RESIN DUCTS. 143 



thicker than those, of the cells of ordinary rays, and especially thickened near the simple pits by 

 which tliese cells coinmnuicate with each other. In Cuban and Shortleaf this thickening is much 

 less conspicuous, and absent entirely in many cases (see PI. XXV, A), while in tlie Spruce Pine it 

 seems wanting altogether. 



These ducts exist even in the very tirst ring (next to the pith), are smaller and more numerous 

 near center, but have essentially the same structiire in the wood of the fifth and later years. 



The tracheids of the pith rays are wanting next to the pith, but occur in all rays in the outer 

 part of even the first ring. The rays in this ring are generally lower, composed of fewer cell rows, 

 but the cells are larger than in the rest of the wood. 



Both shape and size of these medullary rays are very variable; an average of about 0.4 mm. 

 for tlie height of the ray and GO /( for the width at the resin duct was observed. An attempt to 

 utilize the shape, especially the appearance of the two edges, as a means of separating the wood 

 of these species has so far failed entirely. 



The large resin ducts running lengthwise in the wood or parallel to the common wood fibers 

 are much larger than the transverse ducts, measuring, inclusive of the secretive cells, on an 

 average about 0.2 mm. (0.008 inch) on their sm.dlor radial diameter and about 0.3 mm. on the 

 tangential. (See PI. XXI, A, r. d.) Tliey are usually situated in the sumnierwood of each ring, 

 often in narrow rings, causing an irregular outline. They are smaller and more numerous near 

 the pith, here usually forming several series in one annual ring, more numerous in wide rings 

 than in narrow ones, but their number per square inch of cross section as well as their dimensions 

 apiiear to be independent of the width of the rings. In their structure they resemble those of 

 other pines. They are surrounded by tliin- walled resin-secreting parenchyma, part of which 

 often appears as if not directly connected with the duct. (See PI. XXI, A.) In many cases all 

 the tissue between two neighboring ducts is of this parenchyma. Longitndinal and transverse 

 ducts frequently meet and thus form a continuous network of ducts throughout the wood. 



