98 Tue Microscopes. 
MICROSCOPIC STRUCTURE OF THE STEM. 
Bark.—Beginning with the outside of the stem it is noticed 
that no epidermis is present. The cork, or outer layer of the bark, 
is composed of from five to twelve rows of thin-walled tabular 
cells of a brownish yellow color. The green, or middle layer of 
the bark, is of nearly the same width, and is composed of from 
five to twelve rows of oval parenchyma. Next to the green 
layer come large crescent-shaped masses of liber fibre. A cross 
and longitudinal view of a single liber fibre is seen in fig. B, ¢ 
Le. 
Just inside of this liber fibre is found a secondary forma- 
tion of cork and the green layer of the bark, smaller and more 
delicate than the first. Embedded in this second green layer 
are masses of large stone-cells. These have rather thinner walls 
than the majority of stone-cells, still the walls are much thicker 
than all the other cells of the stem excepting the liber fibre. 
(See h, figs. A and B.) Then come other slender, sharply 
pointed, crescent or horse-shoe shaped masses of liber 
fibre. The spaces inside of these inner masses of liber are 
filled up with hexagonal, thin-walled parenchyma. These 
masses of liber and the enclosed parenchyma form the znner 
layer of the bark. 
The cambium separating the bark from the wood is com- 
‘posed of from three to six rows of tabular cells, clear white, and 
filled with protoplasm. 
Wood.—The medullary rays are made up of from three to 
‘ten zows of thin-walled, white tabular cells. Between these 
medullary rays are the small, thick-walled, clear white cells of 
xwood prosenchyma, resembling somewhat the liber fibre. With 
these are numerous large, open, pitted cells. The ends of these 
cells are seen in fig. A, m; while in fig. B, m, the length 
of the.cell is seen with the numerous pitted marks on its surface. 
Two or three annular rings are generally to be seen in 
this stem. 
Between the wood and the pith is the medullary sheath, 
composed of fine spiral vessels. 
