272 ANATOMY OF THE GYMNOSPERMS 



Ray cells oval or round (tangential). 



Pits on the lateral walls of the ray cells 4 per tracheid. 



3. ** P. miocena. 



1. * P. Douglasii, Carr. 

 Yellviv Fir. Oregon Pine. Douglas Fir 



Transverse. Growth rings very variable, and either very thin with a close, 

 compact grain, or very broad with a coarse, open grain. Summer 

 wood very variable, now barely distinguishable, or again upwards of 

 one half the spring wood, often hard and flinty ; transition from the 

 spring wood more or less abrupt. Spring tracheids large, thin-walled, 

 hexagonal, uniform in regular rows. Medullary rays prominent and 

 somewhat resinous, rather few, i cell wide, distant 2-13 rows of 

 tracheids. Resin passages rather few and widely scattering, chiefly 

 in the summer wood, the canal equal to about i or 2 tracheids. 

 Resin cells few and distant on the outer face of the summer wood, 

 not very prominent or resinous, chiefly distinguished by their position 

 and the sieve-plate structure of the terminal walls. 



Radial. Rays sparingly resinous throughout, the tracheids prominent, 

 chiefly narrow and marginal, but sometimes interspersed. Ray cells 

 straight or somewhat contracted at the ends ; the upper and lower 

 walls thickish, irregularly and often imperfectly pitted ; the terminal 

 walls coarsely pitted; the lateral walls with small, elliptical pits, the 

 border narrow, the orifice lenticular, at first 3-7, soon becoming 1-3, 

 and- in the summer wood i, per tracheid. Pits on the tangential 

 walls of the summer tracheids wanting. Resin cells 15-25 p, wide, 

 125-225 p long. Bordered pits in i row, sometimes in pairs, gen- 

 erally elliptical, the orifice large. Spirals generally wanting in the 

 summer tracheids, the angle 82. 



Tangential. Fusiform rays with linear and unequal terminals. Ordinary 

 rays low to medium, the cells oval to oblong. Medullary ray cells all 

 thick-walled. 



This species presents the most striking variations of any of the North 

 American Coniferae. These variations appear as follows: 



1 . The growth rings occur in zones, between which there are pronounced 

 differences in the average thickness of the component rings (52). 



2. The growth rings vary from several millimeters in thickness to less 

 than i mm. In this respect a distinction may be made between the 

 " fine-grained," in which the rings seldom exceed 2-2.5 rnm., usually being 

 much less, and the " coarse-grained " wood, in which the rings approx- 

 imate to 4 mm. in thickness ; the latter is further distinguished by its 

 coarse, open grain, and often very flinty summer wood, thus approximat- 

 ing to the " red fir," as represented by the next species. 



3. The summer wood varies greatly, either in the same tree or in 

 different trees, being in one case barely if at all distinguishable ; or, 

 on the other hand, becoming very prominent, dark, dense and flinty, 

 and often equal to the spring wood. 



