PROPERTIES OF WOOD. 



shrinking of drying wood shows the reverse process, as the 

 water of inhibition gradually leaves the cell-wall. 



If wood were thoroughly homogeneous, as is a wedge of 

 clay or cement, it would stretch or contract equably on all 

 sides ; as it is composed of elongated organs, which alter their 

 shape much less along their longer axis than radially or tangen- 

 tially, the alteration in the whole piece of wood is unequal in 

 different directions. It has been proved that in passing from its 

 green volume to its air-dry volume the length of a wooden rod 

 shrinks on the average by O'l per cent, of its original length, 



whilst in the radial direction, 

 along the medullary rays, the 

 shrinking is from 3 to 5 per 

 cent., and in the tangential 

 direction, tangential to those of 

 the annual rings, 6 to 15 per 

 cent. The greater contraction 

 along the tangents may be 

 studied on any freshly cut piece 

 of wood, as it causes the wood 

 to crack perpendicularly to the 

 direction of shrinking, that is 

 radially. The effect of unequal 

 shrinking is specially noticeable 

 in planks ; the more tangen- 

 tially they are cut the more 

 they contract in width, but the 

 nearer the sections are to the 

 radius of the stem, the less the shrinkage (Fig. 30). 



The fact that, in spite of the saturation of cell-walls with 

 water, if the amount of water in the cell-lumina be reduced 

 there is a shrinkage in the walls of wood-tissue, is true also 

 for standing trees. Kaiser and Friedrich, by measuring trees 

 in daytime at the moment of greatest transpiration and at 

 night when transpiration is arrested and the tissues are gorged 

 with water, have shown that their diameters vary in width. 

 Mayr's observations alsoshowthat the length of a tree fluctuates 

 with its water-contents. 



The amount of shrinkage depends on : 1. The water-con- 



Fig. 



] > I 'lank from the centre of the 

 stem with a section nearly 

 r;i<lial. c (I Planks with more 

 or less tangential sections, which 

 shrink and warp more than a 1>. 



