IN CEYLON. 
33 
The medullary rays occur as radial rows of cells throughout 
the secondary xylem. They are, in the majority of our 
species, only one cell in tangential width, but in some 
species they may be two or even three cells wide, as 
in D. crumenata, D. Toposia, D. insignis, D. pruriens, D. 
Thwaitesii, D. attenuata, and others. The occurrence of rays 
more than one cell in width is never general throughout the 
xylem, and very frequently when the double ray is traced 
outwards or inwards it is found to be continuous with a ray 
only one cell broad. There seems every probability of 
discovering these broader medullary rays in the xylem 
of nearly all our species, providing sufficient material is 
examined. It is usual to find that the differentiation of a 
medullary ray has been going on for many years without 
a break, but, nevertheless, several cases have been seen where 
the differentiation of a medullary ray has been discontinued 
and where new rays have appeared ; in sucli cases the last or 
first formed cell respectively is usually surrounded with fibres 
only, and only occasionally with a wood parenchyma cell. 
As one passes into the younger wood the number of rays, 
not necessarily the individual cells, increases, and in the 
sapwood of very old trees it is not unusual to find the 
medullary rays separated by only one or two rows of fibres, 
whereas in the heartwood they are usually separated by 
broad bands of fibres. 
This increase of parenchymatous tissue lowers the value of 
the sapwood timber, even though the elements may be more 
or less filled with the brown products so abundant in the 
old wood. In some specimens of D. Ebenum the continuity 
of a medullary ray is destroyed by the intrusion of fibres on 
either side of the ray ; this, however, is only local, for the 
same ray can be seen abutting on the inner and outer 
surfaces of such intrusive fibre patches. 
In D. insignis, where the double rays probably reach their 
maximum development, the distribution is very irregular, 
there being from two to twenty single rays between a pair 
8(1)4 (5) 
