40 WRIGHT : THE OKXTJS DTOSPYROS 



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 xylem 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 JX 

 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 JD. ovali- 

 folia and D._montana and the minimum 0*3 per cent., in J). 

 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 trachea] 

 elements are represented by a lower percentage number than 

 any element of the parenchyma type, and this is to b 

 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. acute with 0*4 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, isJX 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, 



