314 BOTANICAL GAZETTE [NOVEMBER 
werefound. Bastin and Trimble™ observed no hypoderm except . 
along the midrib on the lower leaf surface. The presence or 
absence of strengthening, or hypodermal cells, however, depends 
entirely on the part of the leaf from which the cross sections 
are made. Hypoderm cells rarely occur in cross sections made 
above the middle of the leaf, while below the middle they are 
always present. Thus, numerous cross sections at five different 
parts of the leaves show in the following table their distribution 
(see page 315). 
Longitudinal sections of the leaves show that the thick- 
walled hypodermal cells are long and bast-fiber like, with taper- 
ing, oblique, or blunt ends. The isolated cells are tapering at 
their upper ends and extend farthest up into the leaf. Their 
function, as is well known, is for strengthening the leaves. The 
leaves on the terminal shoots are much more rigid than the leaves 
on the lateral branches. This is due to the greater development 
of their hypoderm. Like the epidermal cells they are lignified. 
The walls are unequally thickened and often provided with 
pore canals. In A. dalsamea one never finds them extending into 
the mesophyll, as they do in the leaves of A. firma. The num- 
ber of hypodermal cells decreases from the base toward the tip 
of the leaf; but with the decrease in the number of hypodermal 
cells there occurs a corresponding increase in the number of 
stomata. 
The mesophyll, which forms the chief substance of the leaf 
within the hypoderm, agrees in its main characteristics with that 
found in the leaves of most species of Abies. The two dis- 
tinct layers of palisade cells extend around the rounded angles 
of the leaves. In the center of the mesophyll, and midway 
between the endoderm and outer angles of the leaf, lie th 
circular resin canals, which are large in Adies balsamed. The 
resin canals are lined with the thin walled epithelial cells, which 
are themselves surrounded by one layer of thick-walled strength- — 
ening cells. These strengthening cells differ from the sut 
e two > 
epidermal ones in that they are shorter and not fiber-like. = a 8 
Mids. 556 
