40 
WRIGHT : THE GENUS DIOSPYROS 
to give a systematic value to the percentage composition of 
the xylem. 
Change in Percentage Number of Elements in Old and. 
Young Xylem . —In every species the percentage number of 
tracheal elements decreases with the age of the cambium, the 
first formed xy le m having the maximum percentage number 
and the last formed the minimum. The number of tracheal 
elements in the heartwood may reach 3*43 per cent, as in D. 
insignis, or be as low as 0*8 per cent, as in some specimens 
of D.sylvatica ; this is a very low number, but in the sap wood 
it is still lower, the maximum being 14 per cent, in D. ovali- 
folia and D. montana and the minimum 0*3 per cent, in D. 
affinis. 
Hence we see that the percentage number of tracheal 
elements in young and old xylem. of all our Ceylon species 
does not vary more than 3 per cent., and the relatively low 
percentage number will serve to systematically diagnose the 
secondary xylem of our species of Diospyros. The tracheal 
elements are represented by a lower percentage number than 
any element of the parenchyma type, and this is to be 
correlated, perhaps, with the evergreen nature and very slow 
rate of growth of all the Ceylon species. 
The thickest leaved species have a very low percentage of 
tracheal elements in the sapwood of the mature tree, as for 
instance D. acuta with 04 per cent, and D. affinis with 0*3 
per cent. The thinnest leaved species in the dry zone is D. 
montana, with a percentage number of 14 in the sapwood, 
whereas the thinnest leaved species in the wet zone, where 
the temperature is never very high and the air is always 
moist, is D. attenuata with a percentage number of 0*88 in the 
sapwood of stems 140 mm. diameter. 
Though knowledge of the individual variations prevents 
one from attaching too much importance to similar features 
in different species, yet it is worthy of note that in every 
species growing in the wet zone the percentage number of 
tracheal elements is approaching the minimum, as in D. 
