206 
Journal of Agricultural Research 
Vol. XXX Ni. 3 
cells, the wall of the vessel next to the 
sheath cell is not pitted. The tissue 
between the two large vessels is com¬ 
posed of parenchyma interspersed with 
narrow vessels (pi. 3, B). The latter 
are porous and have reticulate thicken¬ 
ings on the side walls. The phloem 
forms an oval mass of tissue composed 
of sieve tubes and companion cells. 
The protophloem lies farthest to the 
outside; its walls are swollen and some¬ 
what disorganized. This condition is, 
however, more strongly marked in the 
protophloem of the leaf (pi. 4, B), and in 
the stem bundles of Zea (pi. 25). The 
cells of the bundle sheath are in direct 
contact with the protophloem; the 
other phloem cells, however, are sepa¬ 
rated from the sheath by a layer of 
small, elongated parenchyma cells. A 
similar band of parenchyma, one or two 
cells wide, separates the phloem from 
the xylem. 
The bundle sheath forms a layer of 
cells which are devoid of intercellular 
spaces. The cells are elongated, thick- 
walled and sparingly pitted. The end 
walls are pointed, except at the 
junction of xylem and phloem: here 
the sheath cells not only have a 
shorter vertical diameter, but the end 
walls are also nearly transverse. 
The bundles follow a longitudinal 
course approximately parallel to each 
other except in the node where a 
number of them branch or bend 
abruptly and move toward the periph¬ 
ery (pi. 1, A). Many of the bundles, 
however, pass on to the next internode 
with but slight deviation from their 
former course. Nevertheless the struc¬ 
ture of all bundles is altered when they 
enter the node. 
The peculiar appearance of the nodal 
bundles when viewed under low power 
is produced by the parenchyma adjoin¬ 
ing the sheath. The parenchyma cells 
bordering the sheath are greatly 
enlarged radially and arranged around 
the bundle in stellate fashion (pi. 6, A). 
This same arrangement of the paren¬ 
chyma is often found in the cells 
adjoining the protoxylem. The scleren- 
chyma cap of the phloem pole of the 
bundle is greatly enlarged; the walls 
of the cells are very thick and strongly 
lignified. The sclerenchvma cap of 
the xylem pole of the bundle, however, 
has practically disappeared. The cells 
of the bundle sheath in this region are 
lignified, but the walls have remained 
thin. The nodal bundles lack the 
protoxylem lacuna. There is a general 
increase in the number of protoxylem 
elements which extend upward and 
come in contact with the phloem. 
Occasionally the radial row of proto¬ 
xylem cells is topped by a tangential 
band of annular or spiral elements, 
connecting the two pitted vessels. The 
latter also have a tendency to increase 
in number; the supernumerary cells, 
however, are smaller and less regular. 
The phloem shows on the whole an 
appreciable quantitative increase over 
that found in the internode. The 
phloem cells, however, have a tendency 
to become lignified (pi. 6, A), and non¬ 
functional. The extent of lignification 
varies, but seems to be in direct pro¬ 
portion to the size and age of the 
bundle. 
As the vascular bundles leave the 
node and enter the Keimring, they 
change back to their old form. The 
stellate parenchyma disappears, the 
number of pitted vessels is reduced to 
two, and the protoxylem lacuna is 
formed again. Changes in the vascular 
sheath include the development of a 
cap on the xylem pole of the bundle 
and a reduction in the size of the cap 
on the phloem pole. The phloem 
shows little or no alteration. There 
is still a great deal of lignification, 
which is here sometimes more pro¬ 
nounced than in the nodal region. 
Before the bundles definitely pass out 
into the next internode they traverse a 
region which yields more readily to the 
sectioning, razor. This region is the in¬ 
tercalary meristem from which elonga¬ 
tion of the internode takes place. The 
vascular bundles show surprisingly little 
lignification, except where the stem is 
so old that all meristematic tissue has 
completely matured. The chief di¬ 
vergence from the normal structure 
consists in a transformation of the 
sclerenchyma bundle caps into col- 
lenchyma and in a- general reduction 
of the lignified cells of the vascular 
part of the bundle. 
Peripheral stem bundles. —To¬ 
ward the periphery of the stem the 
bundles are more crowded, while their 
form changes from rhomboid to oval 
with the long diameter radial in respect 
to the stem (pi. 7, B). The outermost 
circle of vascular tissue is formed by 
small bundles which are almost spherical 
and practically form a solid ring at the 
periphery of the stem. The oval 
bundles are actually smaller than those 
nearer the center of the stem. The 
apparent increase in size is due to the 
development of a huge sclerenchyma 
sheath which is most prominent on the 
xylem pole of the bundle (pi. 3, A). 
The phloem is greatly reduced. Its 
form is no longer oval, with the long 
diameter in the tangent of the organ, 
but more or less triangular or trape¬ 
zoidal, with the narrow part sunken 
