PITH-RAYS AND PITH-ELECKS 33 



as in " burrs," produced on many trees by the attacks of mites 

 (Phytdptus), causes similar ornamental wa^7ings of the grain. 



One main cause of the elements not being vertical is their growth 

 in length and in diameter after leaving the cambium stage. Such 

 growth in length causes the tips of the fibres to crowd in between 

 those above and below, and become interlaced and oblique in 

 direction. This adds to the toughness of the wood and makes it 

 less easy to spHt, and may produce a visible twisting of stems or 

 branches. 



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 — aSord 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 (Gasuarina) 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 milli- 

 metres, as in Laburnum and Rohinia, to 64 in the same space, as in 

 Oak, or even 140 in the case of Bhododendron maximum. 



Another character of some value in discrimination is the occur- 

 rence of pith-flecks, 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 Hve in the cambium, these passages becoming 

 fiUed up immediately with cellular tissue ; but their origin requires 

 further investigation. We wiU postpone the consideration of such 

 characters of woods as weight, hardness, colour and odour — char- 

 acters that depend little, if at all, upon structure — to a subsequent 

 chapter. It may be noted here that, while it is the Mgnified ele- 

 ments 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 sap- 

 wood generally less durable than the physiologically inert heart- 

 wood. 



