ECONOMIC WOODS OF THE UNITED STATES 31 



In Leitneria floridana numerous resin ducts are found at the 

 margin of the pith, but are not in the wood. The epithelial 

 cells are thick-walled and in a single layer. 

 Resin ducts are features of great system- 

 atic importance. Their presence in Pinus, 

 Picea, Larix, and Pseudotsuga serves as an ade- 

 quate basis for separating the woods of these 

 four genera from other Gymnosperms. Their 

 relative size, distribution, and occurrence, and 

 the character of the epithelium, whether thick 

 or thin-walled, are features made use of in 

 specific diagnoses. 



References 



Penhallow, D. P.: North American Gymnosperms, 

 pp. 109-153. 



Kirsch, Simon : The Origin and Development of Resin 

 Canals in the Coniferse with Special Reference 

 to the Development of Tyloses and their Co- 

 relation with the Thylosal Strands of the Pteri- 

 dophytes, Proc. Royal Soc. of Canada, 1911. 



Foxworthy, Fred W.: Philippine Dipterocarpacese. 

 Phil. Journal of Science, C. Botany, Vol. VI, 

 No. 4, Sept. 1911, pp. 231-287. 



Solereder, H. : Anatomy of the Dicotyledons, Vol. 

 II, pp. 1101-1102. 



Tschirch, A.: Die Harze und die Harzbehalter, Vol. 

 II. 



n 



\r.t. 



All wood elements when first formed are 

 limited by a very thin cellulose membrane, 

 the primary wall. Subsequent development 

 involves an internal thickening which is com- 

 posed very largely of lignin, the secondary wall. 

 This thickening may proceed uniformly, or, as 

 is usually the case, small gaps, called pits, 

 occur. A pit is merely an unthickened 

 portion of the cell wall. Pits are of two 

 principal types, simple and bordered (Fig. 11). 



Fig. 9. — Tangen- 

 tial section of a fusi- 

 form ray from Pinus 

 ponderosa (western 

 yellow pine); r. d., 

 horizontal resin 



duct; e., epithelial 

 cells; r. t., ray tra- 

 cheids; the remain- 

 der of the cells are 

 ray-parenchyma 

 cells. Magnified 



about 200 diameters. 



