CONIFERALES 341 



followed by the appearance of traumatic or wound resin canals as 

 a reversionary feature. In S. sempervirens, the redwood, this is the 

 only mode of occurrence of these structures. It may be added in 

 this connection that the normal seedling of Sequoia (either species) 

 shows no resin canals in the wood. On the fallacious logic that 

 structures absent in the seedling are not ancestral it could be 

 argued that resin canals in the secondary wood are not an ances- 

 tral feature of the genus Sequoia. This genus is the only one in 

 any coniferous subtribe, other than the Abietineae, characterized 

 in living species by the formation of traumatic resin canals. In 

 Sequoia a sound inference based on the principle of conservative 

 organs is that its ancestral forms were provided with resin canals 

 throughout their structure. 



Although only the genus considered in the foregoing paragraph 

 presents the abietineous feature of resin canals in the secondary 

 wood as a result of injury, many genera of both taxodineous and 

 cupressineous affinities revert to the abietineous type of ray as 

 a response to wounding. This is notably the situation in Sequoia 

 and is illustrated in Fig. 53. In another figure (Fig. 52) the occur- 

 rence of marginal tracheids is indicated for the cupressineous species 

 Chamaecyparis nootkatensis. The investigations of Miss Holden 

 have made it clear that the presence of traumatically recalled 

 ray-tracheids is a common feature of the Cupressineae and the 

 Taxodineae. In neither of these tribes are such structures known 

 to occur normally in conservative organs; hence their former 

 presence is revealed only by reversionary phenomena. It is 

 worth while to note that the recurrence of marginal tracheids 

 as the result of injury is usually exemplified, not in the immediate 

 region of the wound where hypertrophy alone prevails, but in a 

 region of the stem more or less remote from the actual injury. 

 This situation is of interest because it is paralleled by conditions 

 found in connection with certain other wound reactions, notably 

 those presented in the case of the rays in certain angiosperms. It 

 is obvious from what has been stated above that ray-tracheids, 

 although of much wider occurrence among the Cupressineae and 

 Taxodineae, probably appear only as the consequence of experi- 

 mental conditions and are no longer a normal feature of structure. 



