284 Forestry Quarterly. 



ber and distribution of the vessels which accompany them, or in 

 the case of the conifers by the particular structure of the early 

 wood of the growth rings; in other words by the distribution of 

 the elements of unequal resistance in the body of the wood. 



The shortening of a block in compression is obtained either by 

 the transverse bending of the fibers with a formation of a swell- 

 ing of rupture, or by curvature of the fibers with the bulging of 

 one or more of the faces of the test block. Among conifers and 

 in a general way among the ligneous species with a fine texture 

 and homogeneous structure the first type is commonly met with. 



In addition to the anatomical structure the degree of humidity 

 of the wood has an influence on the mode of rupture, fresh or 

 green wood behaving quite dififerently from that which is air- 

 seasoned or kiln-dried. The author refers to the experiments 

 made by the U. S. Forest Service as recorded in the appendix 

 of bulletin 70 and states that his own observations are in com- 

 plete accord with Tiemann's. 



It is Jaccard's contention that the direction of rupture is not 

 influenced by the distribution of the medullary rays, which simply 

 curve with the bundles of fibers to which they are attached. 

 When the compression is continued until failure results, the 

 rupture is in the form of longitudinal rents through the fibrous 

 mass aflfecting equally the rays, the elements of which break in the 

 median plane and not along the plane of attachment to the wood 

 fibers. In fact it is very rare to observe a true detachment of 

 the walls along the middle lamella. 



The folding of the fibers or of the tracheids is accompanied 

 by characteristic alterations of their walls. The latter seem to 

 split into thin lamellae or sheets. Under high magnification the 

 walls of the tracheids cut longitudinally present a fibrile or 

 thread-like structure without definite arrangement, while on cross 

 section numerous concentric strata are visible. The author con- 

 cludes that the successive strata of growth of the fiber wall al- 

 ternate with planes of less cohesion which under the compressive 

 stress separate by shear or cleavage. 



As previously shown by Tiemann the folding of the tracheids 

 may be materially influenced by the bordered pits which form 

 weak places in the wall. Sometimes the pit is crushed and again 

 only deformed. 



In test blocks of certain broad leaf species such as Platanus 



