Teb. 1, 1925 
Vegetative Organs of Sugar Cane 
215 
(15), who fully investigated the stoma- 
tal movements of the grasses, explains 
the opening and closing as follows: 
Since the middle portion of the guard 
cell is narrow and the tangential walls 
greatly thickened, any change in the 
osmotic pressure of the, cell would be 
incapable of causing movement in this 
portion of the wall. The enlarged 
ends of the guard cells, however, have 
thin walls which will respond to turgor 
changes. When there is an increase 
in cell turgor they cause the pore to 
open, whereupon the thickened middle 
portions of the guard cells are passively 
drawn apart leaving a slit with parallel 
sides. 
THE BOOT 
The root differs from the stem by 
two structural features, the possession 
of a root cap, and the radial arrange¬ 
ment of the bundles. The root cap 
occupies the extreme tip of the root. 
It is often colored red, which, ordinarily 
a normal condition, sometimes is of 
pathological significance. 
A cross section of a young root 
(pi. 20, A) shows a cylinder of vascular 
tissue limited on the inside by a well 
developed pith, on the outside, by the 
endodermis and cortex. The pith is 
of uniform nature and is made up of 
large spherical cells, interspersed .with 
small intercellular spaces. The cells, 
are elongated vertically and have a 
tendency to become larger toward the 
center. ' Occasionally there are found 
scattered in the pith small, thick-walled 
cells similar in size and shape to the 
interstitial tissue of the stele' (pi. 21, 
A). The cortex possesses on the inner 
and outer. periphery, layers of small 
and specialized cells. Those at the 
outer periphery are narrow, elongated 
and thick-walled and form in their 
entirety a distinct sclerenchymatous 
cylinder. The rows of cells at the 
inner periphery consist of regular 
small square cells between which are 
found rhomboid intercellular spaces 
(pi. 20, B). The larger part of the 
cortex consists of loosely packed paren¬ 
chyma cells which later disintegrate 
with the formation of large 'air 
chambers. 
The epidermis is made up of fairly 
uniform and thin-walled cells which 
vertically have the form of short 
cylinders. Many of the epidermal 
cells have grown out into hairs which 
remain attached to the root long after 
they have ceased to function. Be¬ 
neath the epidermis is the exodermis, 
a layer of thin-walled parenchymatous 
cells, which are larger and more 
elongated than the cells of the epider¬ 
mis. The walls of these cells are 
suberized, and their inner tangential 
surface is always thickened. 
The endodermis as shown in Plate 
22, B, C, and D consists of a single 
layer of cells connected uninterruptedly 
with one another. The individual 
cell is a vertically elongated four-sided 
prism with horizontal ends. The radial 
walls slightly exceed in length the tan¬ 
gential ones, though sometimes they 
are equal. The cells are thin-walled 
in their meristematic condition, but 
upon maturing the inner tangential 
walls and parts of the radial ones be¬ 
come greatly thickened and lignified. 
The Casparian strips are seen in young 
material bqt even here only with 
difficulty; later they become masked by 
the development of secondary wall 
thickenings. The inner tangential 
walls of the epidermal cells .are- dis¬ 
tinctly pitted and possess peculiar 
protuberances which extend into the 
lumen of the cells. The apical region 
of such protuberances as shown by. 
Klinge (9) and Bremekamp (3) con¬ 
tains silica. 
The vascular tissue consists of al¬ 
ternating masses of primary xylem and 
phloem which occupy different radii 
(pi. 23, A). The xylem plates do not 
reach the center. The latter is occu¬ 
pied by a parenchymatous pith at the 
periphery of which there is a ring of 
large pitted vessels. (PI. 20, A and 
pi. 22, A.) Outside of this there is a 
ring of alternating xylem and phloem 
groups. In young roots (pi. 24, A) there 
are almost always eight large vessels; 
in older and thicker roots (pi. 21, A) 
this number is much larger. The 
clusters of protoxylem and phloem are 
very numerous (pi. 23, A), a result of 
the absence of secondary growth which 
is so important a factor in the develop¬ 
ment of the roofs of the dieots. In 
the thin lateral roots and in the root¬ 
lets of seedlings, the xylem plates are 
few in number and meet in the center; 
a pith and the large pitted vessels are 
often wanting (pi. 24, B). The 
groups of xylem are separated from the 
endodermis by a single-layered peri- 
cycle, but in the small roots the xylem 
cells occasionally border directly on 
the endodermis. Between the groups 
of primary xylem and phloem is inter¬ 
stitial parenchyma (pi. 23, B and fig. 
6). Cells of the same tissue also 
separate the protoxylem and phloem 
from the large vessels, then surround 
the latter and form ^peripheral sheath 
around the pith. The character of 
these cells, however, differs with the 
region where they occur. Thus the 
