674 Forestry Quarterly 



One of the most marked characteristics of the tracheids of 

 Douglas fir is the presence of spiral thickenings on the inner sur- 

 face of the fiber. From 1 to 4 of these spirals, or tertiary thick- 

 enings, may show in a single tracheid. They are practically always 

 prominent in the springwood, running at a mean angle of about 

 80 degrees (Fig. 4), but are usually absent or vestigial in the sum- 

 merwood. The only other coniferous tree growing in Canada, 

 the tracheids of which normally have spiral thickenings, is the yew 

 (Taxus) of the Pacific coast, and in this the spirals occur in both the 

 spring and summer tracheids. Furthermore, the pits marking the 

 crossing-field of the ray cells are different in the two genera and 

 in the wood as a whole ; Douglas fir is easily distinguished from yew 

 because the former has and the latter has not resin canals. 



Although we are not dealing specially with the ray cells which 

 occasionally show with the fibers, as may be seen in the very 

 shortest cells in Plate 1, Figure 1, we must consider the pits or 

 openings which occur in the tracheids where they come in con- 

 tact with ray cells. Since the rays are from 1 to 15 cells high and 

 the tracheids have, in the late wood, from 1 to 3, and in the 

 early wood up to 7 pits per each cell, it follows that the 

 fiber will show from 1 to 15 sets of openings, containing up 

 to 7 pits in each set, representing the crossing-field of the ray. 

 The openings are divided into two classes, those leading into the 

 central cells of the ray and those leading into the marginal cells. 

 The difference between these two kinds of pits is not always 

 easy to distinguish on a single fiber, but in general it may be said 

 that the marginal openings are smaller, more rounded 

 and with a wider border than the central openings; in fact, the 

 latter are often extended into quite distinct slits. Plate 1, Figure 

 5, shows very clearly the difference between the three marginal 

 bordered pits at both the top and the bottom of the slit-shaped, 

 slishtly bordered central pits. 



The circiilar marks surrounded by a circular border are the 

 pits which occur in Douglas fir tracheids, as in all coniferous woods, 

 forming openings from one fiber to the next. These bordered 

 pits, as they are called, are always confined to the radial walls 

 except in the late wood, where they occur on the tangential walls. 

 In Figure 2, Plate 1, the bordered pits may be barely seen and 

 Figure 6 shows them under much higher magnification. 



Specialized collections of cells with an open space between them 



