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 xvlem, 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 sapwood 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. 
