80 ECONOMIC WOODS OF THE UNITED STATES 



B Vertical resin ducts normally absent; may be present as result 

 of injury in which event they are arranged in a compact 

 peripheral row (Fig. 10, p. 32) ; horizontal resin ducts (fusiform rays) 

 absent.* Ray tracheids present or absent. f 

 a Tracheids without spirals. (For b, see p. 85.) 



a 1 Woods without aromatic odor. (For b 1 , see p. 82.) 

 a 2 Resin cells absent or few; never visible without 

 compound microscope. Color of woods not pro- 

 nounced, though late wood exhibits a slight purplish 

 tinge. Without sharp demarcation and with little 

 color contrast between heartwood and sapwood. 

 a 3 Ray tracheids normally absent. Fir. 18 



a 4 Color white or pale brown in general appearance, 

 with late wood rather purplish. Wood often 

 coarse-grained, soft and weak. Sp. gr. .29-.4S, 

 mostly between .35 and .40. Balsam or Balsam 

 Fir, Abies balsamea Mill. (N); Lowland Fir, A. 

 grandis Lindl. (P); White Firs, A. concolor Parry 

 (P) and A. amabilis Forb. (P). 

 b 4 Color yellowish-brown with reddish tinge; rays 

 decidedly reddish. Wood moderately to de- 

 cidedly heavy and hard. Sp. gr. .41-.58. 

 Noble Fir, A. nobilis Lindl. (P); Red Fir, A. 

 magnified Murr. (P). 



b 3 Ray tracheids present in single marginal rows and some- 

 times interspersed. Hemlock. 19 

 a 4 Odor disagreeable, though not very pronounced 

 in small dry specimens. Wood harsh and 

 slivery, inclined to split apart at growth rings; 



* Traumatic resin ducts occur occasionally in the woods of various species 

 of Abies, of Tsuga heterophylla, both species of Sequoia, and certain species of 

 Cedrus, but in no other genera of this group. Traumatic horizontal canals 

 have been reported only for certain species of Cedrus and some extinct species 

 of Sequoia (?) not included in this key. 



f Small marginal ray tracheids are characteristic of Tsuga. No ray tra- 

 cheids have been observed in Taxodium, Tumion (Torreya), and Taxus. 

 Their more or less sporadic occurrence in Abies, Sequoia, Chamcecyparis, 

 Thuya, Juniperus, and (very rarely) Libocedrus has been noted by the author 

 or reported by others. (See W. P. Thompson, "Ray tracheids in Abies," Bot. 

 Gaz.,53;4: 331-338; also Ruth Holden, "Ray tracheids in the Coniferales," 

 Bot. Gaz., 55:1:56-65). 



