The Growth of a Tree 



Fibres of oak are also bound together by flattened bundles 

 of horizontal fibres that extend from pith to cambium, insinuated 

 between the vertical fibres. These are seen on a cross section of 

 a log as narrow, radiating lines starting from the pith and cutting 

 straight through heart wood and sap wood to the bark. A 

 tangential section of a log (the surface exposed by the removal 

 of a slab on any side) shows these "pith rays," or "medullary 

 rays" as long, tapering streaks. A longitudinal section made 

 from bark to centre, as when a log is "quarter-sawed" shows 

 a full side view of the "medullary rays." They are often an 

 inch wide or more in oak; these wavy, irregular, gleaming fibre 

 bands are known in the furniture trade as the "mirrors" of oak. 

 They take a beautiful polish, and are highly esteemed in cabinet 

 work. The best white oak has 20 per cent, to 25 per cent, of its 

 substance made up of these pith rays. The horny texture of its 

 wood, together with its strength and durability, give white oak 

 an enviable place among timber trees, while the beauty of its 

 pith rays ranks it high among ornamental woods. 



The grain of wood is its texture. Wide annual rings with 

 large pores mark coarse-grained woods. They need "filling" 

 with varnish or other substance before they can be satisfactorily 

 polished. Fine-grained woods, if hard, polish best. Trees of 

 slow growth usually have fine-grained wood, though the rule is 

 not universal. 



Ordinarily wood fibres are parallel with their pith. They 

 are straight grained. Exceptions to this rule are constantly 

 encountered. The chief cause of variation is the fact that tree 

 trunks branch. Limbs have their origin in the pith of the stems 

 that bear them. Any stem is normally one year older than the 

 branch it bears. So the base of any branch is a cone quite buried 

 in the parent stem. A cross section of this cone in a board 

 sawed from the trunk is a knot, its size and number of rings 

 indicate its age. If the knot is diseased and loose, it will fall 

 out, leaving a knot hole. The fibres of the wood of a branch are 

 extensions of those just below it on the main stem. They spread 

 out so as to meet around the twig and continue in parallel lines 

 to its extremity. The fibres contiguous to those which were 

 diverted from the main stem to clothe the branch must spread 

 so as to meet above the branch, else the parent stem would be 

 bare in this quarter. The union of stem and branch is weak 



