CHEMICAL AXD PHYSICAL PROPERTIES. 15 



more abundant in the younger than in the older wood, [though 

 this is not always the case with woods in tropical countries, where 

 calcium carbonate and calcium phosphate sometimes fill the 

 lumina of wood-vessels in the heartwood. — Tr.] In European 

 trees they joredominate in the bark, bast and cambium, and are 

 most abundant in the upper and outer parts of the tree. 



4. Heartwood and Sapicood.* 



[The inner and older zones of wood are generally termed heartwood 

 and the outer and younger zones, sapwood or alburnum; these 

 should be distinguished by an actual change in the tissues. Thus, 

 in true heartwood, the walls of the woody elements are charged, and 

 frequently their lumina filled, with colouring matter or with gummy, 

 resinous or mineral substances, which render the heartwood heavier 

 than the sapwood ; if there be any at all, much less starch or protein 

 is contained in the heartwood than in the sapwood, and the former 

 is therefore less liable to attacks of insects and to decay than the 

 latter. Sapwood is fully formed, structurally, from the cambium, 

 but continues to carry-on processes vital to the tree, such as convey- 

 ing water and reserve nutritive material. Heartwood, on the 

 contrary, may still act as a reservoir for water, but has apparently 

 lost all vital functions, and cannot be usefully injected with creosote 

 or other preservative materials. As a rule, sapwood contains more 

 water than heartwood, and is usually less darkly coloured than the 

 latter ; but these distinctions do not hold for many kinds of wood, 

 and the mere presence of colouring matter in the central parts of 

 certain woods does not indicate any physiological change. 



In some trees, the transformation of sapwood into heartwood is 

 very rapid, there being only two or three annual zones of sapwood, 

 whilst in others, as in the oak, there may be from 12 to 45 zones in 

 the sapwood. 



It is, therefore, proposed to classify timber as follows : — 



(a) Heartwood trees, with true heartwood as described above. 



i. Broad zones of sapivood. 

 Oaks, elms, walnut, Scotch and other pines. 



ii. Narrow zones of sapioood. 

 Sweet-chestnut, robinia, mulberry, fruit-trees, larch, yew. 



* Gayer's account of heartwood and sapwood in trees is less detailed. 



