12 TECHNICAL PEOPEKTIES OF WOOD. 



of the greatest possible regularity is always a sign of good 

 quality in wood. Mohl states that nil nearly horizontal branches 

 have excentric zones ; in conifers the zones are broadest below, 

 and in broad-leaved trees, above. Large roots, on the contrary, 

 near their junction with the stem, have broader zones above 

 than below, and in their case numerous zones may be absolutely 

 wanting below. In no part of a tree is uniformity of the zones 

 less frequent than in the roots. 



The relative width of the annual zones in the upper and lower 

 parts of stems depends on whether the tree has grown in a dense 

 ■wood, or in the open. As long as a tree is growing vigorously 

 upwards, and, therefore, usually in a crowded wood, the annual 

 zones are broader in the upper part of the tree than below. 

 The stump of the tree immediately above the roots forms an 

 exception to this rule, for here the broadest rings are formed. 

 In the case of trees grown in the open, standards over coppice 

 and trees with large crowns reserved in high forest felling-areas, 

 the bole tapers from the base upwards ; the annual zones may 

 then be uniformly broad above and below, or even broader below 

 than above. In the case of dominated small- crowned trees, the 

 breadth of the annual zones is always broader above than below, 

 and sometimes, owing to an insufficiency of nutriment afforded 

 by their crowns, certain annual zones may be entirely absent 

 below. According, therefore, to the varying effects of density 

 of growth and admission of light on the tree at different periods 

 of its life, there may be a considerable difference in the width 

 of its annual zones. 



Section II. — Chemical and Physical Properties of 



^N'OOD. 



Freshly-felled wood is composed of woody substance, water, 

 and other materials, some of which are dissolved in the water. 



1. The woody skeleton of a tree — i.e., the walling of its 

 component organs — is chiefly composed of cellulose and lignin. 

 In the cambium-zone the walls of all elements arc of cellulose 

 (C,j Hio O.O, but during the very year of their formation the cell- 

 walls become thickened by layers of lignin (C,s !!.,„ 0,i), which 

 contains more carbon than cellulose : whilst cellulose is soft, 



