26 WRIGHT: THE GENUS DIOSPYROS 



transverse sections only differ from the vessels in being 

 usually of smaller diameter. They are generally limited to 

 the neighbourhood of large vessels, frequently flanking 

 the tangential surface, but in D. Embryopteris they may 

 occasionally be seen among fibres. 



The transverse dimensions are often only a trifle in excess 

 of the largest wood parenchyma cells, but in some instances 

 they attain a radial diameter of O04 to O08 mm. In length 

 they come next to the fibres and may attain 0*27 to O90 mm. 

 When of great length the ends taper very strongly, and in 

 such cases it is difficult to distinguish them from the more 

 abundant fibres, and the term " fibre-tracheids " would be 

 very appropriate. The commonly small and constant trans- 

 verse dimensions of true fibres seems to differentiate them 

 from tracheids, but otherwise it often becomes well nigh 

 impossible to distinguish a short fibre from a narrow lumined, 

 long tracheid. 



Parenchymatous Elements. 



This includes the wood parenchyma, fibres, and medullary 

 rays of the secondary xylem. It is in the distribution of 

 these elements that the maximum variation occurs, and even 

 in our Ceylon species it is sufficient to allow of a grouping 

 of the species into sections according to 



(1) Whether the medullary rays are one cell in tangential 



breadth. 



(2) Whether the medullary rays are more than one cell 



in tangential breadth. 



(3) The presence of special radial groups of parenchyma. 



As with the tracheal elements, the general features of 

 distribution, histology, and function may be subject to all 

 manner of variation according to the internal and external 

 forces at work. 



The parenchymatous tissues compose the greater part of 

 the secondary xylem and increase considerably with the age 

 of the cambium. It is obvious that the value of the timber 



