Cross sections of swamp white oak fOii<'>'''"S platanoides) showiui; 

 tlie effect of rate of growth. The specimen ou the right grew rapidly 

 and produced a dense, hard and strong wood with few and small pores, 

 while that on the left grew very slowly and is very porous and light. 

 Both magnified twenty diameters". 



(.'ross sections of hur uuU ((Jucrctis maciutai ija) showing the eft'uct 

 of rate of growth. The specimen on the left grew slowly and is made 

 up of large pores and few wood fibers, while the other grew more 

 rapidly and has smaller pores and a very high proportion of wood fibers 

 which give weight and strength. Both magnified twenty diameters. 



' eTOisa35i aiia*m:^)iw^m»*'H^itw ^ttt*H^*;i;tmi^^ 



W^ood Growth and Properties 



Other things being equal, the strength of wood varies with the 

 weight, that is, the heavier the wood the stronger it is. This of 

 course is true only in case greater weight is due to increase in 

 the amount of wood substance. A wood heavy with resin or other 

 infiltrated substance is not necessarily stronger than, perhaps not 

 as strong as, a lighter specimen free from such materials. 



The weight of wood substance is practically the same for all 

 woods, being a little greater than one and one-half times that of 

 water. The reason that any wood floats is that the air imprisoned 

 in its cells and cavities buoys it up. Leaving out of consideration 

 infiltrated substances, the reason one wood or piece of wood is 

 heavier than another of equal moisture content is because it con- 

 tains a greater proportion of wood substances — in other words, is 

 denser. 



It is a matter of common experience that specimens of wood 

 of the same species and even from the same tree exhibit great 

 variation in density and consequently in strength and stiffness. 

 It is not always possible to explain why this is true. In the case 

 of certain woods, however, there exists a pretty definite relation 

 between rate of growth and the technical properties, while in 

 others no such relationship can always be inferred. 



Hardwoods may, on a basis of structure, be readily grouped into 

 two classes, viz., ring-porous and diffuse-porous. As previously 

 stated in Hardwood Eecoed a tree increases in diameter by the 

 formation between the old wood and the inner bark of new woody 

 layers which envelops the entire stem, branches and roots. In 

 cross section, as on the end of a log, these layers appear as con- 

 centric zones or rings. Each ring consists of two more or less 

 readily distinguishable parts, the inner, ca.lled early wood (spring 

 wood), and the outer, or late wood (summer and autumn wood). 



In ring-porous woods such as oak, chestnut, ash, black locust, 

 catalpa, mulberry and hickory the larger vessels or pores (as they 

 are called in cross section) become localized in the early wood, 

 thus forming a region of more or less open and porous tissue, 

 while the wood fibers preponderate in the late wood, thereby pro- 

 ducing a much denser layer. It is to these wood fibers that hard- 

 woods owe their weight and strength while pores (which are es- 

 sentially holes) are a source of weakness. To quote a British ex- 

 pert: "From a timber merchant's point of view the question of 

 growth is an important one and it must be borne in mind that 



it is wood and not cavities the user wants — the greater the pro- 

 portion of jjores the less wood and the less weight and strength." 

 In good oak these large vessels of the early wood occupy from 

 six to ten per cent of the volume of the log while in inferior ma- 

 terial they may make up twenty-five per cent or even more. The 

 late wood of good oak, except for radial grayish patches of small 

 pores, is dark-colored and firm and consists of thick-walled fibers 

 which form one-half or more of the wood. In inferior oak such fiber 

 areas are much reduced in both quantity and qualit.y. Such varia- 

 tion is very largely the result of rate of growth. 



In diffuse-porous woods the vessels or pores are scattered 

 throughout the ring instead of collected in the early wood. Ex- 

 amples of this kind of wood are maple, gum, yellow poplar, buck- 

 eye, basswood, beech, cottonwood and willow. In such woods, as 

 well as in conifers or softwoods, there appears to be no definite 

 relation between rate of growth and the quality of the wood. 



In ring-porous woods of good growth it is usually the middle 

 portion of the ring in which the thick-walled, strength-giving fibers 

 are most abundant. As the breadth of ring diminishes, this mid- 

 dle portion is reduced so that very slow growth produces com- 

 paratively light, porous wood composed mostly of thin-walled ves- 

 sels and wood parenchyma. Wide-ringed wood is often called 

 "second-growth" and in the manufacture of articles where 

 strength is an important consideration such "second-growth" 

 wood is preferred. 



This is particularly the case in the choice of hickory for handles 

 and spokes. Here not only strength but toughness and resilience 

 are important. The results of a large number of tests on hickory 

 by the United States Forest Service show that "the work or 

 shock-resisting ability is greatest with wide-ringed wood that has 

 from five to fourteen rings per inch; is fairly constant from 

 fourteen to thirty-eight rings, and decreases rapidly from thirty- 

 eight to forty-seven rings. The strength at maximum load is not 

 so great with the most rapid-growing wood; it is maximum with 

 from fourteen to twenty rings per inch, and again becomes less 

 as the wood becomes more closely ringed. The natural deduction 

 is that wood of first-class mechanical value shows from five to 

 twenty rings per inch, and that slower growth yields poorer stock. 

 Thus the inspector or buyer of hickory should discriminate against 

 timber that has more than twenty rings per inch. Exceptions 



—37— 



