134 



ANATOMY OF THE GYMNOSPERMS 



noted in P. nigra, P. pungcns, and P. sitchensis, but they are 

 always thin-walled. Parenchyma tracheids are not obvious in 

 the summer wood, but they are recognizable in the spring wood, 

 where they appear to replace the resin cells, though they are 

 apparently of much less frequent occurrence than in the genera 

 previously discussed. In P. alba, however (fig. 43, pr.t.), we 

 sometimes find a radial series of tracheids which also extends 

 laterally so as to form an inclosing layer. Longitudinally the 

 canal is continuous, but with more or less frequent constrictions, 

 as in Pseudotsuga and Larix. The epithelium consists of narrow 

 cylindrical and much-pitted cells, which increase in length in the 

 outer layers, where they become five to seven times longer than 

 broad, and finally merge with the parenchyma tracheids, which 

 replace them. 



While the general composition of the resin passage in Pseu- 

 dotsuga, Larix, and Picea is the same as that of the resin cyst, 

 it is obvious that the frequent constrictions in the canal indicate 

 a partial survival of the cystic formation. We must, therefore, 

 regard these structures and the three genera to which they 

 belong as forming a transition group between the primitive 

 resin cyst, on the one hand, and the perfectly organized resin 

 passage of Pinus, with its canal of uniform width, on the other. 



In the genus Pinus the resin passages show considerable vari- 

 ation in detail, but they all conform to the same structural type 

 (fig. 44). The central canal is broad and round, often very large, 

 and in longitudinal section it is a perfectly continuous passage 

 of uniform width. The epithelium consists of large but very vari- 

 able and thin- walled cells in from one to several rows. In the soft 

 pines it generally forms a concentric zone of uniform width, but 

 in several of the hard pines there is a marked tendency to exten- 

 sion in a tangential direction and the formation of rather exten- 

 sive eccentric tracts. In all of the pines there is a pronounced 

 tendency for the epithelial elements to become so thin-walled 

 that they are readily broken out in making sections, while in 

 the hard pines, as P. cubensis, P. taeda, P. pungens, etc., the 

 cells are often strongly resinous. In the outer epithelium the 



