238 JOURNAL OF FORESTRY 



brittle wood on the contrary sustains a much greater load without break- 

 ing, and will upon failure, give more slowly, with a prolonged cracking 

 sound and exhibit a lacerated splintered appearance as illustrated in 

 figure 12 which pictures the broken end of a strong specimen of the 

 same species. Some wOods like California redwood are habitually 

 brash, while such a quality is exceptionally rare in hickory or white 

 oak. Other trees show variation in this respect since some timbers are 

 brash and others strong. 



The process used in testing wood for brashness presents only a simple 

 problem in physics which may be found fully explained in any text on 

 the mechanical properties of timber, but as this phase of the problem 

 is really outside the province of this paper a few brief remarks will 

 serve to clarify the connection as between weakness and anatomical 

 structures. Whenever a strong timber is tested either by impact, static 

 bending, or compression, deformation iS proportional to the stress 

 applied up to a definite point known as the fiber stress at the elastic 

 limit. Beyond this, as recorded on the stress-strain diagram, deforma- 

 tion progresses at a faster rate until rupture takes place as that point 

 determined upon as the modulus of rupture. A brash wood tested 

 under such conditions shows a proportionally low opposition to flexure 

 or pressure, and the interval between the fiber stress at the elastic limit 

 and the modulus of rupture is small, and the failure abrupt. 



Unlike many structural materials, wood is a product of growth, and 

 not one fashioned by man for his own uses. In common with all living 

 organisms it consists of cells which have become modified during the 

 evolution of the arborescent type in ways which are suited to serve the 

 functions necessary for existence and growth. This life process may 

 for convenience be divided into three main classes in so far as it has 

 a bearing upon the stelar anatomy. The vertical elements which in the 

 conifers consist of tracheids serve both for conduction and support. 

 Scattered among the tracheids in the Pineous group are tubular pas- 

 sages lined with parenchyma which aid in the transportation and stor- 

 age of resin. These canals are a source of weakness only as they 

 diminish the strengthening tracheary content. The only other vertical 

 elements in soft woods are the septate parenchymateous storage cells 

 evolved during past geological ages to conserve the food supply during 

 a resting season. They affect timber in much the same way as do 

 resin canals only to a less extent. Interjected between the longitudinal 

 cells there are radial plates of thin-walled storage tissue, the wood rays. 



