386 Kansas Academy of Science. 



THE MEDULLARY RAY AS AN ELEMENT OF 

 STRENGTH IN STRUCTURAL TIMBER. 



By Frank E. Jones. Assistant Professor of Carpentry, University of Kansas. 



IN the growth of the exogenous tree, the prime function of the 

 medullary ray, as is well known, is the distribution of thee lab- 

 orated foodstufPs to the active cells in the living zone, or sap-wood, 

 as it is called. 



As the successive annual layers are added and the protoplasm 

 migrates outward, leaving the cells empty, these pith rays eventu- 

 ally harden into plates radiating outward in every direction from 

 the heart of the stem to the bark, apparently forming part of the 

 stiffening framework or mechanical support of the tree. 



Whether or not the pith ray is really an element of strength is a 

 much disputed question. Since most of the ruptures or radial 

 cracks that occur in seasoning a section of an entire log are found 

 to be either in the middle of a pith ray or immediately adjacent, 

 many are led to believe that the pith ray is the weaker element. 



There is a very apparent difference in the shrinkage of a tangen- 

 tial section or along the annual layers over that of the radial section, 

 or in the direction of the medullary ray. This is accounted for in 

 various ways by the different writers on the subject. The reason 

 that seems to me to.be the most logical is, in substance, as follows: 



The shrinkage is directly due to the fact that as a section of 

 wood gives up its moisture the walls of each individual cell become 

 thinner, its diameter smaller, and as a consequence the piece as a 

 whole shares in this diminution of size. Since the length of a fiber 

 is a great many times its diameter, the effect of this contraction 

 along the length of the fiber is inappreciable. The cells of the 

 medullary rays have their length at right angles to the direction 

 of the wood fibers, and this opposition prevents the latter from 

 shrinking as much as they otherwise would, while between the rays 

 or in the direction of the annual layers they are left free to con- 

 tract as they will. With this theory in mind the first experiments 



were made. 



Since hygroscopicity makes the expansion of dry wood-fibers by 

 the absortion of moisture practically equal to the contraction of 

 unseasoned wood by the evaporation of its moisture, this was made 

 use of for the sake of making comparisons in the first experiments. 

 Pieces of equally dry wood were cut to exactly the same dimensions, 



