RESIN PASSAGES 



135 



thin-walled elements may be associated with occasional thick- 

 walled elements with which they are interchangeable, precisely 

 as in the similar relations displayed by the medullary rays of 

 P. pungens and P. cubensis. In the same region also there is 

 a similar association with and transformation into parenchyma 

 tracheids, which also has its parallel in the medullary ray. Some- 

 what more specifically, special reference to two examples may 



s.w- 



SM 



FIG. 44. PINUS REFLEXA. Transverse section of a resin passage from the inner 

 face of the spring wood showing the central canal (C.)\ the thin-walled and 

 resinous epithelium (ep.) \ the parenchyma tracheids (t.) ; the spring wood 

 (Sp.W.) and the summer wood (S. IF.) X 225 



serve to illustrate the general nature of some of the more im- 

 portant variations. In longitudinal section the parenchyma tra- 

 cheids are usually of much greater length than the associated 

 parenchyma cells, with which they are parallel or conterminous, 

 and they occur in large numbers in P. Lambertiana. In P. reflexa 

 they are conterminous with parenchyma cells, which they finally 

 succeed, to be replaced in turn by thin-walled wood tracheids. 

 In P. Lambertiana they are always to be distinguished by the 



