272 TIMBEES AND THEIE USES 



it less easy to split and may produce a visible 

 twisting of stems or branches. 



" Anotber character of some value in dis- 

 crimination is the occurrence of pith flecks, or 

 medullary spots, dark rust-bke patches, which 

 occur in Alder, Birch, Hazel, Hawthorn, and 

 some species of Willow, Poplar and Pyrus. 

 They are supposed by some authorities to 

 originate in passages bored by the larvae of a 

 species of insect which live in the cambium, 

 these passages becoming filled up immediately 

 with cellular tissue; but their origin requires 

 further investigation. ... It may be noted 

 here that, while it is the lignified elements of 

 woods, especially their tracheids and fibres, that 

 give them their chief technological value, it is 

 the stored-up nitrogenous and other more com- 

 plex, and therefore more chemically unstable, 

 substances that are the most combustible, 

 i.e. the most readily oxidized, and also the most 

 readily decomposed by the attacks of fungi. 

 It is these substances, therefore, that have to 

 be eliminated, or at least taken into account, 

 in the processes of seasoning or preserving 

 timber, and it is their presence which renders 

 sapwood (Fig. 82) generally less durable than the 

 physiologically inert heartwood (Fig. 82)." 



We may well examine a longitudinal section 

 of our stem to learn more of the cells composing 



