RESIN PASSAGES 131 



four contiguous or even coalescent reservoirs. The central canal, 

 which is usually small and not infrequently very narrow, is rather 

 more generally rounded than in previous types. The epithe- 

 lium is very clearly defined and consists of one to three rows of 

 thick-walled parenchyma cells, sometimes containing resin, the 

 first row of which are rather small and radially flattened, but 

 in P. macrocarpa they are rather thin-walled. In P. Douglasii 

 the epithelium is commonly extended on the two sides of the 

 resin canal in such a way as to form a tangentially elongated 

 tract which not infrequently extends beyond and involves neigh- 

 boring medullary rays. In P. macrocarpa, on the other hand, 

 the epithelium is concentric with the canal, thus forming a 

 tract of about equal thickness all around. Such a deviation as 

 is expressed in P. Douglasii constitutes the first evidence of a 

 tendency in development which is fully and frequently expressed 

 in Pinus. Thyloses are of infrequent occurrence, and appear 

 to be confined to P. macrocarpa where they are few in number 

 and generally rather thin-walled. Parenchyma tracheids are 

 usually not apparent in a transverse section. This results from 

 the frequent location of the resin passage in the summer wood, 

 which is not favorable to their development, and from the close 

 resemblance which they bear to the tracheids of the spring wood ; 

 and while such elements form an integral part of the resin cyst, 

 their particular disposition cannot be exactly defined, though 

 there is no good reason for supposing that they differ in this 

 respect from what may be observed in other cases. In a longi- 

 tudinal section the canal is found to be more or less continuous, 

 though it presents frequent constrictions and is thereby reduced 

 to very narrow dimensions, or it may even be discontinuous and 

 thereby form cysts. It is this feature which causes the canal to 

 exhibit such marked variations in size, when seen in transverse 

 section. The epithelial cells are narrowly cylindrical and rather 

 long and thick-walled, as well as somewhat strongly pitted. Out- 

 wardly they become much longer and relatively narrower, and 

 they eventually merge with the surrounding parenchyma tra- 

 cheids, by which they may also be replaced (fig. 42). 



