IN CEYLON. 27 



is greatly due to the percentage composition of elements of 

 the parenchyma type and the structure and form of these 

 elements. 



In the poorer timbers the wood parenchyma is usually of 

 the thin walled, wide lumined type, and this alone is often 

 responsible for the inferior quality, for no matter how thick 

 and long the surrounding fibres may be, the soft bands 

 6f parenchyma, when present, lead to early cracking and 

 decay. 



Wood Parenchyma. In transverse sections the presence 

 of these elements is usually determined by the abundance 

 of carbohydrate and mineral contents, together with their 

 relatively thin walls and wide lumina. Often, however, they 

 are not easily distinguishable, sometimes because their trans- 

 verse dimensions are similar to those of the neighbouring 

 cells, as in IX qusesita,; sometimes on account of the 

 proximity of adjacent medullary rays, and again because the 

 surrounding fibres may not have undergone much lignifica- 

 tion ; the latter is often the case in the secondary xylem of 

 young twigs. In some species they are always very 

 conspicuous, either on account of their lumina being very 

 large, or because adjacent medullary rays are separated by 

 patches of fibre of considerable tangential width. 



The difficulty of distinguishing these elements, however, 

 is only when one is limited to transverse sections ; the great 

 length of these elements and their numerous contents serve 

 to isolate them in any longitudinal section. 



The distribution of these elements is somewhat complex, 

 and is such that a continuous system of elements of the 

 parenchyma type is established, together with frequent direct 

 and indirect contact with the tracheal elements. The wood 

 parenchyma cells are distributed as tangential bands, oblique 

 bands, and around large tracheal elements. The tangential 

 bands usually extend from one medullary ray to another, 

 and their significance varies according to the width of this 

 gap ; the tangential bands between adjacent rays in xylem of 



