[Reprinted from SCIENCE, N. S., Vol. XXXV., No. 889, Page* 75-77, January 12, 1912] 



TIER-LIKE ARRANGEMENT OF THE ELEMENTS OF CERTAIN WOODS 



THERE are numerous woods which present 

 on longitudinal section (particularly the tan- 

 gential) fine, delicate cross lines or stripes 

 sometimes called " ripple marks." The dis- 

 tance between these markings varies from 

 0.11 to 0.50 mm., and is fairly constant for a 

 species. On some woods (e. g., Msculus 

 octandra Marsh., Rwietenia mahagoni Jacq., 

 Bombax insigne Wall, and Pterospermum 

 diversifolium Blume.) these lines are very 

 clear and distinct to the unaided eye; on 

 others (e. g., Tilia americana L., T. pubescens 

 Ait., Pterocarpns indicus Willd. and P. dal- 

 ~bergioides Roxb.) they are near the limit of 

 vision or again (e. #., Guaiacum sanctum L. 

 and G. o-fficinale L.) they are invisible with- 

 out the lens. Tn most species showing these 

 markings the feature is constant and of con- 

 siderable importance for diagnostic purposes, 

 though in a few species (e. a., flwietenia 

 mdhagoni) the same piece of wood may show 

 the marking's in one place and not in another. 



This cross-stripinsr of a wood is due (1) to 

 the arrangement of the ravs in horizontal 

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

 wood fibers, vessel segments or other elements, 

 or (3) to a combination of (1) and (2). The 

 lines resulting: from the horizontal seriation 

 of the rays is usually more conspicuous and of 

 more common occurrence than those in (2). 

 Tn the combination of the two forms, which 

 is very common, the junction of the vessel 

 segments or of the fibers is usually between 

 the rays. 



This peculiar arrangement of wood ele- 

 ments is also evidenced on transverse section. 



Where the rays are in perfect horizontal seria- 

 tion 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 con- 

 siderable variation, according to the relative 

 location of the plane of section. Where the 

 fibers are arranged in tiers their apparent size 

 is affected in a similar manner. According to 

 von Hohnel 1 the fibers in Bocoa provacensis 

 Aubl., and a few other tropical woods with 

 cross-striping are also radially disposed, and 

 when the plane of cutting passes near the 

 junction of two tiers the section shows large 

 and small cells in alternate radial rows. The 

 small cells are sections of the tips of the fibers 

 forced by growth between their upper and 

 lower neighbors. It is von HohnePs theory 

 that their appearance in alternate radial rows 

 instead of alternately in the same row is the 

 result of the pressure of the bark upon the 

 cambium while the fibers were being formed. 

 Tier-like arrangement is most common in 

 tropical woods and has been found fairly 

 characteristic of the families CsesalpiniaceaB 



1 Von Hohnel, Franz Hitter, ' ' Ueber stockwerk- 

 artig anfgebaute Holzkb'rper, ' ' Sit&ungsberichte 

 der Mathematische-Naturwissenschaftlichen Classe 

 der kaiserlichen ATcademie der Wissenschaften, 

 Vol. 89, Part 1, Wien, 1884, pp. 30-47. Von 

 Hohnel, Franz Ritter, "Ueber den etagenformigen 

 Anfbau einiger Holzkb'rper, " Berichte der 

 Deutschen Botanischen Gesellschaft, Vol. II., 

 Berlin, 1884, pp. 2-5. 



