282 ANATOMY OF THE GYMNOSPERMS 



Pits on the tangential walls of the summer wood few, often widely 

 scattering and extending for some distance into the summer wood. 

 Pits on the lateral walls of the ray cells 2-6, chiefly 4, per tra- 

 cheid. 



Ray cells (tangential) conspicuously unequal. 



Spring tracheids large, thin-walled, uniform in regular 

 rows, squarish-hexagonal. 



6. P. polita. 



Ray cells (tangential) usually very equal and uniform, oblong or oval. 



Pits on the tangential walls of the summer wood chiefly or wholly 

 confined to the outermost tracheid wall. 



Pits on the lateral walls of the ray cells 2-4 per tracheid. 



7. P. bicolor. 



Pits on the lateral walls of the ray cells 2-6 per tracheid. 



3. * P. alba. 



Pits on the tangential walls of the summer wood not confined to the 

 outermost wall, but chiefly small and inconspicuous. 

 Rays (radial) nonresinous. 



Pits on the lateral walls of the ray cells at first narrowly bor- 

 dered, 2-5 per tracheid, in the summer wood reduced to i. 

 Ray cells (tangential) rather thick-walled. 



Spring tracheids rounded-hexagonal ; the summer 

 wood rather open but prominent, upwards of one 

 half the spring wood. 



4. P. Engelmanni. 



Pits on the lateral walls of the ray cells with an oblong 

 orifice, 2-6 per tracheid, toward the summer wood reduced 

 to 2, and finally to i. 



Ray cells (tangential) thin-walled. 



Spring tracheids hexagonal, very thin-walled ; the 

 summer wood very thin and open, often barely 

 distinguishable. 



8. P. pungens. 



Rays (radial) locally resinous, the resin chiefly confined to the 

 thicker-walled and more strongly pitted cells, more rarely dif- 

 fused throughout the central cells. 



Pits on the lateral walls of the ray cells with a lenticular 

 orifice, at first 2-6 per tracheid, more rarely 2 throughout, 

 in the summer wood reduced to 2, and finally to i. 



Spring tracheids squarish-hexagonal, not very uniform, 

 the walls rather thin. 



9. * P. nigra. 



