IN CEYLON. 35 



In specimens of some species, notably of D. acutajand I). 

 Qardneri, 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 U. 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 



