ECONOMIC WOODS OF THE UNITED STATES 39 



There are numerous woods which present on longitudinal sec- 

 tion (particularly the tangential) fine, delicate cross lines or stripes 

 sometimes called "ripple marks." The distance between these 

 markings varies from 0.11 to 0.50 mm., and is fairly constant 

 for a species. On some woods (e.g., JEsculus octandra, Swietenia 

 mahagoni, and Diospyros virginiana [Plate IV, Figs. 4, 5]), these 

 lines are very clear and distinct to the unaided eye; on others 

 (e.g., Tilia americana, T. pubescens, and T. heterophylla) they are 

 near the limit of vision, or again, they are invisible without the 

 lens. In most species showing these markings the feature is con- 

 stant and of considerable importance for diagnostic purposes, 

 though in a few species (e.g., Swietenia mahagoni) the same piece 

 of wood may show the markings in one place and not in another. 



This cross-striping of a wood is due (1) to the arrangement 

 of the rays in horizontal series, or (2) to the tier-like ranking of 

 the wood fibres, vessel segments, or other elements, or (3) to a 

 combination of (1) and (2) (Plate IV, Figs. 4, 5). The lines 

 resulting from the horizontal seriation of the rays is usually more 

 conspicuous and of more common occurrence than those in (2). 

 In the combination of the two forms, which is very common, the 

 junction of the vessel segments or of the fibres is usually between 

 the rays (Plate IV, Fig. 5). 



This peculiar arrangement of wood elements is also evidenced 

 on cross section. Where the rays are in perfect horizontal 

 seriation a section between two tiers shows an entire absence of 

 rays. In most instances, however, it results in gaps of irregular 

 width, depending upon the regularity of the stories. Where the 

 rays are much wider near the middle than at the margin, their 

 apparent width when viewed transversely will show considerable 

 variation, according to the relative location of the plane of sec- 

 tion. Where the fibres are arranged in tiers, their apparent size 

 is affected in a similar manner. 



References 



RECORD, S. J. : Tier-like Arrangement of the Elements of Certain Woods, 

 Science, January 12, 1912, pp. 75-77. 



VON HOHNEL, FRANZ RITTER: Ueber stockwerkartig aufgebaute Holzkorper. 

 Sitzb. d. Math. Naturw. Classe d. kaiserlichen Akademie d. Wissen- 

 schaften, Vol. LXXXIX, Part 1, Wien, 1884, pp. 30-47. 



