214 
Joumal of Agricultural Research 
Vol. XXX, No. 3 
tively large pitted vessels which border 
directly on the phloem (pi. 17, A.) 
As is usual, however, phloem and 
xylem cells do not communicate by 
pits. The phloem is structurally 
similar to that found in the bundles 
of the leaf sheath. This is especially 
true of the bundles of the midrib 
(pi. 18, A). The sclerenchyma cap of 
the bundles of the midrib abuts directly 
on the epidermis. 
Transverse bundle connections are 
frequent. They are in the nature of 
narrow branches which run somewhat 
diagonally, or at right angles, between 
two veins and effect union with the 
latter. As the leaf becomes narrow 
there is a gradual reduction in the 
number of bundles. The bundles 
which are to drop out either fuse 
with other bundles or they become so 
completely reduced that only a few 
phloem and xylem cells remain and 
finally only elongated parenchyma. 
The chlorophyll-bearing parenchyma 
of the leaf forms a more or less con¬ 
centric ring around the bundles, ex¬ 
cept when the latter are so large that 
they practically fill the area between 
the two epidermis. In the midrib the 
chlorophyll-bearing parenchyma is 
found largely between the small 
bundles. The cells are small, irregular, 
and filled with numerous chloroplasts, 
the color of which is darker green than 
that of the plastids found in the 
sheath cells. This apparent difference 
in color is probably due to the fact 
that in the spongy parenchyma the 
chloroplasts are more crowded than 
in the sheath where they are few in 
number and much larger individually. 
In fixed material the large chloroplasts 
have a tendency to clump together in 
narrow crescent-shaped bands, (pi. 
17, A). Adjacent to and partly sur¬ 
rounding the spongy parenchyma are 
large colorless cells, which have some 
peripheral protoplasm and a watery 
sap. They probably serve for the 
storage of water. 
The cells of the epidermis are brick¬ 
shaped with the largest diameter in the 
direction of the long axis of the 
organ. The walls of the cells are 
strongly undulated, thickened and 
lignified (pi. 18, D). In addition to the 
long cells there are found, just as in the 
epidermis of the stem and leaf sheath, 
short cells singly or in pairs (pi. 5, D). 
One component of the pair is com¬ 
monly thick-walled and strongly silici- 
fied. The epidermal cells above the 
veins differ from the normal type in 
being greatly elongated and often so 
thick-walled that there is hardly any 
lumen. The upper epidermis has in 
addition to the two types of cells 
mentioned, the so-called “bulliform” 
cells (pi. 16, A), which form longitudi¬ 
nal bands and alternate with strips of 
ordinary epidermal cells. The bulli- 
form cells are much larger than the 
other cells of the epidermis and differ 
further in that their walls remain 
thin and of cellulose. They are con¬ 
tinuous with the inner part of the leaf 
through the large colorless cells re¬ 
referred to above. The bulliform cells 
differentiate only after the leaf begins 
to unroll, but then they increase in 
size rapidly and become slightly 
raised above the level of the other 
cells. 
The margin of the leaf is serrate; 
the teeth are formed by stiff setae of 
somewhat triangular form. The rows 
of epidermal cells next to the margin 
are thick-walled, colorless, and almost 
devoid of content. 
Except for the structural deviation 
exhibited by the margin of the leaf the 
lamina is remarkably uniform. The 
veins, which occur at regular intervals, 
divide the blade into narrow bands of 
tissue composed of rows of regular 
epidermal cells, alternating with rows 
bearing either stomates of hairs. The 
latter exhibit two distinct types: short 
one-celled bristles, which attain a 
larger size at the lower leaf surface, 
and two-celled appressed hairs. There 
are commonly three rows of stomates 
between two veins, but on the upper 
leaf surface they are less numerous and 
are commonly limited to a single row 
at each side of a large vein. 
The stomates are phaneroporous, 
lying in the same level with the epi¬ 
dermal cells. They are always found 
in longitudinal rows in such a manner 
that a stomate alternates with an 
epidermal cell. The mature stomate 
is formed by two guard cells and the 
adjacent subsidiary cells (pi. 19, A, C, 
D, and E). The latter become much 
larger than the guard cells, extend 
deeper radially and their outer walls 
slope slightly in the direction of the 
guard cells. The guard cells are 
wider at the ends than in the middle 
region of the cell. In longitudinal 
sections it will also be noticed that 
both tangential walls of the guard cells, 
but especially the inner walls, are 
deeply constricted in the middle, 
which further tends to narrow the lu¬ 
men of this part of the cell. The pore 
of the stomate is elongated lozenge¬ 
shaped, its lateral walls being straight 
and parallel, or sometimes gently 
curved. The stomatal movements are 
correlated with this peculiar construc¬ 
tion of the guard cells. Schwendener 
