26 TIMBER AND TIMBER TREES. 



Fibres are characteristic of dense and tough woods, 

 but there are many timbers which show no traces of 

 them. Thus, true fibres do not occur in Pines, Firs, 

 Larch, Cedar, Pear, Hawthorn, and a few others ; 

 whereas they abound in such woods as Oak, Ash, Elm, 

 Chestnut, etc. In most hard woods the fibres are 

 scattered among other elements, often in characteristic 

 groups or strands, and it is at present impossible to 

 make any generalisation as to the relation between the 

 mechanical properties of wood and the distribution of 

 the fibres in it ; it is clear, however, that much depends 

 on the length of the fibres, and on the degree of 

 thickening and hardening undergone by their walls. 



Tracheitis occur in nearly all timbers, and in some 

 cases — e.g. Pines, Firs, etc. — the wood is composed 

 entirely of these elements. Tracheids are confounded 

 with fibres by nearly all observers except the more 

 modern ones, and the distinctions between them are 

 not always easy to make out. Mechanically they 

 affect the wood much as do the fibres, imparting to it 

 the properties of hardness, toughness, and heaviness, in 

 proportion to their number and length, and the density 

 and thickness of their walls. 



It is obvious that many characters useful in distin- 

 guishing timbers can be obtained from the points of 

 structure referred to, though only in the hands of skilled 

 observers. But these latter also make use of many 

 other peculiarities of structure in woods for purposes 

 of identification. 



Oak and Chestnut can be distinguished by the 

 peculiarities of their medullary rays, and Alder and 

 Birch likewise, and the breadth, depth, and other pecu- 

 liarities of medullary rays are widely employed for such 

 purposes. 



