RESIN CELLS 



tracheids, by their situation slightly in advance of the outermost 

 row of summer wood tracheids, and most particularly by their 

 pitted terminal walls when the latter lie near the plane of sec- 

 tion. This last feature may also be relied upon in all other 

 cases when any element of doubt is involved (fig. 35). In longi- 

 tudinal section the characteristic form of the cell serves to dis- 

 tinguish it beyond all doubt, even in the absence of resinous 

 contents. Whether exposed in radial or tangential section the cell 

 has the form of a narrow cylinder upwards of 300 /JL in length, 

 and always several times longer than broad, except in cases where 



)} 

 S.W. 



FIG. 35. ABIES AMABILIS. Transverse section showing the position and structure 

 of the resin cells (r.c.) on the outer face of the summer wood. X 300 



there is a definite tendency, through aggregation, to the forma- 

 tion of resin canals. 



The resin cells sometimes occur in pairs, but more generally 

 as isolated structures separated by one or more tracheids. The 

 terminal walls are transverse and more or less strongly marked 

 with simple pits. The side walls, especially the radial, are pro- 

 vided with simple pits, though often few in number, and this 

 feature serves to a large extent to assist in their differentiation 

 from adjacent tracheids of similar form (fig. 36, c}. It neverthe- 

 less not infrequently happens that in transitional forms, such 

 as are met with in Sequoia sempervirens (fig. 36, c), bordered 

 pits occur on the lateral walls. 



The resin is in all cases massive and often very abundant. 

 In such genera as Taxodium (plate 30) or Sequoia (plate 36) it 



