2 INTRODUCTION 



odor, taste, and resonance; (6) those determined by measurement, 

 viz., density, weight, water content, shrinkage, swelling, warping, 

 and hygroscopicity; (c) relation of these properties to the use- 

 fulness of wood; and (d) their employment to some extent as aids 

 to identification. 



In Part II attempt is made to use the details of Part I in the 

 construction of an artificial classification of the economic woods 

 of the United States. Unimportant species have in some cases 

 been included where it was felt that their presence would not lead 

 to confusion. This classification has been prepared with two 

 objects in view: (1) for use in practice as a key for the identifica- 

 tion of unknown specimens ; (2) for use in the laboratory as a basis 

 for the comparative study of known specimens. 



As far as considered practicable, the distinctions in the key 

 are based on macroscopic features, those readily visible to the 

 unaided eye or with the aid of a simple lens magnifying 10 to 

 15 times. Owing to the great variation of wood it is usually 

 unwise to rely upon single diagnostic features, and for this reason 

 the descriptions have been extended to embrace all or most of the 

 important characters so far recognized. This method also permits 

 ready arrangement of the key or the fitting into it of additional 

 woods. 



In the woods of many genera the structural variations appar- 

 ently are not sufficiently distinct and constant to assure specific 

 identification. Good examples of this are afforded by the woods 

 of Pinus, Quercus, Hicoria, and Populus, where it is usually 

 difficult and very often impossible to do more than separate them 

 into groups. Accurate knowledge of the botanical and com- 

 mercial range of each species will often serve as a basis for further 

 subdivision of a group in which other distinctions are apparently 

 wanting. 



In preparing a specimen for careful examination either with 

 or without a lens it is highly desirable that a very smoothly cut 

 surface be obtained. If the knife used is not sharp, the cut surface 

 will be rough and the details of structure obscured. Cross sections 

 are, as a rule, the most valuable for comparative study, and in 

 making them it is very important that the plane of section be 

 as nearly as possible at right angles to the vertical axis of the 

 specimen. 



A compound microscope is necessary for the study of the 

 minute anatomy of wood. Sections for immediate observation 



