Nos. 451-452.] ^^V^T'O;^/^'' OF THE CONIFERALES. 539 



the cell is farthest removed from that which is found to enter 

 into the composition of resin passages, whence they are also to 

 be regarded as of a primitive character. This view is supported 

 by the observed fact that those genera and species in which such 

 segregations occur, are also of a relatively ]irimitivc tyi)c. With 

 an advance in organization, there is a tciulciicy to the tormalion 

 of aggregates as expressed in the zonal distribulioii nf Taxotlium, 

 Libocedrus or Sequoia, where we also tiiul the definite Idi niatioii 

 of groups of cells which later exhibit the initial sta-es in tlie 

 formation of a definite canal. Ikit in Sequoia, as also in Abies 

 where similar changes take place, the more eoni])lete a-icre,i;ation 

 of the cells is invariably accompanied by structural alterations 

 whereby they become greatly shortened and more strongly 

 pitted, while they are always accompanied by parenchyma 

 tracheids with which they are interchangeable. In this connec- 

 tion 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 aggrega- 

 tion 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 tiie 

 real succession is in the order already given. Thus while both 

 species of Sequoia are characterized by scattering cells, .S. scm- 

 pervirens also shows them aggregated to form grou])s and 

 eventually imperfectly organized resin canals. Or in Cupressus, 

 the transition is expressed in a more complete form, involving 

 air 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, 



