142 ANATOMY OF THE GYMNOSPERMS 



fails in the case of Sequoia sempervirens and S. Langsdorfii (46), 

 and it likewise appears to fail in the case of S. Burgessii, but in 

 this latter case it is possible that there is the same peculiarity of 

 distribution which Jeffrey has observed in S. Penhallowii, accord- 

 ing to which local areas may be devoid of longitudinal canals 

 while radial ones may be present. 



As presented by existing species, Pseudotsuga, Larix, Picea, 

 and Pinus, without exception, show resin passages in both the 

 radial and longitudinal positions. In transverse section they are 

 scattered throughout, sometimes appearing chiefly in the sum- 

 mer wood, sometimes chiefly in the spring wood, or again about 

 equally in the two regions, and they rarely conform to the pre- 

 cise law stated by De Bary (13, 495) that " they lie scattering 

 in a ring in the external region of every annular layer." The 

 constancy of their occurrence in the four genera mentioned in- 

 volves very few features which call for special comment. In 

 Pseudotsuga and Larix the resin passages are scattering. They 

 sometimes unite in pairs so as to form short tangential series, 

 and they thus approach the type of Tsuga or Abies, while yet 

 again they may become definitely isolated and scattering, thereby 

 approaching the distribution of Picea and Pinus. In Larix occi- 

 dentalis the tendency to a primitive form of distribution is ex- 

 pressed in the formation of a tangential zone essentially similar 

 to that of Tsuga Mertensiana. In both Pseudotsuga and Larix 

 there is an obliteration of resin cells from all parts of the struc- 

 ture except the extreme outer face of the summer wood. In 

 Picea, however, without exception, there is a complete oblitera- 

 tion of all resin cells except such as enter into the structure of 

 the resin passages, and this is directly correlated with a higher 

 type of structure in such passages. 



In the genus Pinus, as already shown, the resin passage 

 reaches the highest degree of organization in all respects. It 

 shows little if any tendency to those primitive associations which 

 are expressed in the formation of tangential series, while it has 

 entirely replaced the isolated resin cells, which are never to be 

 found in that genus. 



