38 



ECONOMIC WOODS OF THE UNITED STATES 



trabecule: sanio s beams 



In radial and cross sections of the wood of all Gymnosperms 

 it is not uncommon to find small bars stretched across the lumina 

 of the tracheids from one tangential wall to another. Occasionally 

 they appear in isolated tracheids, but usually traverse in the same 

 direction the entire length of a long radial series (Fig. 12). While 

 the most common form of bar is a simple cylinder slightly enlarged 

 at the points of contact with the cell wall, they may occur as 



Fig. 12. — Trabecular in Pinus murrayana (lodgepole pine). A, cross section 

 showing tracheids with beams (tr.) crossing the middle row in tangential series; B, 

 radial section showing beams (tr.) which become wider in late wood. Magnified about 

 150 diameters. 



double bars or as constricted plates. These bars, which were 

 first described by Sanio (loc. clt.), originate in the cambium and 

 result from the partial resorption of folds in the cell wall. Their 

 function is unknown. Owing to their general distribution through- 

 out all species of Gymnosperms they are without taxonomic value. 



References 

 Sanio, Carl: Botanische Zeitung, Vol. XXI, No. 14, 1863, p. 117. 

 MtJLLEH, Carl: Ueber die Balken in den Holzelementen der Coniferen, 



Bericht ii. d. Verhandlungen d. achten General-Versammlungen d. 



deutsehen Botanischen Gesellschaft, 1890, pp. (17) to (46). 

 Raatz, Wilhelm: Die Stabbildungen in seeundaren Holzkbrper der Baume 



und die Initialentheorie, Jahrb. fur Wissenschaftliche Botanik, Vol. 



XXIII, 1892, pp. 567-636. 



