136 ANATOMY OF THE GYMNOSPERMS 



bordered pits on the radial, tangential, and terminal walls, while 

 in P. reflexa they are characterized by the large number of bor- 

 dered pits on the radial walls, with very few on the tangential 

 walls. In the former situation the pits are much smaller than 

 in adjacent wood tracheids. Together with adjacent wood tra- 

 cheids the parenchyma tracheids may be more or less involved 

 in bearing resin (P. Lambertiana), while finally, as exhibited in 

 transverse section, their numbers may be so large that they 

 form extensive areas about the resin passage (fig. 44). In such 

 a case the sequence of elements in transverse section would be : 



1 . Canal with thyloses. 



2. Thin-walled epithelium. 



3. Epithelium and cylindrical parenchyma tracheids. 



4. Parenchyma tracheids. 



5. Wood tracheids with thin walls. 



Thyloses are a constant feature in the structure of the resin 

 passages of Pinus (fig. 31, a). They are always thin-walled and 

 completely fill the canal. So constant are these features in 

 association with those previously recounted that they serve to 

 afford a ready means of accurately recognizing the genus under 

 all circumstances. 



The general course of development thus outlined shows that 

 the parenchyma tracheid stands in such relation to the organiza- 

 tion of the resin passage that its more frequent occurrence is 

 directly correlated with a higher type of organization and devel- 

 opment in the plants to which they belong. 



We are now in a position to present a general summary of 

 the relations which the resin cells bear to the organization of 

 the secretory reservoirs -- cysts and passages and the position 

 which the latter occupy in the economy of the plant as follows : 



1. Resin cells, which are of the nature of wood parenchyma, at first occur 



as isolated structures filled with resin, but they show a definite tendency 

 to association and later form definite aggregates. 



2. Parenchyma tracheids become associated with such aggregates for the 



purpose of effecting a more complete nutrition of the secretory cells. 



