Nos. 451-452.] '-iAV/rat/F OF THE CONIFERALES. 531 



extent that the resin is barely recognizable, while in Abies it is 

 wholly wanting. 



A relation of more than ordinary interest is to l)c Inund in 

 the relation of the resin cells to certain forms ol trachcids. in 

 Sequoia sempervirens it commonly happens that the resin t ells 

 he in immediate contact with trachcids of special form. These 

 structures are wholly unlike the wood trachcids anion- which 

 they are found, but they are, in all essential respects, like the 

 tracheids of the medullary rays. They have the form of long, 

 cylindrical elements with abrupt terminations, and they thus 

 bear an external resemblance in form to the wood pareiu h\ ma 

 cells with which they are associated. They differ, h(j\vever, in 

 the distinguishing presence of bordered pits upon their side and 

 terminal walls (Fig. 38^). The relations of these two elements 

 is nevertheless a much more intimate one 

 than is implied by mere association. In 

 Sequoia an interchangeable relation is 

 manifested as already pointed out, in the 

 occurrence of resin cells with bordered pits 

 (Fig- 37), while in Abies amabilis (Fig. 38) 

 resin cells and tracheids also form a coter- 

 minous series. It is thus obvious that we 

 have here precisely the same interchange- 

 able 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 

 associated parenchyma cells, and that in ^^'^^^^^[~^^J^Z\ho^C%"d} 



such types as PodoCarpUS, TaXOdium, etc., the structure of the paren- 



the resin cells occur without tracheids, .trcTureof the'^reiJteiis • 

 while the latter do occur in Sequoia and « and b being ^■^■■'"aiiy 

 especially in Abies, seem to justify the 

 inference 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 distin- 



