FORMATION OF WOOD 277 



4. Wood fibres, rays of two kinds and soft 



tissue. 



5. Wood fibres, rays of two kinds and soft 



tissue of two systems. 



6. Wood fibres, rays of two kinds, no pores, 



but vertical resin canals. 



7. Wood fibres, rays of two kinds, but no 



pores (vessels) or resin canals. 



" Th.e first class may be neglected as being 

 rare, but the others are very constant, and the 

 presence or absence of one or more of these 

 tissue systems may serve as a foundation upon 

 which to build a key." 



Various additional properties of wood are 

 used as a basis for rough classification ; amongst 

 these properties may be mentioned hardness, 

 weight, colour, odour, colour of a solution in 

 water or alcohol and the nature of the ash pro- 

 duced in burning. 



We have dealt with the question of weight 

 and specific gravity elsewhere. Hardness " may 

 be expressed with precision by the number of 

 kilograms required to sink a punch one centi- 

 metre square to the depth of 1-27 m.m. per- 

 pendicularly to the fibres of the wood, or by the 

 number of pounds per square inch to produce 

 such an indentation." 



According to this standard the woods may be 



