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 D. insignis, D. 
hirsuta, and D. 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. 
