REIS^ARCH IN FOREST PRODUCTS 281 



In manufacturing structural timber, it is not practicable to cut the 

 timber so as to eliminate the effect of spiral grain. Consequently 

 where strength is an important consideration, spiral grain is a factor 

 which cannot be disregarded. In a series of tests on Douglas fir and 

 Sitka spruce, recently made at the laboratory, the effect of diagonal 

 and spiral grain upon strength was clearly established. ^ The weaken- 

 ing effect in the two species was the same. There was practically no 

 difference shown between the effect of diagonal grain and the effect of 

 spiral grain. The latter, however, is more difficult to detect and its 

 weakening effect is more likely to be augmented by season checks. 

 For both, it was found that some mechanical properties are affected 

 by very slightly sloping grain, while others are not materially influ- 

 enced until the slope becomes 1 :15 (one inch in a length of 15 inches) 

 or greater. For the aggregate of mechanical properties, the tests 

 established that from the start strength decreases with increase in slope 

 of grain. At a slope of 1 :20 the rate of decrease becomes very rapid. 



The data secured on this subject show conclusively that the tree in 

 the forest which is badly spiral grained is not suitable for structural 

 timber and should not be appraised on that quality. In order to arrive 

 at a close estimate of structural quality of a tract of timber, the ap- 

 praiser will likewise need a ready and reliable method of determining 

 spiral grain in standing trees. None exists at the present time. Some 

 trees, to be sure, show it by the very apparent twist in the bark ; others 

 do not. There may be some reliable surface or bark test which an 

 intensive study would quickly reveal or it may follow certain laws or 

 conditions of site which would find practical application in growth 

 studies and timber surveys. We know less about spiral growth than 

 we know about density or specific gravity of wood. Its cause has 

 never been definitely determined. What, if any, is the relation between 

 spiral growth and soil, light, climate, storms, injuries, species, wind 

 currents, etc.? Is it transmittead from tree to seed and does the 

 secret, after all, lie in the seed? Until some of these questions are 

 answered through comprehensive studies in the forest, the practice of 

 using seed from badly spiral grained trees in nursery and field sowing 

 and in regeneration from seed trees may well be questioned. 



In the foregoing, density and spiral grain have been selected merely 

 to illustrate more specifically the intimate relation between research 



2 "The Influence of Spiral Grain on the Mechanical Properties of Sitka 

 Spruce and Douglas Fir," by T. R. C. Wilson and R. F. Luxford, August, 1919. 



