IN CEYLON. 
35 
In specimens of some species, notably of D. acuta and D. 
Gardneri, this differentiation is only feebly exhibited. 
The same ray may possess members belonging to each 
category, but often one type exists alone throughout a 
considerable length of a particular ray, and is superposed 
vertically on a group of the opposite type. 
The vertical cells usually predominate in the very old wood, 
and as one passes outwards the number of horizontal cells 
increases until in the peripheral sapwood of very old trees 
they are the predominant type. Sometimes the heartwood, 
as in specimens of D. crumenata, shows a fairly equal 
proportion of horizontal and vertical cells. This increasing 
differentiation of the horizontal type with the age of the 
cambium is worthy of note. 
The size of the medullary ray cells invariably increases 
from within outwards, and this increase, since it usually 
results in a radial extension of the cells, leads to an increase 
in the number of horizontal cells. The local occurrence of 
rays more than one cell in width also leads to a rapid 
increase in the percentage number of horizontal cells. 
In the sapwood of D. insignis it appears very probable 
that many of the horizontal cells are parts of double rays 
and the vertical cells parts of single rays, the volume of the 
two types being nearly the same. Nevertheless, one must 
realize that both types of cells may be differentiated in single 
rays, as in D. Ebenum and others, where double rays are 
very scarce or do not exist. 
In the sapwood of old trees the outline of the cells is usually 
sharply angular ; the walls are straight, and the tangential 
walls are frequently inclined at an angle of 45° from 
the vertical ; in young twigs the outline is not as sharply 
cornered, and tends to form a crude ellipse in transverse 
section. 
The following table of measurements will serve to 
indicate the differentiation in wood of various ages in all 
