RESIN DUCTS. 143 



thicker tluin those of the cells of ordinary rays, aud esi)ccially thickened near the simple pits by 

 which these cells communicate with each other. In Cuban and Shortleaf this thickening- is much 

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

 seems wanting altogether. 



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

 near center, but have essentially the same structure 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 the height of the ray and 00 /; for the width at the resin duct was observed. An attempt to 

 utilize tlie shape, especially the appearance of the two edges, as a means of sei)arating tlie wood 

 of these species has so far failed entirely. 



The large resin ducts running lengthwise in the wood or parallel to the common wootl 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 smaller radial diameter aud about 0.3 mm. on the 

 tangential. (See PI. XXI, A, r. d.) They are usually situated in the summerwoud 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 

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

 other pines. They are surrounded by thin-walled resin-secreting parenchyma, jjart 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. Longitudinal and transverse 

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



