THE STEM OF PLANTS, 35 
said, an aggregation of cells, at first nearly circular, which become 
polyhedral from compression as the stem increases in size, and this 
is the medulla or medullary tissue of which the central column of 
Fig. 40.—Polyhedral tissues of the Pith, Fig. 41.—Prismatic cells of the Pith. 
most forest trees is formed. This medullary tissue is totally 
deficient in vital energy in its central parts. 
Between the pith and the bark we find concentric zones, which 
bear the name of igneous layers, the aggregation of which form 
what is commonly called mood. If we examine the trunk of the 
oak, the apple, or the cherry-tree, a very sensible difference is 
observable between the innermost woody layers, which are of a 
darker colour and denser tissue than the exterior ones, which are, 
on the contrary, of a paler hue and softer texture. In Fig. 42, 
which represents a vertical 
section of an oak of eigh- 
teen years’ growth, the 
sap-wood (alburnum) is 
represented by the letter a, 
the wood by the letter B 
the bark by the letter E. 
The pith is in the centre, 
with the starry appearance, 
which, in the oak, it often 
presents. The medullary 
rays, to which we shall 
now return, are very ap- 
parent in this section. The 
name of sap-wood (albur- 
num) is given to the outside sie of wood, and that of heart- 
D2 
Fig. 42.—Vertical section of the Oak of eighteen 
years’ growth, 
