IN CEYLON. 





Gardner!, 0'7 per cent., I), jiirsuta 0'63 per cent., and D. 

 Thwaitesii 0'44 per cent. ; whereas in the majority of our dry 

 zone species the percentage number is considerably increased, 

 e.ff., EL montana 1'4 per cent., D. ovalifolia 1*4 per cent., JX 

 Melanoxjlgn T2 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 

 xy lem of differentages 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. insignia (2O15 

 and 21*14), and, as pointed out in a previous paragraph, it is /. I 

 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) 



