LARIX 



277 



Pits on the tangential walls of the summer wood confined to the 

 outermost tracheid. 



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



Ray cells (tangential) equal, uniform, and oblong, more 

 rarely oval. 



2. * L. americana. 

 Bordered pits in I row, sometimes in pairs. 



Pits on the tangential walls of the summer wood confined to the 

 outermost tracheid. 



Pits on the lateral walls of the ray cells 3-6 ; those on the tan- 

 gential walls of the summer wood numerous and small. 



Ray cells (tangential) equal and very uniform, narrowly 

 oblong. 



3. L. Lyallii. 



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

 on the tangential walls of the summer wood few, small, con- 

 fined to the outermost wall. 



Ray cells (tangential) oblong, more rarely oval, and much broader. 



4. L. leptolepis. 



1. L. occidentalis, Nutt. 

 Tamarack 



Transverse. Growth rings usually broad, the dense and prominent sum- 

 mer wood about one half the spring wood, from which the transition 

 is abrupt. Tracheids of the summer wood large, squarish, in regular 

 rows. Tracheids of the spring wood very large and thin-walled, 

 squarish-hexagonal, in very regular rows, rather uniform. Medullary 

 rays prominent, rather resinous and broad, i cell wide, distant 2-6 

 rows of tracheids. Resin passages few, large, without thyloses, the 

 epithelium narrow, rather thin-walled, the nutritive layer thick-walled, 

 resinous. Resin cells widely scattering on the outer face of the 

 summer wood, but readily recognized by their abundant resinous 

 contents. 



Radial. Rays conspicuously resinous throughout ; the tracheids narrow 

 and marginal, rarely interspersed. Ray cells chiefly straight through- 

 out and equal to 3-9 spring tracheids ; the upper and lower walls 

 chiefly thick and unequal, sparingly pitted throughout, more strongly 

 so in the summer wood ; the terminal walls coarsely pitted throughout ; 

 the lateral walls with elliptical and distinctly bordered pits, with a 

 narrow, chiefly oblong or lenticular orifice, numerous, at first 6-8 per 

 tracheid, soon greatly reduced in size, and in the summer wood 

 abruptly i per tracheid. Bordered pits conspicuously in 1-2 rows, 

 more rarely in i row only, elliptical, the orifice very large. Pits on 

 the tangential walls of the summer wood rather numerous but small 

 and often obscure. Resin cells about 12.5 p. wide and 60-150 p. 

 long. 



