680 A NATIONAL PLAN FOR AMERICAN FORESTRY 



of the direct attacks may be regarded as falling under the broad head- 

 ing of " Properties. " This group includes not only the determination 

 of the values for the physical and mechanical properties, but also 

 special studies of the chemical composition and structure of wood 

 and of the interrelationships of all these fields. Probably the most 

 complete investigations have been carried out in the field of physical 

 and mechanical properties. The strength properties, specific gravity, 

 and total shrinkage have now been determined for all the important 

 native species, and detailed studies have been made of various factors 

 affecting strength, such as specific gravity, moisture content, and 

 previous conditions of treatment or use. This basic information, 

 together with the technique and experience gained in obtaining it, 

 has been applied in the "Grading and Selection" and "Design and 

 Adaptation" groups of investigations previously mentioned. 



The field of chemical composition is much less definite and clear 

 cut, both in its methods of research and in the use of the results. The 

 comparative chemical composition of a number of species has been 

 determined, and some work has been done on relationships between 

 chemical composition and certain physical properties. In the present 

 state of knowledge, however, chemical composition can be stated only 

 in terms of various empirical groups or " constituents" such as 

 cellulose and lignin that are in themselves variable and of only partly 

 known composition. Much of the investigative work on chemical 

 composition is therefore in the stage of developing methods for more 

 completely determining the composition. 



The investigations of structure have progressed far beyond the 

 original stage in which the emphasis was placed on microscopic 

 structure as a means of identification of species. Not only have ad- 

 vances been made in the character and detail of microscopic and sub- 

 microscopic structure, but the field has been enlarged to include 

 measurements of structure of submicroscopic sizes. Beginnings have 

 also been made in determing the effect of these microscopic and 

 submicroscopic structures on the properites. 



Out of a real understanding of the chemical and physical properties 

 of wood may well come some of the most revolutionary changes in 

 the form and character of its future use. What has happened in the 

 coal-tar field as a result of research is one example of the possibilities. 



GROWTH CONDITIONS 



The effect of growth conditions on wood quality involves both 

 silviculture and utilization. Growth conditions vary so widely and 

 there are so many lands of wood quality that this is a very broad 

 field of research. Only a few of the simpler and most readily con- 

 trolled conditions of growth have as yet been studied, and the determi- 

 nation of the effects has been confined largely to the specific gravity of 

 the wood. 



The undoubted future trend toward a more and more intensive 

 silviculture, and the increasing intensity of competition between 

 wood and other materials which will tend to make the requirements 

 on wood more and more exacting, will enhance the value of a thorough 

 understanding of the relationship between growing conditions and the 

 character of the final product. 



