42 WRIGHT : THE GENUS DIOSPYROS 



to be the same, viz., 6 % 3 per cent., and in other cases a reduction 

 in percentage number has occurred in the sapwood. It 

 is therefore obvious that the percentage number of wood 

 parenchyma cells is the most unstable of the secondary xylem 

 elements, since all other elements show a definite increase 

 or decrease. 



The fibres in virtue of their abundance and constitu- 

 tion are the most important from an economic point of 

 view. The most parenchymatous woods, such as those of 

 D. insignis, rarely contain less than 79 per cent, of fibres, and, 

 on the other hand, the xylem of several species contain over 

 90 per cent, fibres. 



From the table quoted it will be seen that the percentage 

 number of fibres gradually increases from the heartwood to 

 the sapwood, and in this respect is almost alone among 

 the secondary xylem elements. The gradual increase in 

 percentage number in passing into the younger wood is 

 concomitant with a decrease in the percentage number of 

 tracheal elements and medullary ray cells. The increased 

 weight of foliage, &c., consequent on age and development 

 is perhaps sufficient reason for the production of an increased 

 percentage number of fibrous elements. 



The percentage number of medullary ray cells comes next 

 to those of the fibres, and varies from 6 to over 18 per cent, 

 of the total number of xylem elements. 



The highly parenchymatous woods are characterized by a 

 particular abundance of these elements, and in all such cases 

 the quality of the timber is inferior, as in JD. insignjs, D. 

 hirsuta, and J). oppositifolia. 



As with the tracheal elements, a decided reduction in 

 percentage number occurs as one passes from the heartwood 

 to the sapwood, but this is accompanied by an increase 

 in dimensions, especially in the radial direction. In the 

 sapwood of most species the enormous size of these elements 

 is apt to create an erroneous idea of their percentage number ; 

 the decrease in the latter is fully compensated by the 

 increased dimensions. 



