DEFECTS AND UNSOUNDNESS. 35 



2, Knottiness and exaggerated waviness ofjihre. 



A knot is produced by an irregular course of the fibres round 

 an independent centre of growth, such as branches or a dormant 

 bud. Owing to the greater pressure occurring at these places, 

 the fibres are also packed more closely together, and compose a 

 denser and harder tissue than that surrounding the knot. The 

 simplest knot is that formed by a single branch that has attained 

 normal development. In a broad-leaved tree such a knot, as long 

 as there is no decay present, detracts from the value of the wood 

 only when thin planks of good quality are required. It is, how- 

 ever, different with conifers, since the wood of the branches is so 

 entirely dissimilar from that of the stem, that if a branch has not 

 fallen off while it was still only a twig, it runs radially through 

 the tissues of the stem merely like a plug, which ultimately shrinks 

 from loss of moisture until it is easily detachable, even falling of 

 itself out of boards and planks. Such knots are known as loose 

 knots. A burr, so much sought after by the turner and cabinet- 

 maker, is a complex knot formed at points where dormant buds 

 show abnormal vigour without being able to develop into branches. 

 In species extremely rich in such buds, as in Celtis spp. and ma- 

 ples, the burrs may attain the size of a man's head. Epicorms pro- 

 duce knotty tissue along the entire length of the stem. If they 

 are numerous without ever getting beyond the size of small twigs, 

 an extremely handsome mottling may be thereby produced. The 

 presence of numerous but weakly-formed latent buds gives rise to 

 a wavy course of the fibres, making the wood well adapted for 

 ornamental purposes. This defect, when exaggerated, always 

 diminishes transverse strength very considerably, and usually 

 renders the wood unsuitable for purposes in which heavy strains 

 have to be withstood. 



3. Twisted fibre. 



In this defect the course of the grain of the wood follows a 

 spiral round the stem, making with the vertical an angle which 

 may sometimes exceed 40°. In most cases this angle increases 

 with the diameter of the stem, the spiral growth being not at all 

 apparent in young saplings. This defect is due to the fibres in 

 each new layer of wood being longer than those in the preceding 

 layer. The cause of this abnormal growth is not yet exactly 

 known. What we know regarding it is that it is hereditary, that 

 certain species {Boswellia serratq, Hardwickia binata, &c.), are more 

 liable to it than others, that it may be prod uced by the wind acting. 



