38 



ECONOMIC WOODS OF THE UNITED STATES 



TRABECULiE: SANIO S BEAMS 



In radial and cross sections of the wood of all Gymnosperms 

 it is not unconunon to find small bars stretched across the luniina 

 of the traoheids 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 



B 



Fig. 12. — TrabcculjE in Pinus murrayana (lodgepolo pine). A, cross section 

 ghowing tracheids with beams (tr.) crossing the middle row in tangential scries; 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. cit.), 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. 

 MuLLER, Carl: Uebcr die Balken in den Holzelementen der Coniferen, 



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



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

 Raatz, Wilhelm: Die Stabbildungen in secundiiren Holzkorper der Baume 



und die Initialontheorie, Jalirb. fiir Wissenschaftliche Botanik, Vol. 



XXIII, 1892, pp. 567-630. 



