208 
J oumal of Agricultural Research 
Vol. XXX, No. 3 
between the larg® pitted vessels. The 
protoxylem lacuna has disappeared 
and the bundle contains thick-walled 
tracheidlike elements instead. The 
most peripheral bundles lack the 
protoxylem and the protophloem and 
consist largely of sclerenchyma. 
In the region of the node these large 
elongated bundles lose their identity. 
They appear to break up into small 
bundles of various configurations, 
which, together with the peripheral 
bundles already present in the inter¬ 
node, form a comparatively broad 
zone of small vascular groups, which, 
like all bundles of the node, are sur¬ 
rounded by stellate parenchyma (pi. 
7, A). 
When leaving the nodal region the 
peripheral bundles become more uni¬ 
form again. The bundles continue to 
show large lignified sheaths, but there 
is no longer that maze of bundle fusion 
and splitting found in the lower zone. 
In the upper part of the Keimring the 
bundles have practically attained the 
structure and arrangement they pos¬ 
sessed in the internode. 
Just as the center bundles become 
temporarily modified when passing 
through the intercalary meristem, so the 
peripheral bundles in this region ex¬ 
hibit considerable deviation from the 
normal structure. The large periph¬ 
eral bundles upon leaving the root 
zone and passing through the meristem 
become so closely crowded that they 
appear like a honeycomb in which the 
walls of the comb represent the paren¬ 
chyma which separates the bundles. 
The crowding of the bundles is the 
result of their increase in size, which is 
produced by the development of a 
huge collenchyma jacket around a 
small vascular nucleus (pi. 8). The 
lignified tissue is reduced to a few 
protoxylem cells. In the mature stem 
the same condition exists, except that 
in addition to the lignified vascular 
cells there is a thin-walled lignified 
vascular sheath around each bundle. 
The sheath is potentially present in the 
young bundles, but since the cells are 
delicate and of cellulose, they are not 
readily distinguished. 
Special bundle structures. —At 
the point of insertion of the leaf sheath 
the node is traversed by longitudinal 
traces which, descending from the leaf, 
penetrate almost horizontally to the 
center of the stem and thence perpen¬ 
dicularly downward (pi. 15, B). In 
their horizontal extent the leaf traces 
deviate from the normal structure. 
This deviation is shown first in the 
arrangement of xylem and phloem, 
which, instead of being distinctly col¬ 
lateral, approaches the amphivasal 
type. Secondly, the deviation is one 
of structure. One notices at first 
glance that the large pitted vessels are 
wanting and that instead there is an 
abundance of narrow elements with 
short articulations and secondary thick¬ 
enings in the form of very close spirals. 
In cross section the bundle is typically 
heart-shaped (pis. 9, A and 10). The 
xylem is limited to the periphery and 
distributed in a manner that the nar¬ 
row spiral elements occupy the tip of 
the bundle, while at the flanges are 
found lignified parenchyma cells with 
scalariform, sometimes reticulate, wall 
thickenings. The bundle sheath is 
indistinct and is composed of thin- 
walled, short fibers of large diameter. 
The phloem is abundant but lacks the 
characteristic structure produced by 
the relative arrangement of sieve tubes 
and companion cells. It is composed 
of nearly uniform polyhedral cells, 
many of which are sieve tubes. The 
elements of the protoxylem are embed¬ 
ded in parenchymatous tissue, the cells 
of which later enlarge and partially 
disintegrate, so that in the mature 
bundle there is commonly seen a large- 
celled and lignified lacunate tissue 
adjacent to the protoxylem. 
In addition to these large leaf trace 
bundles there are numerous small ones 
which run at right angles to the stem 
axis. They often surround the ascend¬ 
ing leaf traces (pi. 11, A), and fre¬ 
quently fuse with them. The phloem 
of these small bundles is relatively well 
developed, while the xylem is often 
reduced to a single large vessel (pi. 
9, B). The bundle is surrounded by & 
sheath, the cells of which become 
lignified. 
The internodes of the mature stem 
occasionally contain bundles which 
deviate from the common structure by 
the absence of the protoxylem and the 
reduction of the metaxylem to a single 
large vessel. The failure of the pro¬ 
toxylem to develop is due to the fact 
that the bundles were differentiated 
after elongation in the internode had 
practically ceased. A second type of 
deviation is caused by inversed orien¬ 
tation. It is not infrequent to find 
bundles, especially near the periphery, 
which have the phloem developed in 
the direction of the center of the stem, 
whereas the adjacent bundles are 
oriented normally, that is, the phloem 
facing the periphery. 
Relationship of the leaf 
traces. —The large number of traces 
entering a leaf and the very complex 
nodal structure make it impossible to 
follow the course of the vascular 
