RESIN CELLS 121 



it is also to be noted that the aggregates in Sequoia, Abies, 

 and Larix, leading to the formation of resin sacs, are always dis- 

 posed in a zonal manner, conformably to the zonal disposition 

 of the separate elements, --a relation which is in direct harmony 

 with the view already advanced, that the zonal disposition of the 

 isolated cells is an advance upon the strictly segregated form, 

 and that it leads directly to the formation of resin passages. 

 Following upon the zonal distribution, a more complete aggre- 

 gation results in the formation of local groups of short resin 

 cells ultimately leading to the formation of a true resin canal. 

 Such a feature of distribution, occurring in genera which, from 

 other data, may be shown to be relatively high in development, 

 is in itself significant ; but we further find that the scattering, 

 zonal, and grouped forms bear such relations to one another 

 that the real succession is in the order already given. Thus 

 while both species of Sequoia are characterized by scattering 

 cells, S. sempervirens also shows them aggregated to form 

 groups and eventually imperfectly organized resin canals ; or, 

 in Cupressus, the transition is expressed in a more complete 

 form, involving all three modes of distribution. In Tsuga there 

 is an obvious tendency toward the elimination of the resin cells, 

 which are now greatly reduced in numbers and confined to the 

 outer face of the summer wood. In Abies a similar tendency is 

 also manifested, but it is expressed in a different way, and just 

 here we must note a fact of more than ordinary significance. 

 Resin cells are present on the outer face of the summer wood in 

 A. grandis, A. concolor, A. amabilis, and A. magnifica. Groups of 

 resin cells are present in A. nobilis, A. concolor, A. bracteata, and 

 A. firma, but it will be seen that in only one case-- A. concolor 

 are the two forms of distribution presented in the same 

 species. This is in direct conformity with the idea that the resin 

 passage eventually displaces the resin cell, bringing about an 

 obliteration of the latter, and it goes far to support the idea that 

 with respect to these particular structures the genus Abies occu- 

 pies a transitional position, standing next to Picea and Pinus, 

 from both of which the resin cells have completely disappeared. 



