218 
Journdl of Agricultural Research 
Vol. XXX, No. s 
Succession 
of leaves on 
the stem 
axis a 
Length of 
sheath 
Length of 
leaf blade 
Succession 
of leaves on 
the stem 
axis a 
Length of 
sheath 
Length of 
leaf blade 
Cm. j Cm. 
Cm. 
Cm. 
First. - _ -. 
1 
Eighth._ 
1. 2 
120 
Second 
Ninth _ _ 
3.0 
155 
Third 
Tenth . _ 
22.0 
185 
Fourth 
Eleventh 
31.0 
185 
Fifth . . 
0.1 i 32 
Twelfth 
33.0 
165 
Sixth_ 
.2 1 50 
Thirteenth.. 
34.0 
190 
Seventh_ 
.2 90 
Fourteenth.. 
33.0 
186 
a The first four leaves are not yet differentiated 
into sheath and blade. 
Beginning with the twelfth leaf 
(or the youngest leaf, the blade joint 
of which is already superficially ex¬ 
posed) the length of the sheath re¬ 
mains practically the same and fluctu¬ 
ates slightly about a mean of 33 cm. 
While the leaf sheath is rather belated 
in its development the blade enlarges 
rapidly as soon as it is cut off from 
the apical meristem. Thus, a blade 
about 1 meter in length (at that stage 
still rolled in the bud) is borne by a 
sheath only 4 mm. long. However, 
although unequal at the start, blade 
and sheath attain their maximum 
length practically at the same time 
with the blade slightly in the lead. 
The development and differentiation 
of the different tissues of the stem 
follow in sequence the '-external differ¬ 
entiation. The first eight internodes 
of the stem, which have a total height 
of a few millimeters only, show the 
early beginning of tissue differentiation. 
The growing point is made up of a 
thin-walled meristem, the cells of 
which are in a state of active division 
(fig. 8). Gradually certain of the 
cells begin to enlarge, setting off 
thereby small groups of cells which 
continue to divide actively. These 
groups constitute the primordia of 
the vascular bundles, and within them 
differentiation proceeds until all the 
components of the bundle are formed. 
The protoxylem differentiates first. 
It is made up of a few narrow, but 
greatly elongated elements, which have 
secondary thickenings in the form of 
rings. The third protoxylem element 
is usually a spiral vessel. While dif¬ 
ferentiation of the protoxylem proceeds 
in centrifugal direction, there appear 
on the opposite pole of the group the 
first elements of the phloem recogniz¬ 
able by the refractiveness of their 
thickened walls. ' Between xylem and 
phloem there is an actively dividing 
tissue which sometimes takes on the 
appearance of a typical cambium and 
from which all other xylem and phloem 
cells are formed. 
Not all bundles in a cross section of 
the growing point show the same degree 
of differentiation. A certain number 
of them are larger and contain a greater 
number of protoxylem cells. The 
larger xylem elements have closely 
ringed wall sculpture and compara¬ 
tively short articulations. In a cross 
section just below the insertion of the 
young leaf these bundles are seen to 
bend more or less abruptly and move 
toward the periphery, where they enter 
the leaf. They constitute the large 
leaf traces which already in their very 
young condition foreshadow the struc¬ 
ture characteristic of the mature traces. 
The peripheral stem bundles are much 
smaller than those nearer the center 
with many of them just in the state of 
procambial differentiation. 
Cross sections through the tenth in¬ 
ternode (just 1 cm. below the growing 
point) show the differentiation of the 
large pitted vessels. The walls of 
these cells are still very thin and easily 
crushed in making a section. The 
tissue surrounding the xylem and 
phloem is composed of narrow, elon¬ 
gated parenchyma cells which gradu¬ 
ally mature into the bundle sheath. 
The development of the bundle sheath, 
however, is very slow and is completed 
only after all the vascular elements of 
the bundle are fully formed. 
In the twelfth internode the large 
vessels have developed a secondary 
wall which shows the pitting charac¬ 
teristic of the mature element. The 
cross walls between the articulations 
of the vessels have also disappeared 
and communication is now established 
through large pores. As the large 
vessels complete their development the 
phloem cells divide longitudinally to 
form the sieve tubes and the companion 
cells of the mature bundle. 
In the next lower internode the walls 
of the large vessels have become ligni- 
fied. The cells of the sheath have 
ceased division and have begun to 
thicken their walls, which is especially 
pronounced in the cells forming the 
bundle caps. 
Finally, in the fourteenth internode, 
with the lignification of the scleren- 
chyma sheath, the formation of the 
protoxylem lacuna and the obliteration 
of the protophloem, the stem bundle is 
completely mature. The formation of 
the protoxylem lacuna may be ob¬ 
served at an earlier stage, but then 
only in individual bundles. 
Hand in hand with the progressive 
differentiation of the different elements 
of the vascular bundle goes an increase 
in the size of the cells. This increase, 
however, is not so marked in the bundle 
