Nos. 45I-4S2-] ANATOMY OF THE COXI FEKA I.ES. 



553 



always to be distinguished by the bordered jiits on the radial, 

 tangential and terminal walls, while in P. rcjlcva arc char- 

 acterized by the large number of bordered 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 tra- 

 cheids. Together with adjacent wood tracheids, the paren- 

 chyma tracheids may be more or less involved in bearing resin 

 {P. lambcrtiana), while finally, as exhibited in transverse sec- 

 tion, their numbers may be so large that they form extensive 

 areas about the resin passage (Fig. 49). In such a case the 

 sequence of elements in transverse section would be : — 



1 . Canal with thyloses. 



2. Thin-walled epithelium. 



3. Epithelium — 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. 30). 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 (le\el- 

 opment in the plants to which they belong. 



We are now in a position to present a general sunmuu N of 

 the relations which the resin cells bear to the organization of 

 the secretory reservoirs — cysts and passages — and the i)osi- 

 tion which the latter occupy in the economy of the plant, as 

 follows : 



1. Resin cells, which are of the nature of wood i)arenchyina, 



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 aggre- 



gates for the purpose of effecting a more complete nutri- 

 tion of the secretory cells. 



