IX CEYLOX. 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, IX Tgposia, 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 such 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) 



