330 BOTANICAL GAZETTE [ NOVEMBER 
soon dry up and fall off, they are seldom found on two-year old 
or older internodes. 
The cuticle is less developed than the cuticle of the leaves. 
The epidermal cells are thickened more on their outer than 
inner walls. The periderm or corky layer of the outer bark of 
the one year old shoots is composed of from 4 to 6 rows of 
cells, but through the activity of the cork cambium more layers 
are formed the second and following years. A. dalsamea forms 
cork scales only after reaching a certain age. Thus one usually 
finds trees 20 to 50 years old with a smooth bark and without 
any distinct cork scales. The original epidermis is still present 
and can be peeled off in small scale-like layers. 
Primary cortex (middle bark).—The outer part of the primary 
cortex, or middle bark, beneath the peridermal layers, is com- 
posed of from 4 to 6 strengthening collenchyma cells. The 
greater part of the chlorophyll parenchyma remains thin walled 
during the first year. Many of the cells, however, become 
thick walled and sclerenchymatous. This thickening of the 
parenchyma and differentiation into mechanical tissue continues 
during the second and following years. The sclerenchyma 
cells soon form short branches, the cell lumen of the original 
cell extending into them, giving them the appearance of 
branching bast fibers. Hartmann” calls these cells in A. pee 
tinata bast fibers. That they are not bast fibers is shown by 
their parenchymatic origin and profuse branching. Their cell 
walls are laminated and pitted. They are usually aggregated 
together in nests of from 3 to Io cells, forming areas of mechan- 
ical tissue. . 
Many of the parenchyma cells of the primary cortex function 
as secretion reservoirs for tannin and mucilage. Crystals of 
calcium oxalate are seldom found in the primary cortex. The 
tannin cells are scattered over the whole primary cortex, and 
do not vary in size and form from the chlorophyll parenchyma 
cells. The mucilage cells are spherical, ovoid, or elliptical, 
and vary in diameter from 30 to 240m. The largest ome ar 
3 [bid., p. 27. | ae 
