THE VEGETATIVE ORGANS OF PLANTS. 279 
as have been described. To answer the purpose of air 
and sap-channels, the wood-cells which constitute the con- 
centric rings of the old wood are constructed in a special 
manner, being provided laterally with visible pores, through 
which the contents of one cell may pass directly into those 
of its neighbors. Fig 
53, B, represents a por- 
tion of an isolated wood- 
cell of the Scotch Fir, 
(Pinus sylvestris,) mag- 
nified 200 diameters. 
Upon it are seen nearly 
circular disks, x, y, the 
structure of which, while 
the cell is young, is 
shown by a section 
through them length- 
wise. A exhibits such 
a section through the 
thickened walls of two 
contiguous and adhering 
cells. w, in both A and 
B, shows a cavity be- 
tween the two primary 
cell-walls; y is the nar- 
row part of the chan- 
nel, that remains while 
the membrane thickens 
around it. This is seen 
in B, y, a8 a pore or 
opening in the cell. In a 
A it appears closed because the section passes a little to 
one side of the pore. 
In the next figure, (54,) representing a transverse sec- 
tion of the spring wood of the same tree magnified 300 
diameters, the structure and the gradual formation of 
