66 ECONOMIC WOODS OF THE UNITED STATES 



of a piece of dry wood. The character of the demarcation in color 

 between heartwood and sap wood, whether sharp or gradual, is 

 often an important feature, though usually not exhibited on very 

 small specimens. The character and amount of coloring matter ex- 

 tracted by treatment with NaOH is sometimes made use of in 

 identification. 



Abnormal discoloration of wood usually denotes disease. 

 The black check in Tsuga heterophylla is the result of insect 

 attacks. The reddish-brown streaks so common in Hicoria are 

 mostly the result of injury by birds. The bluing of the sap wood 

 of many soft woods is due to the attacks of fungi. Many fungi 

 can be determined specifically by the characteristic color they 

 impart to wood. 



References 



ROTH, F.: Timber, Bui. 10, U. S. Div. For., p. 24. 



GAYER, K. : Schlich's Manual of Forestry, Vol. V (1908), pp. 43-46. 



HANAUSEK, T. F.: The Microscopy of Technical Products. 



MELL, C. D. : Fustic Wood, Cir. 184, U. S. Forest Service. 



EXNER, W. F. : Lorey's Handbuch der Forstwissenschaft, Vol. II, pp. 105-111. 



NORDLINGER, H. : Die Technischen Eigenschaften der Holzer, pp. 46-51. 



GLOSS OR LUSTRE 



Gloss or lustre of wood refers to the manner in which light 

 is reflected by the wood elements. The fibres of the bast are 

 more lustrous than the wood fibres. The fibre of flax is highly 

 lustrous, while that of cotton is dull. Similar variation occurs 

 in the elements of different woods. For example, the woods of 

 Fagara, Rhus, and Toxylon are highly lustrous; those of Acer, 

 Betula, and Robinia less so; while those of Juglans nigra, Sequoia, 

 Fagus, and Platanus are dull. The wood of Picea possesses a 

 pearly lustre; that of Guaiacum and Taxodium is rather greasy 

 or waxy. In some cases the lustre varies in different parts of 

 the wood or on different planes. The late wood of Juniperus 

 virginiana exhibits a frosted lustre on tangential surface. The 

 rays on quarter-sawed wood of several species, particularly the 

 oaks, are so lustrous in contrast tq the other elements as to give 

 rise to the term "silver grain," while the rays themselves are 

 called " mirrors." Woods with high natural lustre are usually 

 capable of taking a high polish. Lustre is a sign of soundness 



