8 TECHNICAL PROPERTIES OF WOOD. 



summer-wood, but also pass along the medullaiy rays, whicli are 

 described in tbe next paragraph. The two kinds of ducts open 

 into one another, and their contents have important effects on the 

 technical properties of wood. 



5. Medullary rays (tig. 1) consist of woody cells, which in 

 winter usually contain starch ; they form bands, cither running 

 radially from the i)ith to the bark, or not reaching as far as the 

 pith, but originating from some of the later annual zones of wood. 

 The number and size of these rays have much inllucnce on the 

 technical properties of wood. 



As regards the dimension oi a ray, c d (fig. 1) is its height, 

 a h its breadth, and in n its length. The oak and beech 

 have very broad rays ; the oak and alder very high rays. 

 These species are also characterized by possessing a large 

 number of small rays besides their large ones. Maple, ash, 

 elm, plane, teak and hornbeam,* have moderately broad rays. 

 Most European woods have narrow rays, which may, how- 

 ever, be clearly seen on thin transverse sections of the wood, as 

 in lime, birch, robinia, horse-chestnut, sweet chestnut, hazel, 

 alder, ai)plc, cherry, &c. ; in the case of willows and poplars, 

 however, it is difficult to distinguish the rays without a 

 magnifying-glass ; in conifers they are extremely narrow and 

 crowded together, giving a characteristic silky gloss to a thin 

 transverse section of the wood. 



From fig. 1 it is clear that, in order to ascertain the structure 

 of a piece of wood, sections of it in three difterent directions at 

 right angles to one another should be examined. The section 

 cut at right angles to the axis of the tree is termed transverse, 

 the radial section is parallel — and the tangential section at right 

 angles to — one of the medullary rays. The medullary rays, 

 vessels, wood-fibres and cells, may be seen in all their dimen- 

 sions from the above three sections if cut sufficiently fine and 

 observed either through a magnifying-glass or a microscope of 

 low power. 



6. Annual zones. — The structure of the annual zones of a 

 piece of wood has considerable intluence on its properties, and is 



* Hornbeam-wood np] tears to liave very broad ra3s,, because its rays arc often 

 crowded together in bundles. 



