IN 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 



