IK CEYLON. 
51 
Usually the ebony or heartwood occurs as a fairly regular 
circular mass at any particular level in the stem ; when the 
ebony is not limited to xylem of the same age the projec¬ 
tions towards the sapwood are associated with wounds or 
old branches. 
To explain why the limiting line of the central ebony is 
so sharply defined, or, in other words, to explain why the 
coloured substances occur so definitely in xylem of the same 
age, we must consider the distribution and pit communica¬ 
tion between the different elements. In the first place we 
must admit that the wood parenchyma, medullary ray cells, 
and tracheal elements are abundantly supplied with large 
pits on all surfaces, whereas fibres are but sparsely pitted. 
The tangential bands of wood parenchyma serve as 
excellent means for the transference of coloured substances 
through xylem of the same age, from like cells or those of 
the medullary ray to the tracheal elements or the short 
parenchyma surrounding them. 
The frequency of the pits between the wood parenchyma, 
medullary ray cells, and tracheal elements is one reason why 
the circulation of the coloured substances, originating in the 
parenchymatous elements, should tend to keep within these 
elements. The fewness of pits in the fibres does, on the 
other hand, account for their sparse contents and the fact 
that they are the last elements to have their lumina filled 
with the coloured substances. 
We therefore see that the maximum facility is accorded 
for transference of the coloured substances tangentially 
through the wood parenchyma and tracheal elements rather 
than radially through the wide patches of sparsely pitted 
fibres, and this will perhaps serve to explain why the 
coloured substances tend to first fill the elements in xylem 
of approximately the same age and thus lead to the sharp 
line of distinction between the outer limit of the ebony and 
the white sapwood. 
At the same time the transference of coloured materials 
can and does take place in the radial direction, from one 
