ECONOMIC WOODS OF THE UNITED STATES 43 



because growth begins considerably later at the base. The 

 strongest, densest, and toughest timber is that grown in the open 

 where conditions are favorable to rapid growth. 



In diffuse-porous woods, such as Acer, Betula, Liriodendron, 

 and Fagus, there seems to be no definite relation between ring 

 width and density. In Gymnosperms, as a rule, wood of medium 

 to fine grain contains a greater proportion of late wood and con- 

 sequently possesses greater weight and strength than when very 

 fine or very coarse grained. 



In this connection the following statement of H. Mayr * is 

 interesting: " Assuming identity of soil, the specific weight and 

 hardness of wood decreases with distance from the optimum 

 climate of its production both toward cooler or warmer climates. 

 It is indifferent whether the annual zones consequently increase 

 or decrease in breadth, or whether the wood is broadleaved or 

 coniferous. Within the natural habitat of any tree the centre 

 of its habitat produces the heaviest and hardest wood." 



Various theories have been advanced to explain the formation 

 of early and late wood. Penhallow (following Sachsf) says that 

 the elements of the early wood are "formed under a minimum 

 tension in consequence of which they rapidly attain to relatively 

 great size, and it is therefore found that the first tissue of the 

 season is always most open. In consequence of the great excess 

 of nutrition supplied during this period of growth, and the very 

 rapid process of construction which follows, secondary growth 

 of the walls is limited, and these structures remain thin, while the 

 lumens are correspondingly large." 



R. Hartig maintains that the thin-walled early wood is due 

 to poorer nutrition and the necessity of forming conductive tissue, 

 while thick-walled late wood results from better nutriment during 

 the warm and sufficiently moist summer. Wieler, on the other 

 hand, claims that the more unfavorable the conditions of nutri- 

 tion, the slower the development of assimilating organs, hence the 

 more late wood. 



References 



PENHALLOW, D. P. : The Relation of Annual Rings to Age, Can. Records of 

 Sci., Vol. I, p. 162. 



: North American Gymnosperms, pp. 24-32. 

 DEBARY, A.: Comparative Anatomy, pp. 475-478; 500-507. 



*Schlich's Manual of Forestry, Vol. V, rev. ed., p. 54. 

 t Text-Book of Botany, p. 575, foot-note. 



