88 GUIDE TO TIMBERS OF NIGERIA 



3. The " metatracheal " parenchyma which we shall call P. (6) is that 



which is characteristic of the common Walnut and Hickories, being 

 very prominent in the latter, where it is sometimes visible to the 

 unaided eye. In the European Beech and in the Hornbeam, this 

 kind of Parenchyma lacks contrast with the woody fibres and can 

 only be seen in a transparent section. Rarely if ever does the 

 P. (6) exceed one cell in (radial) thickness ; thicker zones are com- 

 posed of P. (a), see previous note and compare fig. 5, PL I. 



4. The concentric parenchyma (c) which is histologically the same as 



P. (a) frequently closes the season's growth, hence it is often called 

 " terminal " parenchyma (cf. figs. 1-4, PL I and fig. 2, PL III). 

 An example of a wood in which it may be readily seen is the Cigar- 

 box Cedar. We treat this third form of P. (c) separately for the sake 

 of convenience. 



5. Alstonia is a rare case (the only one that we have yet met with) in 



which the concentric zones of P. are so closely arranged that as 

 many as three zones may appear in the space equal to the radial 

 diameter of a pore-group. 



6. Festoons. In this arrangement of the vessels the latter are in oblique 



lines inclined towards each other and joined at their ends by P. (a), 

 thus forming a strongly-waved line (see fig. 3, PL I, and fig. 6, 

 PL II). 



7. When the lines of P. (a) are only gently undulating, but uninterrupted 



the pores look like beads strung upon a string. Figs. 2, PL I, and 

 1 and 2, PL II. 



8. " Herring-bone " arrangement of the vessels. This is well shown in 



fig. 1, PL III. The vessels are disposed in oblique, more or less 

 parallel lines. The lines may reverse their direction from zone to 

 zone. Herring-bone is apt to be lost in badly developed rings. 



9. Particular care should be taken to search for the widest rings and to 



ignore the narrower ones. 



10. The rays in tangential section when all of the same height are found 



to be arranged either in parallel rows which produce a fine stippling 

 that is visible to the naked eye, or they are in 6chelon, which may 

 be a modification of the other produced by spiral grain. See fig. 

 10, PL I, in parallel, and fig. 13, PL I, in echelon. 



11. Rays are more usually of very diverse heights, in which case they 



are never arranged in parallel. The height runs from a certain 

 maximum characteristic of the species down to a single cell. When 

 the extremes are considerable a different appearance is observable 

 in tangential section to that produced when the difference in height 

 is little (cf. fig. 11, PL I, proportionately low; fig. 12, proportion- 

 ately high). ^ 



12. Unless otherwise stated, proportion of parenchyma to the total mass, 



includes the area occupied by the pores. 



