CONIFERALES (PINACEAE) 233 



tracheids found occasionally in Cunninghamia, Jeffrey has found 

 to be due to wounding, and he infers that this response brings back 

 an ancestral feature, and that the ray of Cunninghamia has been 

 derived from a more complex one. Since the ray tracheids described 

 for certain Taxodineae and Cupressineae resemble those induced 

 in Cunninghamia, the suggestion is made that this evidence favors 

 the origin of these groups, in which ray tracheids are a vanishing 

 character, from Abietineae, in which ray tracheids are an established 

 character. In a recent study of the origin of ray tracheids by Thomp- 

 son (170), complete transitions were observed, in the young root, 

 from short tracheids extending between the rays to ray tracheids, 

 both marginal and interspersed; the inference being that ray tracheids 

 originate from tracheary tissue. From a comparison of living and 

 fossil forms, the conclusion is reached that while traumatic ray tra- 

 cheids are evidently vestigial in Abies, their absence is an ancestral 

 feature of the pines. It is evident that the whole subject of the 

 phylogenetic significance of ray tracheids is more suggestive as yet 

 than definite. 



The resin canals have become of great service in discussions of 

 the interrelationship of the tribes. In Pinus, Picea, Larix, and 

 Pseudotsuga they form an anastomosing system in the secondary 

 wood and cortex of both shoot and root, and also occur in the outer 

 margin of the primary xylem of the root. In Abies, Pseudolarix, 

 Cedrus, and Tsuga resin canals do not ordinarily occur in the second- 

 ary wood of either shoot or root, and are present in the center of the 

 primary xylem of the root. Jeffrey (90) has discovered that in this 

 latter group resin canals sometimes occur in the wood of the axis 

 of the ovulate strobilus and in the first annual ring of vigorous shoots, 

 and also that they may be induced in the secondary wood by injury. 

 The inference is that resin canals in the secondary wood are a primitive 

 feature of Abietineae; that they persist longest in the reproductive 

 axes, leaves, and first annual rings; and that they may be recalled 

 as a traumatic response. In Pseudolarix and Tsuga they have dis- 

 appeared even from the cortex of all organs except the ovulate stro- 

 bilus and leaves. This reasoning leads tp the conclusion that Pinus 

 and its associated genera are the most primitive, and that the other 

 genera are derived from the condition of Pinus by the disappearance 



