IN CEYLON. 23 



tangential width. Sometimes one group is separated from 

 its neighbour only by a single line of medullary ray cells ; 

 usually, however, there is a wide band of fibres and 

 parenchyma between parallel rows of vessels. 



There does not therefore appear to be much regularity in 

 the distribution of the vessels in the secondary xylem, and 

 one may find as many as eight radial groups in one field, 

 while adjacent parts of the same section may not show a 

 single vessel, though the field of view is 0*6 mm. in radius. 

 Usually, in passing from within outwards the vessels become 

 more widely separated, tend to be more as single members, 

 and are considerably increased in size. 



In the sap wood of D. Moonii the vessels are nearer to one 

 another tangentially than in the heartwood, yet in other 

 species the space between the vessels increases from within 

 outwards. 



They are usually surrounded with fibres or flanked with 

 short wood parenchyma or medullary ray. 



The size of the members is never very great. The 

 transverse and longitudinal dimensions show a regular 

 increase as one passes from the heartwood to the sapwood, 

 and it may be taken for granted that the larger the dimensions 

 of the trunk the larger will be the dimensions of the vessels 

 in the youngest part of the sapwood. This regular increase 

 occurs in all our species, and is probably correlated with 

 the physiology of the plant, particularly the storing and 

 conducting of water. This increase of dimensions in the 

 products of the old cambium is not characteristic of all the 

 secondary xylem elements, and with the vessels we have to 

 note that the increase of dimensions is concomitant with a 

 decrease in percentage number and a more complete isolation 

 from like elements. The fact that there is in every case a 

 decrease in the percentage number when there is the maxi- 

 mum amount of foliage, and therefore probably maximum 

 transpiration, renders some increase in size absolutely neces- 

 sary when the heartwood is blocked up with gum and resin. 



