ECONOMIC WOODS OF THE UNITED STATES 29 



THOMPSON, W. P. : On the Origin of the Multiseriate Ray of the Dicotyledons, 



Annals of Botany, Vol. XXV, No. 100, October 1911, pp. 1005-1014. 

 : The Origin of Ray Tracheids in the Coniferae, Bot. Gaz., Vol. L, 



No. 2, 1910, pp. 101-116. 

 KNY, L.: Beitrag zur Kenntnis der Markstrahlen dicotyler Holzgewachse, 



Berichte d. deutschen Bot. GeseUschaft, Vol. VIII, Berlin, 1890, pp. 



176-188. 

 ESSNER, BENNO: Ueber den diagnostischen Werth der Anzahl und Hohe der 



Markstrahlen bei den Coniferen, Halle A. S., 1882. See also Bot. 



Centralblatt, Vol. XII, No. 12, 1882, pp. 407-408. 

 ZIJLSTRA, K.: Die Gestalt der Markstrahlen in sekundaren Holze, Rec. 



Trav. bot. ne*erl., V, 1908, pp. 17-20. . 



RESIN DUCTS 



Resin ducts are long, narrow, intercellular channels surrounded 

 by parenchyma cells and filled with resin (Fig. 8). Unlike vessels, 

 they have no walls of their own, but are limited by a layer of cells 

 called epithelium. The epithelial cells are thin-walled in Pinus 

 and mostly thick-walled in Larix, Picea, and Pseudotsuga. When 

 thick-walled the cells are rounded and show clearly in cross sec- 

 tion, while those with thin walls are compressed and very likely 

 to be torn in sectioning. 



Resin cysts are very short, duct-like, intercellular spaces very 

 common in Sequoia, Tsuga, and Abies. Not infrequently they 

 are in longitudinal series, but differ from a true duct in having 

 numerous constrictions. 



Resin ducts are largest and most abundant in Pinus, where 

 they are fairly well distribut )d throughout the growth ring, though 

 usually more numerous in the transition zone between early and 

 late wood. They are comparatively large in most species, averag- 

 ing about 0.25 mm., and are readily visible to the unaided eye. 

 On longitudinal surface they appear as long, delicate lines like 

 pin scratches, filled with resin. In Larix, Picea, and Pseudotsuga 

 the ducts are smaller, sometimes invisible without lens, fewer in 

 number, and irregularly distributed, often more or less grouped. 



In addition to the ducts extending in a vertical direction, there 

 are horizontal ducts in the fusiform rays (Fig. 9). The two series 

 are united at infrequent intervals. 



Resin ducts very commonly develop as a result of injury, 

 not only in genera in which they occur normally, but also in others 



