IN CEYLON. 
41 
Gardneri, 0*7 per cent., D. hirsuta 063 per cent., and D. 
Thwaitesii 044 per cent. ; whereas in the majority of our dry 
zone species the percentage number is considerably increased, 
e.g ., D. montana 1*4 per cent., D. ovalifolia 1*4 per cent:, D. 
Melanoxylon 1*2 per cent. 
The facts of histological differentiation are therefore in 
general accord with the varying climates and the transpiration 
which probably occurs under these conditions. 
As previously shown, the decrease in percentage number 
from within outwards is concomitant with a regular increase 
in transverse dimensions. 
The parenchymatous elements compose the remaining 
part of the secondary xylem, and are mainly represented by 
fibres. The fibres constitute from 74 to over 90 per cent, of 
the total secondary xylem elements. 
The wood parenchyma, together with the medullary ray 
cells, tend to form an approximately constant proportion in the 
xylem of different ages within the same tree. Generally speak¬ 
ing, the percentage number of medullary ray cells decreases 
from within outwards, this decrease being concomitant, as in 
the case of the tracheal elements, with an increase in size ; 
whereas the percentage number of wood parenchyma cells, 
though generally increasing from within outwards, is apt to 
show a steady percentage number, or even a decrease from 
within outwards. The approximation to a constant propor¬ 
tion in the old and young wood is seen in D. insignis (20*15 
and 21*14), and, as pointed out in a previous paragraph, it is 
often very difficult, owing to feeble lignification of surround¬ 
ing fibres, to be absolutely certain, in transverse sections, 
whether one is dealing with fibres or wood parenchyma. 
Though the percentage number of wood parenchyma cells, 
in the majority of our species, increases in the younger wood, 
the difference in percentage number is rarely over 3 per cent, 
of the total number of xylem elements. In the xylem of D. 
ovalifolia, the percentage number in the wood of a young 
twig and the sapwood of a tree 240 mm. diameter proved 
8(1)4 (6) 
