28 



ECONOMIC WOODS OF THE UNITED STATES 



pitch pines, affords a constant diagnostic feature of much im- 

 portance in separating the two great groups of Pinus (Figs. 4-7). 

 Ray-parenchyma cells in general communicate with each other, 

 with the ray tracheids, and with the adjacent wood elements by 

 means of pits always simple in the wall of the parenchyma cell, 



r.tr 



Ir^f- 



Fig. 7. — Radial section of a ray of Pinus palustris (longleaf pine), showing the 

 dentations (d) or reticulations on the upper and lower walls of the ray tracheids 

 (r. tr.J, and the presence in the lateral walls of the ray-parenchyma cells (r. p.) of 

 small simple pits (s. p.), communicating with the wood tracheids (w. tr.) adjacent; 

 b. p., bordered pit. Magnified about 250 diameters. 



but commonly more or less bordered in the other. Often certain 

 cells of a ray have thicker vvalls and more numerous pits than the 

 others. 



References 

 Penhallow, D. P.: North American Gymnosperms, pp. 78-108. 

 Bailey, I. W.: On the Origin of the Broad Ray in Quercus, Bot. Gaz., Vol. 



XLIX, No. 3, March 1910, pp. 161-167. 

 : Notes on the Wood Structure of the Betulacese and Fagaceae, 



Forestry Quarterly, Vol. VIII, No. 2, 1910, pp. 178-185. 

 : The Relation of Leaf -Trace to Compound Rays in Lower 



Dicotyledons, Annals of Botany, Vol. XXV, No. 97, January 1911, 



pp. 225-241. 

 Groom, Percy: The Evolution of the Annual Ring and Medullary Rays of 



Quercus, Annals of Botany, Vol. XXV, No. 100, October 1911, pp. 



983-1004. 



