered interfascicular 
segments, and the aggregate rays are supposed to be continuations of 
portions of the" fundamental tissue, included between the primary bundles. 
Furthermore, it has been supposed that masses of protoxylem subtend the 
larger, but not the smaller segments of the stem. However, we have seen 
above that seedling Red and White Oaks and the stem of the primitive Live 
Oak do not show this segmented character of the woody cylinder, but 
possess instead, in the first annual ring, a continuous woody cylinder. This 
unexpected condition is explained by a consideration of the origin and 
development of the large rays in oak. The so-called primary rays have 
been shown to originate in the vicinity of the leaf- trace, and to be formed 
from the secondary wood by an aggregation and fusion of numerous uni- 
seriate rays and included fibres, and to be in no way related to inclusions 
of fundamental tissue between so-called primary bundles. As has been 
shown above, seedling plants elucidate the phylogenetic history of these 
ray structures, and illustrate the early stages of their development. In the 
mature portions of the plant the aggregate rays are seen to become highly 
perfected, and to extend considerable distances above^ and particularly 
below, the leaf-traces. The depressed segments which occur in the mature 
twigs are, in reality, caused by the concentrated retarding influence of long 
lines of paired aggregate rays upon the growth of a portion of the stem. 
That these depressed segments do not correspond to tissue formed by an 
interfascicular cambium is further shown by the fact that they are subtended 
by protoxylem elements, similar to those which subtend the larger so-called 
fascicular segments. Similar conditions occur in other genera of the 
Cupuliferae. In fact the primitive central cylinder consists invariably of 
a continuous woody ring, and only with the development of highly special- 
ized ray structures, in mature ramifications of the plant, does the central 
cylinder appear to originate from so-called fascicular and interfascicular 
segments. In Fig. 17 is illustrated the primitive condition of the central 
cylinder in the Cupuliferae. The protoxylem elements form an unbroken 
line around the pith, and there is no indication of the division of the central 
cylinder into the putative fascicular and interfascicular segments. However, 
as the two lateral leaf-traces pass off in this section, it may be seen that 
a gap is left in the protoxylem elements of the central cylinder. In the 
