PITH-RAYS AND PITH-FLECKS. 39 



Up to a certain age the segments or chambers (original cells) 

 of the vessels, the tracheids and the fibres, gradually increase 

 each year both in length and diameter. 



The pith-rays as seen in cross-section afford a very useful 

 distinctive character, varying much, as they do, in number and 

 in width. In Willow, Horse-chestnut and Ebony, as in Conifers, 

 they are either only one cell in width, or are at least so incon- 

 spicuous as to require a lens for their observation, whilst in Oak 

 and in the so-called She-oaks (Casuariua) they are conspicuous to 

 the naked eye. They vary in width from -005 millimetre to a 

 millimetre ; and in number from 20 or less in a breadth of 5 

 millimetres, as in Laburnum and Eobinia, to 64 in the same space, 

 as in Oak, or even 1 40 in the case of Rhododendron maximum. 



Another character of some value in discrimination is the 

 occurrence of pith-fleck^ or medullary spots, dark rust-like 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 Tipula (wire-worm) which live in the cambium, these 

 passages becoming filled up immediately with cellular tissue ; 

 but their origin requires further investigation. We will postpone 

 the consideration of such characters of woods as weight, hard- 

 ness, colour and odour characters that depend little, if at all, 

 upon structure to a subsequent chapter. 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 complex, 

 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 generally 

 less durable than the physiologically inert heartwood. 



