100 



PROPERTIES OF WOOD. 



without distinct colour, as in spruce, silver-fir, or birch ; 

 for heartwood contains no easily decomposable albuminous 

 constituents, and is always drier than sapwood. 



The presence of colouring- matter in heartwood, however, 

 increases durability greatly ; woods with coloured heart- 

 wood are always more durable than woods with heartwood 

 and sapwood of a uniform colour. Mayr also states that the 

 more intense the colour of the heartwood the more durable 

 it is. The imperfect heartwood, or internal sapwood, of oaks, 

 that occurs occasionally in annular zones in the midst of 

 perfect heartwood, characterised by the absence of thylosis 

 and tannin and the presence of starch and so faintly coloured 

 that it resembles sapwood, is no more durable than is normal 

 sapwood (cf. p. 142). 



In the following tabular form, woods are grouped according 

 to the colours of their heartwood : 



COLOURS OP HEARTWOOD. 



The exceptions in this list to Mayr's law about the corre- 

 spondence of high colour and durability are the two cypresses, 

 holly and alder. The heartwood of all cypresses contains 

 ethereal oil and is very durable ; in alder-wood, though 

 the heartwood is of a high colour, this is due to an oxidised 



