326 ANATOMY OF THE GYMNOSPERMS 



The following determinations are after Bovey : 

 Coefficient of strength in pounds for : 



Bending 5,400 



Elasticity 1,430,000 



Torsion 11,500 



Compression 3,900 



Shear 380 



Weight per cubic foot 33 



Rather a widely distributed but somewhat localized tree flourishing partic- 

 ularly in the poorest soils ; Nova Scotia, New Brunswick, Quebec, and 

 westward to Lake of the Woods (Macoun) ; through the New England 

 States to northern Pennsylvania and westward to Michigan and central 

 Minnesota (Sargent). 



22. P. tropicalis, Morelet 



Transverse. Growth rings narrow, unequal. Summer wood chiefly rather 

 dense but somewhat variable, usually exceeding the spring ; the tra- 

 cheids hexagonal-oblong, uniform, and in regular rows ; transition from 

 the spring wood abrupt or more rarely gradual. Spring tracheids 

 large, conspicuously squarish or hexagonal, the walls rather thin. 

 Medullary rays prominent, resinous, i cell wide, broad, distant 2-8, or 

 more rarely 12, rows of tracheids. Resin passages numerous, large, 

 resinous, and chiefly confined to the summer wood ; the epithelium of 

 1-2 rows of thin-walled cells which often form tangentially extended 

 tracts. 



Radial. Ray tracheids rather low and rather sparingly dentate, never reticu- 

 late ; numerous and interspersed, often predominant. Medullary rays 

 resinous, the cells all of one kind ; the upper, lower, and terminal walls 

 thin and commonly much broken down; the side walls with large, 

 oval, oblong, or lenticular pits, 1-2, chiefly i, per tracheid, in the 

 summer wood often reduced and vertically lenticular. Pits on the 

 tangential walls of the summer tracheids wholly wanting, but often 

 appearing on the tangential walls of the first spring tracheid. Bor- 

 dered pits large, in i row or in pairs, the latter often approximating so 

 as to form 2 rows ; in the summer wood becoming much reduced and 

 rather distant in I row. 



Tangential. Fusiform rays somewhat numerous, medium ; the terminals 

 somewhat prolonged but usually acute and composed of few, thick- 

 walled tracheids ; the cells of the central tract all thin-walled and much 

 broken out. Ordinary rays numerous, very resinous, medium ; the cells 

 oval, uniform, and equal but much broken out ; the higher rays some- 

 what contracted by the interspersed, rather smaller, and thick-walled 

 tracheids. 



Cuba and Isle of Pines, West Indies. 



A careful comparison of this wood with that of P. resinosa will show that 

 while they are very closely related, there are, nevertheless, essential struc- 

 tural differences which compel us to recognize them as distinct species. 



