552 THE AMERICAN NATURALIST. [Vor.. XXXVIII. 



making sections, while in the hard pines, as P. cubcnsis, P. tceda, 

 P. pungens, etc., the cells are often strongly resinous. In the 

 outer epithelium, the thin-walled elements may be associated 

 with occasional thick-walled elements with which they are inter- 

 changeable, precisely as in the similar relations displayed by the 

 medullary rays of P. pungens and P. aibcnsis. 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. Somewhat more specifically, special reference 

 to two examples may ser\^e to illustrate the general nature of 

 some of the more important variations. In longitudinal section 

 the parenchyma tracheids are usually of much greater length 

 alld ^^^^^/''■''''"^.^^^^ parenchyma cells, with which they are par- 

 or eotermmous, and they occur in large numbers in P. 

 . lii y . icjtexa they are cotermmous with paren- 

 cnynia cells which they finally succeed, to be replaced in turn 

 thin-walled wood tracheids. In P. lambcrtiana they are 



