212 Bulletin 194 



/. Transition (cross) from spring wood to summer wood 

 gradual, the growth rings distinguished by narrow lines 

 of darker colored summer wood ; wood soft and light, 

 sHghtly resinous; lateral zualls of ray cells (radial) of 

 spring wood with 1-2 large pits to each tracheid. 

 g. Walls of ray tracheids {radial) smooth; pits {radial) 

 on the tangential zvalls of the summer-zvood cells 



numerous Pinus strobus, p. 215. 



gg. Walls of ray tracheids {radial) conspicuously 



toothed; pits {radial) on the tangential zvalls of the 



summer-wood cells absent, Pinus resinosa, p. 217. 



ff. Transition (cross) from spring wood to summer wood 



more or less abrupt, the growth rings distinguished by 



broad bands of darker colored summer wood ; wood 



medium hard and heavy, strongly resinous ; lateral 



zvalls of ray cells (radial) of spring zvood with 2-6 



small pits to each tracheid. 



g. Growth rings (cross) wide, the resin ducts small, 

 widely scattering; fusiform rays (tangential) having 

 the cells of the inflated portion all or mostly thick- 



zvalled, not broken out Pinus banksiana. 



gg. Growth rings (cross) narrow, the resin ducts large, 

 chiefly in the summer wood; fusiform rays (tan- 

 gential) having the cells of the inflated portion all or 

 mostly thin-zvalled, all broken out, Pinus rigida. 



dd. Heartwood not distinct in color from the sapwood ; resin 

 ducts (cross) not conspicuous, of same color as surround- 

 ing wood; fusiform rays (tangential) narrozv, not promi- 

 nent, zvith only one small resin duct. 



e. Growth rings (cross) wide; resin ducts (cross) rather 



numerous, the epithelium cells thin-zvalled. 



f. Resin ducts (cross) scattered through both spring and 



summer wood, with tyloses usually absent; pits 



(radial) on the tangential walls of the summer wood 



chiefly confined to the outermost tracheid zvall, 



Picea canadensis. 

 ff. Resin ducts (cross) confined chiefly to the summer 

 wood, zvith tyloses present; pits (radial) on the tan- 

 gential zvalls of the summer zvood not confined to the 

 outermost zvall, but usually numerous, Picea mariana. 



