RESIN CELLS 



(fig. 36), while in Abies amabilis (fig. 37) resin cells and tra- 

 cheids also form a conterminous series. It is thus obvious that 

 we have here precisely the same interchangeable relations that 

 have been found to occur in the medullary rays, and it is evi- 

 dent the one element must arise through modification of the 

 other. The precise order of this sequence is not altogether clear 

 from the available data, but the fact that 

 ray tracheids are derived from their asso- 

 ciated parenchyma cells, and that in such 

 types as Podocarpus, Taxodium, etc., the 

 resin cells occur without tracheids, while 

 the latter do occur in Sequoia and espe- 

 cially in Abies, seems to justify the infer- 

 ence that here also they are derived forms, 

 having their origin substantially in special 

 modifications of the parenchyma elements. 

 In view of these relations it is necessary to 

 distinguish such elements as parenchyma 

 tracheids in order to establish their proper 

 identity and differentiate them from the 

 wood tracheids which have a wholly dif- 

 ferent origin, as well as from the ray 

 tracheids which have a wholly different 

 location. 



chyma tracheids also serve a similar pur- 

 pose to the ray tracheids with respect to 

 the distribution of nutrient fluids. The 

 origin of the parenchyma tracheids as sug- 

 gested finds support in the statement of 

 Eichler (15) that the wood parenchyma arises through the activity 

 of the cambium cells, abundantly in the Cupressineae and Abie- 

 tinese, forming in exceptional cases the epithelium of the resin 

 canals, since it at the same time shows how the parenchyma 

 tracheids arise, and how they may be intimately connected with 

 the wood parenchyma ; but it finds additional support in a knowl- 

 edge of the genesis and structure of the resin passage. 



FIG. 37. ABIES AMABILIS. 

 It is probable that the paren- Radial section showing 



(a) the structure of the 

 parenchyma tracheids; 

 (t>) the structure of the 

 resin cells ; a and b being 

 normally conterminous. 

 X 200 



