5S PROPERTIES OF WOOD. 



wanner Germany, region III w, has an average sp. weight 

 of between 38 and 41. The region II c the uppermost 

 spruce region, above which (III c) its cultivation becomes 

 impossible, as the top of II c is the upper mountain limit of 

 tree growth is characterised by the production of narrow- 

 ringed " resonance " wood (used for violins and other stringed 

 instruments), which has also a low sp. weight, as low as 40 

 and averaging 42. 



Thus Mayr's law explains the exceptions to the above prac- 

 tical laws regarding coniferous and broadleaved woods, which 

 are no exceptions to the greater natural law that is here 

 enunciated. 



This law that the sp. weight of woods is diminished when 

 they are produced beyond the optimum climatic region of the 

 species of tree is especially interesting to those who main- 

 tain that the strength of timber is dependent on the sp. 

 weight of wood. Schwappach (1897) is one of these, and 

 states that the transverse strength of timber diminishes as a 

 tree grows beyond its optimum region, which is only an asser- 

 tion of this law. Hartig states, that in beechwood neither the 

 breadth of the annual zones nor the climate exercises any 

 influence over its sp. weight, which depends solely on the age of 

 the tree; that in coniferous woods the sp. weight increases as 

 long as the annual increment is increasing and diminishes 

 when the latter decreases. 



(5) The tending of a crop of trees must influence the sp. 

 weight of the wood, for cleanings, thinnings, and a free posi- 

 tion of a tree are merely alterations in the environment of 

 trees as regards light and heat. In a dense wood dominated 

 trees suffer from a deprivation of both light and heat. Giving 

 a tree a free position removes it, as it were, from a cooler to a 

 warmer climate, while for a suppressed tree these conditions 

 are reversed. Hence, by thinning, the optimum climate for 

 oak may be approximated to or receded from in the case of 

 spruce. Spruce, for instance, grown in II w or III w, when 

 suppressed has, it is well known, heavier wood, while suppressed 

 oak grown in II c or III c has lighter wood. 



(6) The specific weight varies with genera, species, or indi- 

 vidual trees. As regards genera, woods that are produced 



