10 ECONOMIC WOODS OF THE UNITED STATES 



off scale-like portions of the cortex — scale-bark. The results are 

 subject to very wide variation. 



In Platanus and Taxus the outer bark is shed annually in 

 the form of comparatively large, irregular, thin flakes which, falling 

 away, leave the surface smooth. In species of Betula thin, exfoli- 

 ating layers are produced, marked with horizontal lines of lenticels. 

 In many species of Pinus, the outer bark of mature trees is made 

 up of small, irregular scales in very intricate pattern. In Hicoria 

 ovata and H. laciniosa the outer bark peels off in long, flat, reddish- 

 brown strips, while several other species of the same genus have 

 bark that is not flaky. In a great many woody plants the layers 

 of bark persist for many years, and, as the stem increases in size, 

 become more and more cracked and furrowed. Such is the case 

 in Quercus, Robinia, Liriodendron, etc. In Sequoia, Juniperus, 

 Taxodium, and others of the Cedar group, the bark is character- 

 istically fibrous. These examples are sufficient to indicate the wide 

 variation in the bark and its importance as an aid to the identi- 

 fication of a specimen upon which any portion of bark remains. 



The bark of many trees is of high technical value. A very 

 great number are used for medicinal purposes. Tsuga and species 

 of Quercus possess barks which contribute very largely to our 

 tannin supply, upon which the leather industry is dependent. 

 Some barks contain coloring principles; others (e.g., Hicoria ovata) 

 are highly valuable for fuel. Birch bark was formerly used for 

 canoes. The inner barks of some woods (e.g., Tilia) are sometimes 

 used in manufacturing fibre cloth. The highly-developed corky 

 layers of Quercus suber furnish the cork of commerce. 



References 



Stevens, W. C. : Plant Anatomy, Philadelphia, 1907, pp. 37-39; 56-58; 72-82. 



DeBary, A.: Comparative Anatomy, pp. 108-114; 519-566. 



Sachs, Julius: Text-Book of Botany, Oxford, 1875, pp. 90-92. 



Gregory, E. L.: Elements of Plant Anatomy, Boston, 1895, pp. 133-142. 



Henkel, Alice: American Medicinal Barks, Bui. 139, U. S. Bu. Plant 

 Industry, 1909, p. 59. 



Hill, Arthur W. : Sieve-Tubes of Gymnosperms, Annals of Botany, Vol. 

 XV, Dec. 1901. 



: Notes on the Histology of the Sieve-tubes of Certain Angiosperms, 



Annals of Botany, Vol. XVII, Jan. 1903, pp. 265-267. 



Moeller, Joseph: Anatomie der Baumrinden; Vergleichendc Studien, Ber- 

 lin, 1882, p. 447. 



